Pseudo conjunction in the cyclopropane ring

; ~ 'I ,_ 'tl

',l;; '\' \

. 1-/\ (, AJ3STRNJT

INTRODUGTION

EXPERIMENTAL SYNTHJI:SI!}S

KINETICS OF DINITROPHENYLATIONS 01l1 _{PfiENYLCYC LOPR OPYLAMINES}

KITf:lnfJ.1_{ICS OF DIPHENYU!Il!:THYLATIO:NS}

OTI1 _{BI!lNZOIC &} PHENYLCYOLOPROPANE-1

41

67

76

CAHBOXYIJ:W ACIDS ( R7)

D!BCUSSION (I 0 0)

SUMMARY

I?.EPmHJiJNQ]IJS

his comment

Dr~ .A" Campbell, University Otago, helpful micro-ana.lyt serv

Dr.. B., H.. ) of the University Aucklanr1,

tsio:n,

and helpful commen.ts on interpret.ing, some of n~ m. r ..

the Pund lows hip

Unless otherwise ment melting pointe other general bElen t from Beilete:ins 11_.~..,<AJ.I

... J der

s-and struc·tural data 't~''.Ha.Hdbook

of' Organ.:tc Structural Analysia11

9 or in.

Unless otherwise ment ultraviolet

A lable data on the properties

cyclopropane shows that the common assumption that cyclopropane can conjugata with unsaturated systems is tionable.

and a e of a study was the

phenyloyolopropylamines wer~ prepared, of the basicity and nuoleophi ity reactivity Df the 'former, and the

r::J both.

It war:~ shown on the ni

ar to c

·1.1'/ith the

A

po

sidaration

aanno·~ be

i

It i cone

not with

are not

ility, the

that

the eye

more polarizable

es

con-INTRODUCTION

Bonding in Ozclopropa~!·

TH!: LIBRARY

Ci\NTEIU~UI\Y CHi~lSTCHURCH, N.Z.

Since the concept of the tetrahedral carbon atom be-came accepted, the cyclopropane ring has been something of an enigma; its internal angles are far from the tetrahedral angles, yet it is readily formed and it is quite stable. Because of this it has attracted considerable attention from theoretical chemists who wish to describe its bonding and predict its properties.

The earliest, and possibly the most obvious explan-ation, was offered from the Baeyer strain theory, and in the 1920s, what could not be explained by the strain theory was at least thought to be a consequence of the strain present .. However, by the 1940s the startling success of quantum mech-anics seemed to lead to the widespread. belief that all

chemistry could be understood by solving the necessary wave equations, and that a g_uali tative understanding could be ob-tained simply by considering the electronic factors present~

Thus H.

c.

Brown1 quotes from a standard organic textbook of 1941, 2 "Steric hindrance ... has become the last refuge of the puzzled organic chemist .. " Of course, cyclopropane presented difficulties to a wave mechanical treatment, since, following the assumption that the energy levels of all atoms are similar to hydrogen, carbon was allowed only s and p electrons for bonding, as these are the only reasonably low lying orbitals available. Since this limits the minimum angle between any two orbitals to 90°, the bond angle of 60° for cyclopropane became somewhat exotic, and demanded a

special treatment.. In the late 1940s two such treatments were puhlished3,4, and although these seem rather different, they both lead to some common conclusions ..

assumptions

a

be seen

lo-to

to

Thus for the solvolysis of cycloalkyl halides or sulphonate esters, since the transition state will have ionic nature, the geometry around the carbon atom will prefer to go from tetrahedral to trigonal. (This will apply to both SN1 and SN2 reactions, since the latter transition state is assumed to involve a trigonal bipyramid.) For the small ring sys-tem, where there is already strain in deforming the bonds from the tetrahedral state, the strain in deforming these bonds from the trigonal state will be markedly increased,

and the amount of increase of strain energy will be greater the higher the original strain. This will result in the solvolysis rate for cycloprop~rl haliCtes being rrru.ch smaller than cyclo"butyl, and this in turn being smaller than other cycloalkyl halides.. As will be shown later, this is observed.,

Between 1925 and

1945

the were numerous treatments of cyclopropane, 6 V\falsh himself contributing one of them, and most of these containe1i features that reached a height of'

so-phistication in Waleh's final treatment .. Waleh conaidered that each CH

2 group was similar to the CH₂ in ethylene.. l~ach carbon forme three sp2 hybrids in one plane normal to and bisecting the plane of the cyclopro-pane ring, and one p orbital in the plane of the r•ing. Two of the three sp2 hybrids are used to bond the hydrogens, while the third forms a three-centred bond in the centre of the ring. The remaining p orbitals then overlap with each other, the axes of these orbitals making angles of 60° with each other.,

For the carbon-carbon 'bonds Walsh considered that there were three molecular orbitals available :f'or both the sp2

elec-trons and the p elecelec-trons. These may be represented by:

_1

~

r

1 +

r

2 +

r3 )

(3)2

The fi:rst of these represen·ts a wave having no nodes, the second a wave having a node through atom

3

and bisecting the 1-2 bond, and the third a wave having a nodal line

through the centre of gravity and at 90° to the other nodal line. Orbitals ( 2) and ( 3) ar•e degenerate because of the two-fold symmetry of the available space. The sp2 atomic orbitals will f'orm one stable MO of type (1 ), but canno·t use types (2) or (3) as these are strongly antibonding. For the 2p electrons, o:t•bi tal ( 1) is antibonding, but

orbitals (2) rmd. (3), although they have antibonding

charac-ter~ hetve a nett bonding effect, and will be useH:t.. Thus

Walsh stattes that the cyclopropane rtng is stable since there are s electrons and. three bonding orbi tale in which to

place them ..

Since the cyclopropane carbon atoms are essentially in sp2 hybl"'idization, Walsh predicted that ·the cyclopropane ring should OO!ljUgate With an Unsaturated system if there is overlap of a cyclopropane p orbi·tal with a p orbital of the neighbouring unsaturated group, transmit it if there are two such groups, and that the conjugating :power should be 'between that of alkanes and alkenes.

Walsh did. not specifically state the size of this con-jugating power, but he made the following observations.

Originally he had assumed sp2 hybridmzation with completely non-localized bonds. This should lead to conjugating power

equivalent to that of alkenes and an H-C-H bond angle of 120°, but Walsh did make the following qualifying comraents,

presumably to allow for bond localization ..

thus the overlap is neither sideways nor endways. In so far as it is the latter, aecol,.ding to Walsh, th~ra is a tendency towards sp3 hybridization and tetrahedral bond angles., Thus, ste.tes Walsh, the H-C·H bond angle and the conjugating power of the cyclopropane ring will be reduced. relative to an ethenoid group. In Walsh9_{s only assessment}

of the magnitude of these effects, he states that sideways overlap, with the resulting non-localization, will be

greater than endways, so that the carbon valenc;v towards hydrogen will 'be nearer to sp2 the.n sp3 ..

Since the axes of the p atomic orbitals make an angle of 60°, it seems reasonable to assume, if' this angle does ac·tually affect the hybriclization which is an assumpt-ion that Walsh does :not justify, that since the overlap is

66%

of a sideways nature, then the cyclopropane p electrons will have approximately 66% non-localiz<~d character. This would result in a conjugat.i:ng power for the cyclopropane ring of approximately

66%

of an ethenoid group assuming that the nuniber of' electrons in the system has no effect on the conjugating power, and. an H-C-H bond angle of approx-imately 116°. However~ since then"e are 1 ~ p electrons per carbon atom, it is possible that the co:nuugating power could rise to near that oi' an ethenoid group ..

(b) Treatment of Coulson and Moffitt ..

4

4

b., The theory of Coulson and Moffitt is basically dtfferent. They postulate a model involving four single bonds f:r.>om each carbon atom, the C-C bonds being bent .. They assume that the axes of the orbitals are not directed tetr.>ahedrally, and that the angles are determined by hybri-disation d.iffering from sp3. · They consider• the hybridization

6 ..

