CONJUGATED DOUBLE BONDS

3.6.1 Aryl-Substituted Ethylenes

Zartman and Adkins hydrogenated various phenyl-substituted ethylenes with Ni–ki-eselguhr and copper–chromium oxide as catalysts.¹¹⁴ The pressure of hydrogen as well as the temperature had a marked effect on the rate of hydrogenation, which de-pended on the structure of ethylenic linkages. Phenylethylene (styrene) was readily hydrogenated over the nickel catalyst at 20°C and a low pressure of 0.25 MPa H₂ (eq.

3.14). Hydrogenation of 1,2-diphenylethylene (stilbene) (18 g, 0.10 mol) over 2 g Ni–

kieselguhr at 20°C required 15 min at 9.3 MPa H₂, 30 min at 3 MPa H₂, and 80 min at 0.27 MPa H₂, while hydrogenation of 1,1,2-triphenylethylene at 20°C required 150 min at 9.5 MPa H₂ and hydrogenation of tetraphenylethylene required over 2 h even at 100°C and 12.5 MPa H₂ (eq. 3.15). In hydrogenations at 125–170°C, these phenylethylenes may give the corresponding cyclohexylethanes. Over copper–chro-mium oxide these phenyl-substituted ethylenes are hydrogenated rapidly at 125–

150°C and 12.5–13.5 MPa H₂ without affecting the phenyl groups. An example is shown in eq. 3.16.

TABLE 3.9 Effects of Additives on the Hydrogenation of Methyl Linoleate over Palladium Catalysts^a,b

Octadecenoate Concentration

(%)^e

Catalyst Solvent Additive^c V_D/V_M^d

At

Maximum After 5 h

5% Pd–CaCO3 THF — 25 95.4 0.0

5% Pd–CaCO₃ THF Benzene 59 97.5 18.9

5% Pd–CaCO₃ THF PhCHO 45 97.7 63.8

5% Pd–CaCO₃ THF PhCH₂CHO 520 99.1 92.4

5% Pd–CaCO₃ THF Ph(CH₂)₂CHO 56 97.9 29.8

5% Pd–CaCO₃ THF Ph(CH₂)₃CHO 43 98.0 45.3

5% Pd–CaCO₃ THF CH₃(CH₂)₂CHO 24 96.0 0.0

5% Pd–CaCO₃ Cyclohexane PhCH₂CHO Very large 98.6 98.6

5% Pd–CaCO₃ t-BuOH PhCH₂CHO 58 97.3 59.4

5% Pd–CaCO₃ THF Quinoline 3 84.6 64.6

Pd black THF — 3 93.2 0.0

Pd black THF PhCH₂CHO 230 98.8 90.7

aData of Nishimura, S.; Ishibashi, M.; Takamiya, H.; Koike, N. Chem. Lett. 1987, 167. Reprinted with permission from Chemical Society of Japan.

bMethyl linoleate (0.3 mmol) was hydrogenated with 6 mg of 5% Pd–CaCO₃ or 3 mg of Pd black in 1.6 ml of solvent at 25°C and atmospheric hydrogen pressure.

cThe additive (0.2 mmol; 0.02 mmol for quinoline) was added to prereduced catalyst before the addition of substrate.

dV_D: the rate of disappearance of methyl linoleate; V_M: the rate of disappearance of methyl octadecenoates.

eGC analysis.

Over palladium catalysts, phenyl-substituted ethylenes are hydrogenated more readily than the corresponding alkyl-substituted ethylenes, as noted previously. Poor activity of palladium toward the hydrogenation of the aromatic ring at low temperature allows the olefinic bonds to be hydrogenated selectively. Stilbene is hydrogenated smoothly to 1,2-diphenylethane over palladium oxide in ethanol at 25°C and 0.2–0.3 MPa H₂ (eq. 3.17).⁵ Anethole (p-1-propenylanisole) was hydrogenated faster over palladium oxide (8 min) (eq. 3.18) than over platinum oxide (14 min).⁵ Raney Ni behaves simi-larly to palladium for aryl-substitutions, although to a lesser extent than in the case of palladium.^9,10

3.6.2 a,b-Unsaturated Acids and Esters

The C–C double bonds conjugated with carboxyl functions are usually much more readily hydrogenated than usual olefinic bonds, especially with nickel and palladium catalysts. Ethyl cinnamate is rapidly hydrogenated over Raney Ni under mild condi-tions (eq. 3.19).¹¹⁵ It is also hydrogenated over palladium oxide much faster (eq. 3.20) than over platinum oxide with which 2.9 h were required under the same conditions.⁵ Cinnamic acid was hydrogenated smoothly to dihydocinnamic acid as the sodium salt over Urushibara Ni in water under ordinary conditions (eq. 3.21).¹¹⁶

