ISOLATED DOUBLE BONDS: GENERAL ASPECTS

In general, the ease of hydrogenation of an isolated double bond depends primarily on the degree of substitution with respect of the double-bond carbons; the ethylenes sub-stituted to a lesser extent are hydrogenated faster than are more highly subsub-stituted ones, and tetrasubstituted ethylenes are hydrogenated at the slowest rates. The nature and the size and degree of branching of the substituents are also important factors, the effects of which, however, may vary with catalyst, solvent, and impurity or additive.

Kern et al. studied the effect of substituents on the rate of hydrogenation over Adams platinum and palladium oxides in ethanol.⁵ It is seen from the results summarized in Scheme 3.2 that, over platinum oxide, monosubstituted olefins (1–4) are hydrogenated most rapidly but a second substitution has little effect on the rate when unsymmet-rically located, that is, one of the double bond carbons having two hydrogens (5, the side chain, and 6), although 6 with two phenyl groups is hydrogenated more slowly than 5. With the symmetrically disubstituted olefins, the rate of hydrogena-tion depended considerably on the character of the substituents. Dialkyl (7) or a methyl and an aryl substitutions (8–10) had only a slight effect on the rate, while a

pro-312 (0.05 g); 153 (0.10 g)

O MeO CH CHMe PhCH CHPh

Me₂C CHMe Ph₂C CHMe

Scheme 3.2 Time (in minutes) for the uptake of 1 molar equivalent of H2 in the hydrogenation of substituted ethylenes (0.1 mol) over Adams platinum oxide (or palladium oxide) (the amount in parentheses) in 150 ml 95% ethanol at 25°C and 0.2–0.3 MPa H2.

3.1 ISOLATED DOUBLE BONDS: GENERAL ASPECTS 65

nounced slowing up of the hydrogenation was observed when two phenyl groups were present (11). With the trisubstituted olefins, trimethylethylene (12), α-pinene (13) and the second double bond in limonene (5) were hydrogenated rapidly, whereas diphenylmethylethylene (14) was hydrogenated definitely more slowly.

Over palladium oxide, it is noteworthy that the trialkyl-substituted double bonds in 5, 12, and 13 are hydrogenated much more slowly than the disubstituted ethylenes other than 5 and 11. Rather slow uptake of the first mole of hydrogen in 5 might be due to extensive isomerization of the disubstituted to the tetrasubstituted double bond. Similar effects of substituents were also obtained by Lebedev et al. with platinum black (Willstätter) in ethanol at atmospheric temperature and pressure.^6,7 In most cases the rates of hydrogenation over platinum remained fairly constant until hydrogen nearly equivalent to the olefin had been absorbed,^6,8 indicating the zero-order kinetics with re-spect to the concentration of substrate.

Different effects on the rate were observed over palladium catalysts with respect to the phenyl group substitution. Thus, Kazanskii et al. obtained the following order in the rate of hydro-genation of trisubstituted olefins I–IV: III > II > IV > I over palladium, in contrast to the order: I > II > III > IV over platinum.⁹ In the hydrogenation of binary mixtures of I with II, III, or IV over palladium, the phenyl-substituted ethylenes were selectively hydrogenated.

Raney Ni behaved similarly to palladium. The rate of hydrogenation was the greatest with III, which was hydrogenated as fast as on palladium and much faster than on platinum. Simi-larly, the rate for IV over nickel, which was as great as for II and much lower than for III, was greater than on platinum, but smaller than on palladium. As on palladium, the most slowly hydrogenated compounds were the aliphatic derivatives I and trimethylethylene.¹⁰ Tetraphenylethylene, which was not reduced over platinum at room temperature and pres-sure, was hydrogenated slowly in the presence of palladium.¹¹ A similar effect of the phenyl group over palladium was also observed in the stereochemistry of hydrogenation of tetrasubstituted ethylenes V and VI. When R was phenyl, 1,2-cis addition occurred al-most exclusively over palladium,^12,13 but when R was carboxyl, 1,2-cis addition decreased to 86 and 70% with V and VI, respectively,¹² and with 1,2-dimethylcyclohexene (V: R = –(CH₂)₄–)¹⁴ and ∆^9,10-octalin,¹⁵ apparent 1,2-trans addition predominated. The charac-teristic effect of the phenyl group in palladium catalyzed hydrogenation has also been ob-served in a marked decrease in racemization from 60% with an alkyl-substituted ethylene 15 to 10% with α-phenethyl-substituted ethylene 16, where racemization is expected to occur via isomerization to a phenyl-substituted ethylene.¹⁶ Similarly, in the deuteration of phenyl-substituted unsaturated compounds over palladium, the deuterium distributions in saturated products were more symmetrical and dideuterio species were more prevalent.¹⁷ Further, it was observed that methyl cis-2-butenoate (methyl isocrotonate) (17) readily isomerized to the trans isomer during hydrogenation, while methyl cis-cinnamate (18) did not isomerize to the trans isomer over Pd–C.¹⁷