The hybrid.ization ratios that are thus obtained are 1 .. 51 for the c~-H bonds and 2. 03 the

c-c

bonds ..

(These may be compared with 1 .. 73 for sp3, 1 .. 41 for sp2 and 1. 00 for sp hybridzt_zation).. Hence in cyclopropane; these calculations predict more p character in the skeletal bonds t?n.cl less in the suberti tuent bonds than is found in normal alkane!3., Havi11g found the hybridization, they plot certain properties or hydrocarbons of lcnown hybridization against hybri<lization, and from these they calculate values i"or the corresponding properties of cyclopropane.

Their calculated force oonste.nts are in good agree-ment with those measured, ana. they calculate the H·-C~H bond angle as being 116°.. They also calculate that the axes of the atomic orbitals forming the ring bonds are inclined at 105° to each other. From the fact that these bonds have

ater 11_{p oharacter}1_{~ they preCI.ict that the cyclopropane} ring should conjugate and transmit conjugation, although not as well as an ethenoid oup., ':Phey· also calculate a reson-ance energy of 3 ev P•~r molecule, which is approximately 70 Kcals per mole, and a "strain energy11 _of

_{1.5 -3}

_{av per}

molecule, 'Nhich is 35 - 70 Koals per mole., Since the

experimental value is 27.5 Kcals per mole, this can be des-cribed as fai~ agreement, especially since the calculations were somewhat approximate.,

Coulson and Moffitt regard their treatment as funda-mentally different to that of Walsh, which they claim ha6 too many assumptions and is of no value for calculating energies.

claim:3 that his treatment, although less rigorous, is more p~.ctorial ancl better sui ted. for tatbre asBessments. Coulson and Moffitt claim that Walsh t s ccmc.~ept o:l:" vtu~ying

as Coulson and Moffitt's, as shown above.. A fur>the:r similarity between these two models is that both claim that there is a high charge density at the centre of the lling. Thus thane two models, although seemingly quite dH'ferent, lead to ma:ny similar conclusions.

(c) Treatment of Coulson and Goodwin~

These two theories agreed well with the available structural information, but in 1958 revised electron diffrac-tion work on cyclopropane was published,.

In 1962 Coulson and Goodwin published a paper calcu-lating the bondi of cyclopropane using the ·theory of

maxirnurn overlap. This was essentially only a slight modi-fication of Coulson's original theory- the bent. bond con-cept was retained, but th.e new H-C-H angle waa asf:~umed. to ·be correct and was used in the calculation to f certain

parameters. Unfortunately no physical properties were mentioned1 nor was any reference to conjugation macle; only

certain wave-·mechanical parameters were calculated., r-rowever, one o:f' these was of interest.. ':Phe angle between the axes of the orbitals used in C-C bonding was 109°, which is vil•tually the tetrahedral angle.. Presumably under this th(:lory, con-jugation would not be expected; at least no more than would be expected from ordinary single bonds ..

Gon.o~J2:l. of conjugation"

It iE~ J':'ound expe:rimen·!;ally that when lone pairs

or

double bond.a ElJ.ternate Vvi th single bonds there is a modi-fication of the J:ll"operties, and. the resultant system fre-quently does not. behave as if it consisted of the two

ind.epende.nt functional groups, lmt ra.ther as one new system., Thus bu.tadiene cen add halogens by a 1 ₁~. mechanism,

8 ..

to those expected of a keto-amine and. a keto-e~lcohol

res-pectively. This non-additivity of properties is termed con juga ti on ..

Many of the modern bonding theories postulate that electrons are not necessarily in localized bonds, but may be clelocal1zed over a whole system of bonds~ and this is

termed conjugation, or resonance. Since this

wave-mechanical conjugation occurs mainly when unsaturated systems are either cumulative~ or alternate with single bonds, and since these theories have had remarkable success

at explaj.ning the phenomena o1merved expex•imenta.lly, the

two uses of' this term have been assumed to be syn<)nymous~

:rhJ.s has led to thE: 1)E:Jlief tha:t the pr.•opc:n.•ties of systems contain:tng unsaturated functional groupr:; alternating with sj.ngle bonClJ::J have to be explained in terms of non-localized electrons, or Ftt leaGt any explanation must be consistent

with such a theory"

./l,s d1scuG above, the wave meeh.anical theories for cyclopropane pred.ict that there should be conjugation between

the cyclopx•opane ri:ng and any suitably placed unsaturated.

l::iem on the grounds that there should. be electron d.elocal-b;ation between the two sy~:~tems., Hence the problem to decide vvhethex• the proper•ties of cyclopropane compounds need

to l;e explained by conjugation in the sense,

and if so1 what the theoretical picture is that best accounts for it~

S -~.I:}ll~ 1D!.ral Data J.Q]Z_Qy~~-nt;J..J]~.1!llil~.

(1) Force Constants~

Forcr~

con,gtants8 for methylene C-·H Btretches have r)ee:q.

determined, and three releva.nt fcl~fCH~s cwnstants are, in

dynes/em: (a)_ .Alkane L1 .• 6 x '105, (b) Ethylene 5 .. 1 x 105, (c) Cyclopl:>opane 5., 05 x 1

o

5.. This su1::,:gcsts a, t'ltrong simj_lari ty

(2) Bond Lengths.

The bond length for the C-0 bond in cyclopropane9 has been found by electron diffraction to be 1.535 A.

Micro-wave spectral studies on 1,1-dichlorocyclopropane10 give

the following bond lengths: 0-0 1 .. 534t 0-H 1 .. 085, 0-01

1.734. The close agreement between these two determinations

of the

c-o

distance is striking, and shows that there is

a very slight shortening of the

c-o

bond compared with

alkanes, but only very alight~ and this suggests that the

c-c

bond is essentially a normal single bond..

on

the

other hand, the C-H bond length is almost identical with

that expected of a sp2 C-H (1..086). Perhaps a better

com-parison is from the C-Cl bond.. ~1_{he corresponding}

bond lengths for methylene dichloride and

1,1-dichloro-ethylene are 1.77 and 1 .. 69 A otively, and this would

indicate that the C-Ol bond in dichlorocyc is

mid-way between the lar bonds tc'> a centre ..

( 3) Bond angles ..

Prior to 1947, when the wave mechanical theories came out, the accepted bond angles for cyclopropane were9: H-C-H

118

±

2°, H-C-C 116.4°. This strongly suggests sp2 hybridizat.:.

ion, and Walsh originally strongly criticised Coulson awl

Moffitt's preliminary treatment of cyclopropane since they ob-tained a H-C-H angle of 113°, which they later revised to

116°. However, Walsh made no mention of the ClwC-01 angle in

1,1-dichlorocyclopropane11 , which is 11 , and was available

at the time., This latter value was determined by electron

diffraction and has since been redetermined by microwave

speotroscopy10 and found to be

114°38'~

The same studies

gave a H-C-H angle of 117°35', which agrees with the

elec-tron diffraction data above.. A microwave study of

cyalo-propyl ch.loride12 gave the H-C-H angle as 116.2° and the

H-0-Cl angle as 115 .. 8°., Again the general agreement is

good, but this study also gave C-O distances of 1.513 A,

1

o ..

l~'inally, in 1958 Pevised electron diffraction results were published foP cyclopropane 13, and the I.I-0-H angle

was found to be 113

±

2°~ Hence it can be seen that the bond angles lie between those expected of tetrahedral and

tri-gonal carbon atoms.,

( 4) Q,UFJdrie{pole Coupling ..

In vinyl chlor>ide there ls a differenc(~ of' 10 Me/sec betwec;n quadrUp)ole coupling uarameters v _{· - ~ ~ ~'-xx} and 7C _yy' • and

this is taken to indicate that the p orbital perpendicular to the plf~_ne of symwetry is sign1.f'ican:tly invc>lved in the

C-Cl bondo In chlorocyclopropane12, this difference ie

I

2/Mc/sec which is identical to thB.t found for· alkyl halides .. Becaur.':l!e tht3 q11adrupole coupltng consta.nt c1id not fit an empirical plot Hgainst C-Cl length, these au.thors aseumed that either the bond length was too short or there was less ionic ohar:-'lcter in the honcl. They assumc~cl t.hs.t the seconrl was the correct reason~ unCI. d.ecld.ed. that this would be cause(l by bsC'J<-eonju.gation from two_ p orbital.s, each con-tributing eqw=tlly. Thir3 ts a most unusual postulate as it demands overls~p with two p Ol''bi tals on the r..:arbon atom,

which in t\n'"'n implt~Hi sp hybrio.ization. Thus it could quite well be that th.e lack o:e fit o:C' cyclopropyl chloride on

this plot could be due to some other cause, ln \lllhich case this dEtta must be taken as strong evidence against conjug-ation between the chlorine and the cyclopropane ringa

c

l}!?m! o al_~.PJ?!!•

'I'he idea that cyclopropane can conjugate pro1Jably ru-..if:.;es in ptwt from the ol:mervatton that cyelopropane under-goes additioTh feactions with many of the reagents that can add to ethylene. Thus cyclopr,YQfme :!.s "like an olef'in" a.nd

hELnoe 11 can COt1 ;jugate". However~ the facility with whioh

1 1 "

hand with the lower reactivity of larger cyeloalkanes seems support for regarding ethylene as a highly strained

two-membered ring compound.. The fact that addition

re-actions occur with cyclopropane does not necessarily indicate that the ring bonding electrons are available for delocalization into attached unsaturated systems ..

Furthermore, in the case of ring opening reactions with an unsaturated group adjacent to the cyclopropane ring, if

one of the ring bonds to the tertiary carbon atom can

break by an ionic mechanism, any charge on the carbon atom will be stabilized by resonance as it is conjugated with

the unsaturated group. This will be so irrespective of

whether the cyclopropane ring can conjugate or not., This

observation alone 'JV:l.ll explain Mar•kownikoff E:i.o.d.i ti onf and

tend.s to nullify such 8tatements as "r•eactions involving ring opening go f8.ster when an u.nsatura.ted group is adjac-ent, and this proves that the cyclopropar:te ring is in conjugation" ..

Some papers have ·been published reporting evidence for conjugation by cyclopropane, though it is extremely difficult to see just why the results indicate conjugation. One such paper deals with the reaction between substituted

cyclopr·opanes and. mercuric acetate in methanol'l5. This

reaction involves the breaking of the

1,3

bond as if by an

ionic mechanism, with a methoxy group being added to the moat substituted position and a mercuroacetate group to the

least substituted position., The percentage reaction after

3 hr at 20° for the substituted cyclopropane was : phenyl

71%, trans 1,2-dimethyl 48.5%, trans 1-ethy1~2-methyl 18.5%,

ethyl 3.5%, isopropyl 1o5%. After 21 hr: trans 1,2-diphenyl

3 .. Br~, cis 1 t 2-diphenyl 3., 5%, isopropyl .3%, 1, 1-diphenyl 2~ 5~

methylcyclopropylketone O%. After~ hr : 1,1,2,2 ....

tetra-methyl 100%, 1,1,2-trimethyl 87%, 1,1-dimethyl 79115%

1, 2-o.imethyl 79 .. 5%. The authors claim that these results

of conjugation and hyperconjugation. \

H:ydrogenol;y:sis.

One of the most studied reactions is the

hydro-genol~sis

of cyclopropane6• Many earlier workers re-ported that catalytic hydrogenation of cyclop:r•opane pro-ceeded at rates comparable to those of alkenes, but as Lukina pointed out, most of the catalyst carriers in this work rapidly isomerized cyclopropane rings to alkenes, and it was the hydrogenation of the alkenes that these workers were measuring. Among the substances quoted_ that rapidly isomerize cyclopropane to alken~s are silica 9 alundnoBilicate, lt;i.eselguhr, nlumh1imn oxide, and at ele-vated_ tElillperatm..,es~ ;:;mmice ana. ~:w·tivated. carbon ..

'1'her13 havo b~c:n n:mnerous reportR of oyclopropime :rings conjugo.t(~d to n double bond 'being hydrogenated. j.n cond.i tione v\rhere [l nghl eyclopropane ri:ng would not reF.Jct,.

It is claimed that thir::J t:~hO\'VS ·that the cyclopropane ring conjugatt:)s el<:3e'trorJ.ically, and allows 1-t~ ad.di tion.. A

typical exam.ple is when 2-cyclopr•opyl-1-alk:enes and 2-cyclo-p:ropyl-2-alkenes are hydrogenated vd i:;h ba~C'ium pr•omoted

copper chromi te catalyst.. The. former hyd.l"'Og6lnater3 readily to cyclopropyl alkanes, whtle the latter hydrog€lnates

slug_9:h1h.ly to give cyclopropyl alkEJ.nes &nd open chairl

e.lkfmes., The authors claim that. this is becau1:11e the latter

- 4th 1 ,.., 1 I. d--'~·'t' 16 cun UJ'lfiergo e;. J . er -..,::- or -y.-· Z'J ttJ. JJ.on. _

Howe"irer the ideR of 1-'-t-ad(H tion is not shared b~r Ul1man17, who ,9:E'ter examtning a lBrge amount of data con-siders thot all the ~prodJJ.Ctfl c~om~ from a ccnnrnon tntArme

iate. The rnechaniRm he proposes is that firstly the organic compound ts chemisorl!ed by the c;I'J.ta1yst, and this is :follow-ed by a hyclridc: i:lLtar:,k on the double bond, f'nrming a ce.rban-ion--metal com:plex~ VIi th tho :negative charge on the carbon

open and form an alkene with a negative charge on the

terminal atom, or it may remain as it is. Either of these two ions can then abstract a proton. If the ring opened form abstracts the proton, the remaining double bond can be further hydrogenated

IAlkina 6 also

hydl~ogenated

cyclopropane compounds with pallarlium black catalyst at room temperature. Only cyclopropane rings conjugated with a double bond are reducedt and hlkina claims that this indicates the exis-tance of conjugation.. However partial hydrogenation shows

·that it is the ~~yclo:oropane ring that is hydrogenated first, nnd thut th~ roaction in specific for p~lladium. As d1s-cu8aed eerlier, this may merely indicate that a polar (or

Enren racl :.:11) centPe may be fo:r.'med. tn the tran::3i tion state at a ~jJ_£l, con~]ugated vrt th the clm.lble ·bcmd. ·- it d.oes not

pr•ove thRt thv: cycJo;wopan.e ring elr:H'\t:t'on;::5 co:njugating Yli th the double boncl., Phenylc:~rcloxn'(Jparw 1I;yd:ro?:en.ates 90

ttmen mort'3 slowly with plntinum b Dnc!. t.he be:n.zen~;) ring is reduced as well as thA cyclopropane ring. !~kina also cluime(l tha.t 1, ~~-cUphenylo:-,rclcYp:r'opane behHYes ,9.s a o.on~Jugat­

ed cyclopropane spectrosoopioally anrl towards hydrogenation,

while 1 ,, ·l ... diphenylcyolopropane d.id not., I.·cukina argues that this is because in 1, 1-di.phenylLcyclcrpJ'opane, the special ai'Ninp;ement of the two phenyl groups is one where the axes

of' the clouds a.r·e not parallel wi t.h t:.h~) plane of the three

membered r:tng~ However the spectral evid.ence was based on increase of' intensities only, and. was not accompanied by a frequency change. At:. will be discussed more fully below, the :i.ntf;nsi ty depends on mony rnor·e ·things than (Jon juga t ion, a:nd is very dependant on steric envix•onment. 't'he

Hydrogenation would. cel"ta:inl;y be affected by st~rj.o

environ-m•::nt1 and if' the :i:'eac·tion went by hyd.ride attacl{ and t:1 posi tivo metal oen:l;re, allowance munt be 111ade :eor tho fact

that one molecule hRs two rj.ng atoms with phenyl group

which would be somewhat sterically hindered.