C C C C

Brown et al.^18,19 and Brunet et al.²⁰ studied the rates of hydrogenation of various olefinic compounds over P-1 and P-2 nickel borides and over a nickel catalyst designated Nic, obtained from NaH–t-PeONa–Ni(OAc)₂, respectively, in ethanol at 25°C and 1 atm H₂ (Table 3.1). Over P-1 Ni, the decrease in the rate of hydrogenation with

2-methyl-1-18

TABLE 3.1 Rates of Hydrogenation of Alkenes over P-1 and P-2 Nickel Boride and Nic Catalysts

3-Methyl-1-butene 45 0.63 44.8 0.38 — —

3,3-Dimethyl-1-butene 56 0.78 11.9 .10 — —

2-Methyl-1-butene 36 0.50 — — — —

2-Methyl-1-pentene — — 2.9 0.025 — —

2-Methyl-1-hexene — — — — 9.5 0.28

2,3-Dimethyl-2-butene 2 0.03 0 0 — —

Cyclopentene 56 0.78 13.4 0.11 23 0.68

Cyclohexene 31 0.43 1.8^* 0.015 10 0.29

1-Methylcyclohexene — — — — 0.15 0.0044

Cycloheptene — — 47 0.40 22 0.65

Cyclooctene 43 0.60 15 0.13 3 0.09

Cyclododecene — — — — 2.8 0.082

Norbornene 80 1.1 125 1.06 33 0.97

Styrene 63 0.88 — — 44 1.3

α-Methylstyrene 49 0.68 5.6 0.047 28.5 0.84

aData of Brown, C. A. J. Org. Chem. 1970, 35, 1900. Reprinted with permission from American Chemical Society. The substrate (40 mmol) was hydrogenated over 5.0 mmol of catalyst (0.29 g Ni) in 50 ml 95%

ethanolic solution at 25°C and 1 atm H₂.

bData of Brown, C. A.; Ahuja, V. K. J. Org. Chem. 1973, 38, 2226. Reprinted with permission from American Chemical Society. The reaction conditions were the same as for P-1 Ni.

cData of Brunet, J.-J.; Gallois, P.; Caubere, P. J. Org. Chem. 1980, 45, 1937. Reprinted with permission from American Chemical Society. The substrate (10 mmol) was hydrogenated over 0.5 mmol of catalyst (0.029 g Ni), obtained from t-PeOH as activating agent, in 15 ml ethanol at 25°C and 1 atm H₂.

dAverage rate from 0 to 20% reaction in ml H₂ at STP⋅min–1 (^* values measured between 0 to 5 or 10%

hydrogenation).

3.1 ISOLATED DOUBLE BONDS: GENERAL ASPECTS 67

butene, an unsymmetrically disubstituted ethylene, is not significant, compared to 1-octene, but significantly marked with 2-methyl-2-butene, a trisubstituted ethylene. In con-trast, over P-2 Ni the corresponding di- and trisubstituted ethylenes, 2-methyl-1-pentene and 2-methyl-2-pentene, were hydrogenated in the relative rates of only 0.025 and 0.002, respectively, compared to 1-octene. Over P-1 Ni 2,3-dimethyl-2-butene, a tetrasubstituted ethylene, was hydrogenated, although very slowly, but over P-2 Ni it did not react at all.

Thus, the hydrogenation over P-2 Ni was found to be markedly more sensitive to the alkyl substitution, compared to the hydrogenation over platinum, Pt–C, and P-1 Ni.

The order in the reactivity of cycloalkenes was C₅ > C₈ > C₆ over P-1 Ni and C₇ > C₈

≥ C₅ > C₆ over P-2 Ni. The differences in the rate between the cycloalkenes were also much greater over P-2 Ni than over P-1 Ni. It is noteworthy that norbornene was hy-drogenated more rapidly than any other cycloalkenes and even more rapidly than 1-octene over both P-1 and P-2 nickel borides. It is also of interest to note that over P-1 Ni the zero-order kinetics in concentration of substrate apparently holds for the hydro-genation of 1-hexene, 2-methyl-1-butene, and cyclopentene while over P-2 Ni only the hydrogenation of norbornene is zero-order, and with the other alkenes, especially even with 1-octene and cyclopentene, the rates tend to decrease with conversion. The effects of substituents on the rates over Nic appear to be similar to those over P-1 Ni rather than over P-2 Ni. Cyclohexene, however, was more reactive than cyclooctene over Nic, in contrast to the results over P-1 and P-2 catalysts.