It has sometimes been assumed that the cyclopropane ring must have ~ electrons because hydrogenolysis proceeds at all, but this argument is not valid. Lukina6 found that under identical conditions ethyl cyclopropane was completely hydrogenated at 50°, methylcyolobutane at 250°, while the reaction was just commencing for cyolo-pentane at 250°. As stated earlier, such reactivity

differences are quite simply explainable in terms of diff-ering strains in the oycloalkanes.

Addition Reactions.

Another reaction that has been favoured for showing that "the cyclopropane ring has olefinic properties" is the 1-3 addition of halogens and hydrohalic acids. Here again the reactions are explainable in terms of strain energy, and the consequential increased reactivity in ri opening reactions.

(1) Addition of Halogens.

It has been shown that iodine adds to cyclopropane to give 1,3-diiodopropane at 250°, and that the reaction was not influenced by light18• However the analogy does not carry over to the other halides. Bromine reacts with cyclo-propane slowly, and only in the presence of lightt giving a mixture of products, some of which are ring opened... A free radical mechanism has been postulated1

9.

Chlorine seems to be unable to open the cyclopropane ring, and sub-stitution occurs mainly forming gem dichlorocyclopropane 20 • Hence only iodine really shows the expected reaction. A

weakness of this bonding, this could explain why the iodine reacts with the cyclopropane ring. If the cyclopropane ring has a high electron density, it could appreciably polarize an iodine molecule in a collision, and if the polarization was sufficient, ring opening could occur with the departing iodine ion being electrostatically attracted to the forming carbonium ion. The reasons for iodine re-acting this way and the other halogens not could be that iodine is more polarizable, has a weaker binding energy, has a higher apparent positive charge because of more im-perfections in screening, ana. possiblrr because its velocity is lower for the same kinetic energy.. In any case, there does not appear to be any definite evide~ce for the presence of a~ bond from these reactions, although there is definite evidence for the cyclopropane ring having higher electron density than have alkanes.

( 2) Ad_di tion of Hydrohalic Acids.

A similar reaction is the reaction between cyclopropane and the hydrohalic acids. This had been observed many times, but it has been shown that there is no reaction at all if both reagents are pure and anhydrous, even at 300°. 22

It has also been found that the kinetics of acid attack on cyclopropane are similar to that of olefine; and in fact the cyclopropane reacts five times faster. 23 In both cases the initial and rate determining steps is the addition of a

proton. If it is granted that the strained hydrocarbon could attract a proton and form a complex, it would have to be a ring-opened complex, and the strain theory would adequately explain these results. If, on the other hand, an explanation of this reactivity towards a proton is sought in terms of the polarizability of the cyclopropane ring electrons, then the somewhat unpalatable conclusion is reached that the

Metal Complexes

Since olefins f'o:t?m f( complexes with various metals, several attempts have been made to form complexes with cyclopropane. Although no complexes are formed. with cuprous chloride, mercuric acetate, or silver nitrate2

4,

all of which form complexes with olefins, cyclopropane does form a complex of empirical formula PtC1

2

c

3

H

6.,

This is

formed by bubbling eyclopropane through chlo:r•oplatinic acid~

and the cyclopl"Opane seems to be intact in the complex, as decomposing the complex with cyanide ions results in pure cyclopropane being evolved25 • The formation of this complex was taken to be evidence of delocalization of the ring

electrons, but this seems somewhat doubtful, since propylene itself did not form a similar complex. This would again indicate that cyclopropane is more olefinic than an olefin. It is also interesting to note that

4

moles of cyclopropane were required to every 1 of acid, and that no complex was formed by bubbling cyclopropane through chloro:Platinite ions.,

Lewis Acid Oomplex~s

F.vtdence for cyclopropane reacting like a d_ouble

I

bond is claimed with a Diels-Alder addition of malet·Cu anhydride to oyclopropylstyrene28• One product was the compound shown, and was assumed to have been formed by 1,5 addition. However, again the reaction involves

ring opening, and again csan be regarded as a consequence of' strain.

()

II

/"""

·-r

II

o

~

₀

0~1'\e-t'

t

_{'? '/" o civ~-c.-"f 5}

An interesting reaction of keten is the acldi tion

:1f' water which gives acetic acid~ rt'his addj. tion across the car1Jon-carbon daub bond is usually attributed to mesomer-ism, where a lone pair on the oxygen tends to form a triple bond and pu.t a negative charge on the carbon atom., If cyclopropane were also able to conjugate it might be

expect-that a similar reaction might occur with ring opening of ·cyolopropanone to give propionic acid., However this.does

not occur. Tnst the strain is relieved partially by the formation of a stable hydrate, 1₉1-dihydroxycyclopropane2

9.

Unlike alkenes, cyclopropane compounds are generally inert to mil!~ alkaline potassium permanganate, ozone, and other comn1on non-acidic oxidizing agents. Furthermore, it should r~membered that many of" the addition reactions mentioned above are alae found, though to a lesser degree,

18 ..

into the theoretical picture of the cyclopropane ring having 11_half-alkene11 _{properties. Addition of acids} pro-ceeds at a rate greater than that of alkenes1 while the

addition of halogens follows the opposite order to that t:ound. for a.lkenes. For alkenes,. chlorine adrls faster than bromine, both by a heterolytic mechanism, and iodine is relatively inert. For cyclopropane, iodine is the only halogen to add by what appears to be a heterolytic mechan-ism, bromine adds by a free radical mechanmechan-ism, and chlorine cannot add. This seems to indicate that the cyclopropane rtng in fact does not have "olefinic properties", but is simply a stra1.ned hydrocarbon.,

Rea(1tivity o_:r Cyclopropane Compounds.

The solvolysis of cyclopropyl halides and sulphonate esters 'i.s t.'nttremely slow3°, and this has been taken as

tWid.ence of' ·1\ bond.ing l)e'Gween the halogen or oxygen and the cyclopropane riiLg~ but these results are just as easily explained 'by Brown in terms of' strain 1• Brown's theory is well supported by the fact that addition reactions to cyclo-propane are extr•em®ly rapid~ and this same sort of reactiv-ity is found in the cyalo'butane series for both halides and ketones, but to a lesser degree.

On the other hand~ cycloprop;vfcarbinyl halid.es sobrolyse

with unusual rapidi ty3°, and. give virtually the same products as the cyclobutyl halides., The products for the cyclopropyl-carbinyl system are

48%

cyclopropylcarbino1,

l.J.7%

cyclobutanol, and

5%

allyl ca.rbinoL For the ethanolysis of benzene sui-phonates; it has been found that the cyclopropyloarbiny1 rate is the ethyl rate. This has been explained by a non--claSLdcal O&lr•bonium :ion as an j_nter·mediate in the cyclo-. Pl"Opy1ca:rcyclo-.'biny1 case; the so-called bicyclobutonium ioncyclo-.

When eyentually clata was· found that dlo. not fi·t a single non-classical ion, Hoberts31 postulated a series of rapid.ly

this aould just as caused

0

At

the moment is considerable

whether there such an ent 1 ty a a a lHm ... o

iurn

ion3J. If'

they

in

t

at,

th~y

equili bra:ting

controversy over ical

carbon-not

arily prove that the cyclopropane can conjugate, as

naan.y of the postulated ions involve molecular :rearrangement,

with formation of' electron :tent bonding similar to that

found in the isoelectronie boron hydroiclEtHh

Solvolysis 3-subatitu'ted

34 _

indica. tea that vex•y lit

la.yed to thnt poai ti

em.

•.rhe

cwolopropylc arb inyl

charge is re-reactivity

was ois-2-phenyl (

o.

) ,

unr:mb i tut~~(l { 1) trans-?.-phenyl

( 19)., 1'hese are rela.t i3

(3

-napthalene-sulphonate est 'V~Jar:~ foun.d for

trt-fluoroaaeta e confirmed

ers solvolysed ~.t"t vir·tual

pro :pyla iny 1 "

little contribution t

oarbinyl

as the

oyalo-sti t11ent me

stab:tl:tzatinn

indue-tively, but makes t.l aonai<l~n"'abl~ contribution meaomer:taally,

these results must indicate that there is no conjugation

be-a.nd the <:Hl.rbonium

T~e.eul.ta that have yet to 'be

on ana reaativit ate that the

does not conjugate vd th a ctarbonyl

detail32,

l.op1•opane ring B:r>own., without

giving details, quotes the follow iva rate

retard-ations for ketone l"lftactions: acetone 1, i.Bopropylmethylketone Lj .• 3, diisopropylketone 11 ayolopr,opylmethylketo:ne 68,

d1oyclopropylkatone 6000. Brown

are

quite oonsi ent with the

and sterio eoure$Se

n th~t these results

ted. fr-om lnd.uotive

on

t

in the cyclo'};J:taopyl

to

nases the ortho

method

( ·oiseo

one

1

phenyJ-do nat ...

th~

ty

both

lo-21 ..

pr·o:pane l"'ing and the other is free, while in the non-bisected form; both are equj.valent .. ) However~ assuming

the ring curr·E:mt mod.el, thE;.~ bisected conformer is claimed to be the most ste.ble37,

Other workers have claimed that the r•ing current model is not good for predictlng chemical shiftt~, and

that better results would be obtained if a value of molecular anisotropy different from the usual carbon-carbon single bond value was used., They suggest that the value) for cyclopropane> shoulc'l be multiplied by 3 .. 638., It

is difficult to se.y v,rhat ef'fect this woulo. have on the above confol'mer identif'ic&tion ...

Morl::l tnridence i~ol"' the btuectect f'ol .. m being the Btable conformer has come fl~om rates o:t' Bo1vo1ysis of' para substituted t-cumyl chlorides39.. 'fhe p·~cyclopropyl

ou:n has a m~:1rked fH::Celerating effect on the rate, whtch is maintained i:t.' there iE~ one mett1 methyl t~r·ottp. In the first case, th€1! cyclopropane :r•ing can b.ave Ed.ther

cont'or-mation~ hnt :tn the second case, it is probably more in the bisected form with the cyclopl .. opane ring being away f'rom

the methyl gr•oupa When there are two methyl groups ortho

tr,) the cyclopt•opane ring, however, steric strain is almost cer•tain to force the cyclopropane into the non-bisected. situation, a:nd this should red:uce possible eongugation .. J1jxperimentally it we,.s fonnd that the :rate wa.s reduced by a factor of'

4 .•

5 when the second n1(:}thyl wa.s introduced., but as will be discussed. mor·e fully late:r;', a. Hammett plot of

their data shows conaido:rable scatter1 and their

3,5-dimethyl-t-ournyl chloride, whioh was used t"l.s a standard, shows a

similar retardation. This data is also in direct contra-diction to the n. m. r~ data quoted above~

cyclo-propane ring were attempting to be coplanar with the vaca.nt. p orbital on the trigonal carbon atom, and this was taken to be strong evidence :t'or conjugat:i.on. However,

it is found generally that a cyclopropane ring and a tri-gonal substituent invariably have only two-:eold r•otation fl.bout the single bond, and thiB demands that the methyls are non-eqtdvalent., rl'his will be discussed in more

detail later.·

A study of the stabllity of carbonium ions formed from 1, 3-d.isubsti tuted. cyclopentadienes is interest.ing41 • The 1- and 3- substituents, and. the ;~ sulphuric· acid in which the cati one are half f'or•med arc: dicyclopropyl-( 1. 2%), me·thylcyclopr•opyl-· dicyclopropyl-( 121~), cLilflethyl~( jjj·~), methyl-phenyl- (50%), diphe]J.Yl-(545'0~ rl'he authox' claimo that these r•esulte indicate th.at the cyclopropane ring is better at coni:iugating ·than tr1e benzene x~ing, since the cyclopropyl group is bette·!' at s-tabilizing the car'bonium ion. However,

he does not explain just why the methyl group is also superior to the benzene ring~

The n.m.r., of cyclopropane itself :ts interesting,. Prom the quantum theor:tes., i·t .might be expected that the chemical shift of' cyclopropyl protons would be between alkanes and s.1kenes.. Ra.ther unexpectedl:.),t, however, it

appears just downfield from teti•arnc3thylailane ..

N. M~ R. CQ~lP.Jinz._.Q.c>~J! ..

Oo:nsicteration o:t• the n .. m. r .. coupling constants has

al~o 1H;;en confusing. rrhe followfng long range coupling

com1tant.s led. Po'bertA4 2 to bel:teve that cyclopropAne ring 2

hnrl elrnost tru.e sp n8.tu:r>e : · acetylene 249 cps, ethylene

156

cps, ethane

125

cps, cyolopropanes

158-166

BecauBr~ of' the sl:i.rsht increase in value over ethylene;

23 ..

these coupling constants had been empirically related to hybridization but it may be questioned whether this empirical correlation is applicable. Usually as the s

character increases, both the C-H and the c~c bonds shorten, but in the cyclopropane system, the

c-c

bonds retain the alkane

c-c

bond lengths.. These same authors found an empirical relationship between the lo~arange

eoupling constant arid the

C-H

bond length4

3,

which is

r

~:::

( 1 .,133 - 3 .. 12

x

1 o·"'4JC-H). Substituting the value

for cyelopropan.e leadJ! to a value -of 1. 083 A, in good agreement w:tth that found by o·th.er means.. Binoe i~his

value is so de:Pendant on bond lengths, it seems that it

ce.nnot be ·f"aken as further inctepenclant evidence of' con-jugation.

On the other hand, ~ onsiderl!lt ion oJ' r;.ou-pltng c:o11stan1js led Hutton and. Sc:hr.l.efer 1io t.h.e optJosi te con-clusion44

~

As8um.tng the F:T-0-H bo:nr1 angle to be 11 "3. 8°,

they clf;dmed that they could. Buccessf'ully account for vic ine.1 and gemtne.l co1.rpltng nonsta:nts, e.na. lifJ 01ridence

for vei'Y little 1Y bonding pre Gent they show that there is no apprec:l.able subst:t tuent electronegativi ty effect as is found in ethylenic compounds~ nor is there ar,prec-iable coupling to methyl substttuent.s through f'our bonds as is usually observed when the C-O bond has it' character ..

.Q:Y£:.:lO'Pr'Q11Y.:J. .§.;.f.l J!.. W11h.9J.:m!.iPS ... £1:1:tb§1lli~,.

Usually, tllf~ cyclopropyl group is considered to be an elcctr•on dcmai:;ing grott:p, 'but there are at least two references in the literature to lts ·being a wt thdr•B.11lling

41~

group,. The first ? quotes the dirole moment of

cyclo-propyl chlol:'ide, which is 0 .. 3 D lower than that of iso-propyl chloride. rrhis was simply explained by conjugation; the lone pair interacts with the delocalized systen~ A

more sophisticated explanation is that the trtgonally

i>etrahedrully hybridi24ed atom. ~rhe second reference is to the effect of size on the properties of acid or grou:ps ac1js.cont to the ri:ng46.. . The f'ollmving table

illnsi:r•ate~:J thcl0.\Cl p1'opr:n ... t:tes. ~-~he f'j.I'!Tt cclu.nm BhOVVi3

the seoond the <:liS;3ooiation oonsta:rd:; for the C:'trboxylic ac :td., the thh~·d. tJ-,€~ reJ.o.tilre Y.'r\1~1') of

reaction of Lh<.3 B.oicl vd.th <liphen;ylCU.z;c:.:;ometh~:inet the f'our'th

th~ base strength of the amine nnd th.e fifth the llase strength of the

»T,

N- _dimeth;vleyeloalkylamine~ ~,he

cUssr)cintion oonste.nts al"'e o.ppLU"E'mt ~ th.ey hnve not heen corrected for ionic strength$

N Ka X 107 X _r•e1B.tiv~ ~

2 2~96 5

:;

0.62 _{1"' 3.3} !~. 6 0 .. 5

LJ,

o.

~1 & !28 22 _) 1-

..

9

5 0.33 0.92

89

8.,6

6

o.

1

68

15

ii.B can be S'3en, cyclopropylam:l.ne appears to be the weakest base, and ·hhis is elaimed to l1e becaur::e the i~1:u:•ee lfti£1mbered r'ing oopjugates w:t th the lone ps.ir on the n:i.t.:r.•oge:n, ap.c1 f!1H.kes 1 t leGs available~

In~ra-red S~ctrosco~.

Since it was shown that the integra. ted inte:nsi ty of the o-H stretch in alcoh.ols is mai.nly dependant on the total inductive ef':fects rather than on polartzability

e:t~f'ectsh

8

,

this has been applied to the measurement of' the inductive

'O

ef'f'ect of the cyclopropane ring 1 ~ $ In the following list,

lop~opyl

oyolopropanes

tion was a function the substituents.

an identical

In the

authors

:moves

Cyolopropylbenzene Btyrene

t\nisole

on

0-H

that on

sum of the

methyl

that

iso-ituted

Thus 1-oyano-1phenylcyclopropane to 1-oyano-2-phenyloyolopropane.

or

,.A commonly

A.. max: (;'max

5

7,000

8,400

245 16,000

5

7,400

217 Lj,OO

and the to an electron

1 to the

should

ultraviolet conjugation is usually

an absorption tion

co-is as

f'ollo~~~

i1-

max: tmax

261 5

274 280

290

550

254

204

1480

The compound is not

1:r

it is

is int an a.lkyl

magnitude

ani

cally with

the extinct However, it is known that

two pB that can int t me13omet•i··

the ring, and. it is interesting to\ note that; these have less than cyclopropane.

Another example is the pyridine

The following 2-su"I:Jsti tuted are lis·t the

\JVav-elength in millim.iorQns of their" absorption maxim.a53:

277., 5, 2-<'lyclopt•opyl- 269, 2-n-propyl.- 262, pyr•:tdi:ne

253.

In add.it i.o11, f'or 2-vinylpyridine was a strong short-wavelength absorption

(:!Ub itute pyridines did not show, th.

p:r:•ob 1y by the tatic:ms the instrument.

Whl il max of' the lopropyl pyridine is about miclWfJ.Y

en. the vinyl pyridine, it :i.s qui

notioeab that. the 1.J in. series is b

ne the

A ss tensity

Phenyloyclopr•opyHcetone

!--max

6

0

study is the lovt

t.

m®.x

42

50

In thi13 case, a shift s an

in-ly d.oe~1 not

ion, then the eye

the conjugation.,

J\:nothet• stucly of l4.-substi tut

foll

iSu.bst0 P~rridine

4-:pic:oline

4-benzylpyridine

}-max

8

Acdrl form

t

:1

ma~x:

5 .3 .. 59

3&47

254.5

3 .. 67

5 l~. 11

L~ .. 2

4.

authors t eoeffioient in

the cyolopt'opyl

'rhey ap].,arently does not do this

view the increase inetion

from the to the·ao!d form for.

I

is a for conjugation.

t the fact that the phenylpyridine

:iably. 'l'hey a lao not seem to

find the that the phenyl~yr:idine a a marked

bathoohromie

undergo a

on p:caotonation while other compounds

'I'hi. a seri ea,

whether or not ld hypsoahromio shift

a out

thel'.'e is conjugation from

of poai

~ in the

A sea lar tab

ut

Pyridine se form

)v,....,.j( I o.~ 1:,

2-piooline 262

55

2 ... eyclopropylpridine270

2-S ... lutidine

268

2-methyl-S ... cyelopropylpyridine

277 .. 5

the

s of 2-suberti

t-Acid form

)v IM-C<-Jl

I

0 ~ ~

e5 3., 62

278

Tl t the oyoltrpropyl group gives the bathoehromio

i methyl rather surprisingly gives no

shift. However, it should be remembered that in these

com-pounds the nitrogen sterieally ~~~w~ crowded. This in

the acid form ially, should have marked effects on the

solvation. Solvation is known to

sorptions considerably56 and

ultraviolet

ob-28.,

served changes.

As evidence for conjugation of' oyclorn.•opane with an unsatut'ated centre, Waib::!h3 quoted the following spectral data on 2-4-dini trophenylhydrazo:nes of' the f'ollowing

carbonyl compounds.

Fo;rmaldehyti"'

CyolopropylmethylkE:~tone

. acrylalaeh;rde

;l max

349

367

3T1

fmax

18, 200 23,500

2·~J~ 600

While tl!o::Jt';~ f J.gu.x•er3 might suppor•t; -~v.9.lsh' s theory, i·t should be nute(l, that wl~tt>X'e<:<s the cyclopropyl compound is a ltetone,

the two t't~fe:c'(.:;nee compounds are ald.ehydea ..

l\(id:t:ng !~he 1l.~1ta t'm.~ two ketones makes this proof' somewhat J ,. ~'"''"' ,JrJ~~ ,..Ol'"JV"in<·• l;. --·· ~".. . . . J ~ .t... l'r:t J(.,:,5 "'1~

J\cetone

Di-isopropylketone

11 1 lV:ethylethylket.one

(2-) MethyJ :!.soprop;y~lketone

(3}Di-isoproiwlketone [tr) Methylv~n;srlk.etonE:~

(S"j Methylr~yc1opro];ylkt~tc•W:)

C6)

Diey<:..loyn"o·pyll;eto:n.e

360

.368 368 369

37LI

375

21 100

22;-000

tmax

2L~₉ 000

22,900

2l.j., 1 00

27~400 ~26, BOO 25,600

'rl:t.e£3~) X'esults, accord.i·n.g to IIavvtl1or~:ne, e.x~e 1tu3.icB tive· cxe

the ability of the cyclopropane ring to conjugate.

compounds 5 and 6 would lead to the opposite conclusion~

Furthermore, it is difficult to see why compound 2 should

have such a low extinction coeffioient.. It seems reason~

able to believe that these differences are in fact

insig-nificant.. If the frequencies derpend on conjugation, then

these results indicate that the cyclopropane ring is

slightly better at conjugating than a double bond" \<Vhat

pe:rhL~:ps is more interesting~ though is to eompare these

figures with those quoted abo'fe. FoP the di-isopropylketone

derivative, there is a 9}b difference in extinction

coef'f'ic.-ient and a difference of

6

millimicrons in the position of'

th~ pee.k maximurs1! The only difference in expe:t~imentnl

condi tiona employed. by these worl<:.ers was that the solvent for the first series waa

95%

alcohol, while for the second

aeries it was dichlor•omethane.. This clearly illuatratee

the sensitivity of ultraviolet spectra to solvent,

solvat-ion, and hence undoubtfll!dly local elec:da"'onic factors ..

It is, perhaps, at this stage worth mentioning the

spectral properties of cyclopropane itself. It is found

that cyclopropane i taelf begins to <iibsorb at 195 m

p ,

and this has led to suggeations t:h.et c;:talor;ropane has an

electronic system resembling alkenes, since alkanes do not

absorb until much further into the vacuum ultraviolst.

However, this is neatly explainable in terma of strains,

as it has b<Zlen shown that the exoi tecl state .is a trimethylene

diradieal, and that this leads to :sm open chain

:pol~ymer

75 ..

The kinettcs of. this rE:!action have been followed and the

mechtmlsm fully elucidated.,. 76

It has also been noticed that ahlorocyolopropane

ebsorbs at longer wavelength than cyolopropane77. The

anthor>s Cllaimed_ th.o,t; th.e:re was no evidence of' photo

deoompor.Ji tion, and e laimed. that this was ev:i.de:nce for

(J)

UJ

(.3)

However. they gave no details as to how they disproved the presence of ring-opening, and in view of the detailed work mentioned above, it would be very surprising if

there was none ..

The ionization potentials of certain hydrocarbons which were: cyclopropane 10.14 ev, propylene 9.7 ev, and propane 11.1 ev, led Field and Hinkle78 to believe that the cyclopropane ring must have an electron system be-tween that of an alkane and an alkene, but it should be noted that~ if the cyclopropane ring will open under ultra-violet radiation, it is more likely to under the more

energetic conditions employed h~)re₉ ~nd the 1 ev dl"OP in the ionization potential is very similar to the strain energy, which is 1~2 ev,

A more significant test for conjugation is to put a cyclopropane ring between two unsaturated groups, and see whether the system behaves as two independent chromo-phores or> as one extended conjugated system.

One such measurement has claimed to find conjugation?9 Both '1-benzoylethane and benzoylcyclopropane showed absorp-tion maxima at 238f>'lj while 1-benzoyl-~~-phenylcyclopropane

showed a maximum at 242~1- While this is a small shift, the authors claimed that it was significant. However a closer look at a larger s~lection of compounds does not necessarily support this view. In the first columns some benzoyl com-pounds are quoted with their uv maxima and extinction co-efficients, and in the f-inal two columns are the data f'or corresponding 4-phenylbenzoyl compounds

4-x-Benzoyl cpd X :::: H _:X:

=

Phenyl

,'I. f:: ). b.

Benzoylethane 238 11,500 276 23,400

Benzoyl ethylene 247.5 1

o,

500 291 22,200 2-Methyl-benzoylethylene

31.

(tf) Benzoylcyolopropane 238

u-)2-Phenyl-benzoylcyolopropane 242

14,400 18, 200

276 280

26,000 28.000 tranti;i

23,000

cis

The comparison quoted is between compounds 1,41 and .5 in

either series.. However, if compounds 1,2, and

4

are oom-paredt on frequency shif't considerations the opposite con-clusion must be reached. The mild increase in extinction coefficient of compound

4

over compound 1 nmst be neglected after consideration of compounds 2 and

3.

Finally~ com-paring compounds 2 and 3 in both oolwnns it is seen tha:t it might be extremely risky to place an interpretation on a shift of

tt-

millimicrons in ).. max.

Evidence against conjugation in the cyolopx•opane ring has come from an examination of the spectra. of the following compounds60

The first compound behaved as a conjugated ketone, while the other two gave similar spectra and both different to the first. Furthermore~ the following compounds showed very little spectral similarity and this was taken as strong evidence for the inability of the cyclopropane

61

ring to conjugate •

b

that

phenylcycl

no

ion with the benzene

"

Since mei.le of' the con:f'igurati on of phenylcyclo:pro:pane by showing tha.t it into the best conformer for c 11_{even though}

it

_{is aterioally}

unfavourable, 11 _{i·t wo1lld ted that on twi ing tha}

aye into configurations unf'avcJUr19ble

c tion~ mark~!l(l would ob the

ultra-violet er,r;(l':otru.m., thJs ie

the f (j

A ttl traviol€. t

ca oniu.m

lo);)entyl o rmium ions

tra

- 63

J.e "'

is found that a.lkyl

:no 1

ultra-violet m1111miarons, whi the

nddi tion of' one lopropyl group se to an

t1on that ia similar to an allyl c&~bonium ion., There,

however. similar! ty endso ltn a.llyl oar•twnium ion

an ion maximum at 305, e. die:nyl at 397, a:n.d io

4

70 millimicron::h Below are ~twme dt.c1ta for some oyolopropy1 carbonium :tons.,

CrJ.rlwntum i.on

Dimethyle;vc loyn:"opyl

DioycloJn~opyl

~:ricyc 16propyl

Phenyldim~thyl

phan:ylmethylcyclo-pT•opyl

l;heny!cUeyc

,"L. max

289

273

270

6

Phenylmet.hyley·clobu:t,yl

~max

10,800 12,200 22,600

11,000

15;200 1 ~,

, 000

390 1,400

~: 11-!0

1 ,600 285 16,600

33 ..

That there is any absorption at all above 220

fl.j

seems to be convincing evidence that the carbonium ion centre is interacting with the cyclopropane bonds, yet the hypsochromic shift seems to be evidence against a

con-jugative mechanism. Another odd feature is the small

increase in extinction coefficient in the first three com-· pounds. Sin.ce the cyclopropane ring is obviously partioi-pa:ting in the spectral transition, it would be expected that by increasing the number of participating groups, even if there was no conjugation, the extinction coefficients would show proporti.onal increa$es. one possible explemation

roight be that' the carbonium ion centre is interacting with one cyclopropane ring at a time by a mechanism other than conjugative, and one that depends on the presence of a positive charge. The positive charge would interact with a cyclopropane ring to form either a bridged ion or even a cyclolJutonium ion.. This could absorb to give an excited state that is in all probability very similar to the

excited state for an allylcarbinyl ion, as either of the former ions can readily be converted to the latter.. If the excd ted state x•equired a transfer of the positive charge into the rearranged systEJm, then fur·ther cyclopropane rings would not assist this, as they are electron donors, and would best help to stabilize the ionic charge in the ground

t~tate

Thermooherni~]J.>X ..

.A standard.. mcrrthod of testing for conjugation has been to measure the heats of combustion of a group in a

Harmnett Equation .•

Another standard method has involved the application of the Hammett equation, which is log k/k₀

=

f

0"" .. Rho is the constant that determines, for a given reaction, how much effect a specified substituent will have on the re-action site.. It is thus a measure of the ea.se of trans-mitting electronic effects across the. molecule, and will depend on the polarizability of the molecule between the sub-stituent and the site. Since a conjugated system is

generally much more polar:tzable than a non-conjugated one, the magnitude of the

f

VRlue con be UGed aA Fl tent for a

conjugated system. The IY veJ:ue attempts tio,aFlrc~ess the relative effect that a substituent will have on a reaction site, irrespective of the reaction.. Since there ar•e two mechanisms for transferring this effect~ j_n<J..u.ctj.ve and conjugaM.ve, it can be seen ·the.t the (t" value for· certain groups will vary from system to syatt:ZJtn, depending on

whether conjugation is possible.. For example, in the pro-tonation of" amines, if there is e. conjugative route to a mesomeric withdrawer, as there is in p-nitroaniline, in

the base form the nitro group ~rill tend to w;i thdraw the

lone pair• on the amine and lower the basisi·ty by a con-jugative as well as by inductive effect. If, however, con-juga-tion is not :present.~ as in benzylamine only ·the in-ductive effect p:c•esent and. a different value of

0"'

is

found.. Hence the \J value can also be a test for conjugation. Attempts have been made to separate the CT' values

66

into indu.cti ve a:nd :L•,::Jsonance values • The inclucti ve

v.9lues are ur~ually obtained. from measurements in aliphatic series.. These are assumed to be equi-valent in the ax•omatic series, and hence the resonance contribution to sigma can be d.erived,. This is accepted as being the. distortions

or indueto-elr3ctromeric effect~ (b) resonemce :tnteractions of the ~ electrons of the ring either with electrons or a vacant or•'bi tnl, an the q~v atom of the substituent~ ( o)

a repulsion of ring "f) electron.s by non-bonded electr.ons, (d) direc~t conjngatton of the substi tuE.mt with the reaction st te., There also seems no reason why ( c~) should be lim1 ted to non-bonded electrons. In

amr

case 1 t can 1H' SE';en that this resonance involves other faoto:r:•s as well as d.ireot o on jug; at ion~

It haf3 been J:'ou.nd that the squa:r.e root of the

inte-grated intenslties of' aromatlc inf'rared stretches corre-lates well ·with ()'~ as deri vec1 by other means, and these values have en interesting tr•emd. The following substi t-uents are listed. vvith their

6'-"R_

values: l\Il\!Ie

2 (0.535), OMe (O.tL3), J? (0.34), CH.O (0.24), Br (0,23L COMe (0 .. 215),

N0

2 (Oe17), t-Butyl (0~·125); CF₃ (0.'11~5), Me (0~10.5),

Ph (0.,10), CN (OQ09), vinyl (0-035)" It is obvious that this measures more that! conjugation as has b0en discussed above, since i t not generally accepted chHt a bromo can conjugate more than a nitro, or a n1ethyl mo:t•e than a phenyl. However, in trds department, a meacuremont o:e the

inte·-grated area o:t' the aromatie 1600 em·~i abso:r·ptit)ns has been done on phenylcyclO})ropane~ and a ~~ vs:ltH!l of 0.174 has been obte.ined68• The :veason

~or

this high value

for phenylc;rclopro:pane need not be conjugation, and it certainly does g<:)i :follow the trGnd from alkane to alkene subatl tuent o.

Integr•al il1tens:L ties have also been measured tn the Raman, although unfortunately too much_ ::1ttenM.on w·as

foc.umd on the eye lopropo.ne r:Jtretches~ whioh vii 11 be modi-fled by e.c1jacent groUJJB in the expectec:l fashion. .However, in the 0:0 stretches me&s1lrec11 the inteni'.d tien of' vinyl and

intene:i.ty of :phenylcyclo:propane wes three times that of isopropyl"benzene. However, since the author• states that an adjacent double bond usually increases the intenstty by a factor between ten and f:l,fteen, and

since the factors applying to the inf'ra;red intensities may also apply here; this is not a'bsolutely convincing.

(Distortions of the ·f( cloud that al t~)r the polar•i ty will presumabl-:>r alter ·the polarizabili"ty during a trans:!. ti on).

The standard use of the Ha.rmnett equation he.s been

to aompare the (J value for ionization and ester solvolysis

of phenylcyclopropanecsrboxylic ac:td~--J with i;he corrc:3spond.-ing

I

va1.u€'ls for cinnamic and 3·-.:phenyh:;n"opion.ic aeid.s.,

~:'he first :3't1Ch stu.d;}r was of the ionization con-stants of' the ucid, in water9 and the yo.o values were

c:l.rmam:l.e, O.,L1.66, 3-:phenylp:r>op:i.onic b" 208, tra.ns-2-phenyl-oyc:lo:prop~.:U'leear'boxylic

o.

1 82~ The authors claimed that this showed that there was no conjugation in the cyclo-p:t"'opane r:i.ng69 ..

Measurlng the ionizo.ti.on constants f'o:t" the same

70

acids in 50% ethanol led to the opposite connlusion ~

Solvolysis of the estera of these acids supported the latter• work.. Htn.•e th.e values were :;·-·phenylpropion.ic 0 .. 59, tra.nr;-·:2-pherJYlcyclopr•opanecarbo:;{ylic 0., 81, cis-2-phenylcyclopropanecarboxylic acid 1. 02" trans-c:l.nnan:d.c

1.31,

cis-cinn~mio 1~27

1

$

V\11ile: the cyclopropane acidf:l here lie b~t\vee:n the

saturated. and the unsatu.rated groupr~, thor·e is 2 peculiar

poiT!t about i:.he cis cyclopropane serieBv wll.ieh has n.n abnormally high value in the eBter solvolysis and almost ap:proElche:::. ols cimJ.amie. In theor·y, conjugation should

Examining the data for the ionization constants in water, it found that all the points lie neatly on the line quoted exaept the m-nitro substituent, which lies well away (nearly

o .. o4

pK units). If' the meta nitro subsJci tuent is included, the line has a poo!'er f'i t, but within experimental error, for• all the substi tuents except

two. One is the p-ni tro, and the other is the p··methoxy,

which th~ autho:rs assigned a (value of'

-o.

26. Now for

a non-conju.gated system, the p-methoxy group has a ()"

constant of -0.12, and this vab1e is in excellent agree-ment with the rev:t13ed. lint~. This revi.eJec1 line would

give a rho value of 0.25, whlch is .more consistent with the other data... Also, it seems X'€H:tsonable to put less weight on the p-nitro group, since the acid is very insolu'ble in water~ an.d f'.:!lthough the authm~·s claimed to have oo:r•:r•ected for this, the result might pex~haps be

more lik<~ly to be in erPor.,

;3imilar examination of the seeond t·wo papers shows that the authors made no conunent on the

o"

values obtained, probably because at the time of publication, the concept of exalted ()'-' was not established. Plotting their quoted V€tlues, the <t" constant for the para methoxy group is -0.1 '1 f'or the cis acids, and -0 .. 12 for ·the trans acids

f'rom the quoted dissociation constants. ~rhese figures seem to be incticative of the cyclopropane ring not conjugating, but consider•ing the ester solvolysis, a ~ value for

of -0.17 is cfutained for the trans series, and no value is

quoted fo~ the cis series. This could be indicative of conjugation, although it om1ld also be misleading, as the scatter in this snries is greater.

It is also interesting to consider cyclopropane as a substl tue:n.t o:n. 'the aromatic ring. If it is a conjugating

dis-38 ..

sociation constant of p-cyclopropylbenzoic acid and the rate of solvolysis of the ethyl ester~ sigma values of -0.24 and -0.19 were found72• The dissociation constant of

~-cyclopropylpyridine

has also been determined55, and on fitting the quoted value to a pyridine Hammett plot73t a sigma value of -0.20 is obtained, which is in good agreement. A stronger demand on a mesomeric

donator would come from the ionization of the t-cumyl chlorides39. The relative rates are quoted below. This data was intended to show the steric demands as mentioned previously, and so the number of substituents is

dis-appo$ntingly small., The substituent is quoted, followed by the log of its rate relative to the unsubstituted compounds: H (o), p-isopropyl (1.25), m-methyl (0.30), p-cyclopropyl (2.,195), 3-5-dimethyl (0.59), 3-methyl-4 -cyclopropyl (2.235); 3-5-dimethyl-4-cyclopropyl (1.57).

If these are plotted against their ~ values, the

correlation is exceedingly poDr .. If the unsubstituted and the two methyl s1.fbsti tuted cumyl chloride!'$ are taken as correct, then the cyclopropyl is conjugating strongly, as its rate is ten times faster than it should be.

However, the rate for the isopropyl is also five times too fast. The line of best fit gives a rho of -10.6, with all rates within 0.2 log units of the line except

the dimethyl which is 0.6 log units off and the dimethyl-cyclopropyl which is 1.1 log units off, if it is assumed that

«s

are additive. The sigma value for p-cyclopropyl was found to be -0.20, although this is only based on

a line through three other points. However, it does appear that the ~value of the p-cyclopropyl group is at least consistent with the absence of conjugation, and this

analysis also casts doubt on the value of the conclusions concerning steric effects since the 3-5-dimethylcumyl chloride is also reacting abnormally slowly"