PhCH₂CH₃

75 ml C₇H₁₄ quantitative 2 g Ni–kieselguhr

100 ml 95% EtOH solution 2 g Raney Ni (W-6)

8.8 g (0.05 mol)

CH CHCO₂Et CH₂CH₂CO₂Et

(3.19)

3.6 CONJUGATED DOUBLE BONDS 93

3.6.3 Conjugated Dienes

Conjugated dienes are usually more reactive than simple olefins. However, selectivity in the formation of monoenes depends greatly on the catalyst employed. Kazanskii et al. studied the selectivity in the hydrogenation of isoprene over platinum black, palla-dium black, and Raney Ni in ethanol at room temperature and atmospheric pressure (Table 3.10).¹¹⁷ Selectivity for monoenes was much higher over palladium and Raney Ni than over platinum. The monoenes were a mixture of three isomeric methybutenes formed by apparent 1,2, 3,4, and 1,4 additions of hydrogen to isoprene over all the catalysts. The high selectivity for the monoene formation of palladium and Raney Ni was also demonstrated in the hydrogenation of 2,5-dimethyl-2,4-hexadiene in ethanol

17.6 g (0.1 mol)

0.1 g Pd oxide

25°C, 0.2–0.3 MPa H₂, 26 min 150 ml 95% EtOH

CH CHCO₂Et CH₂CH₂CO₂Et

(3.20)

3.4 g (0.02 mol)

Urushibara Ni–B (0.5 g Ni) 50 ml H₂O

25°C, 1 atm H₂, 16 min for 100% conversion

CH CHCO₂Na CH₂CH₂CO₂Na

(3.21)

TABLE 3.10 The Products (%) in Half-Hydrogenation of Isoprene over Platinum, Palladium, and Raney Ni^a

Product Pt Pd Raney Ni

7 25 16

26 30 40

15 41 40

26 2 2

26 2 2

Selectivity for monoenes (%) 65 98 98

aKazanskii, B. A.; Gostunskaya, I. V.; Granat, A. M. Izv. Akad. Nauk SSSR, Otdel. Khim. Nauk 1953, 670 (CA 1954, 48, 12664a).

at room temperature and atmospheric pressure.¹¹⁸ Addition of one molar equivalent (1 equiv) of hydrogen to the diene gave the monoenes in 92 and 99% yields with pal-ladium and Raney Ni, respectively, compared to 75% yield over platinum. However, in contrast to the results with isoprene, the greater part of the monoenes was 2,5-di-methyl-2-hexene, the product resulting from 1,2-addition of hydrogen, which amounted to 86% over palladium and 90% over Raney Ni.

Bond and Wells studied the selectivities of supported group VIII (groups 8–10) metals in the vapor-phase hydrogenation of 1,3-butadiene.^29,30 Palladium, iron, cobalt, and nickel were all perfectly selective for butene formation, and the selectivity de-creased in the order: Pd >> Ru > Rh ≥ Pt > Ir. The trans/cis ratios of the 2-butene formed from 1,3-butadiene over cobalt and palladium were much greater (>10) than those of the 2-butene obtained in 1-butene hydroisomerization. The results sharply contrasted to those obtained over the other metals where almost the same trans/cis ra-tios of much smaller values (nearly between 1 and 2) were obtained from both 1,3-bu-tadiene and 1-butene. Formation of the high trans/cis ratios of 2-butene over cobalt and palladium was explained by 1,4 addition of hydrogen to the 1,3-butadiene ad-sorbed in s-trans conformation as shown in Scheme 3.14.

Later studies by Wells and co-workers, however, showed that the trans/cis ratios of the 2-butene formed from hydrogenation of 1,3-butadiene over nickel and cobalt catalysts depended on the reduction temperature employed for catalyst activation.

High trans/cis ratios of 3.5–8 were obtained over the catalyst reduced at 400°C, while the ratios decreased to ~2 with the catalysts activated below 350°C.^119,120 The charac-teristic properties of the nickel and cobalt catalysts activated at 400°C were attributed to a modification of the catalysts caused by the sulfur compounds contained in the sup-port that occurred at such a high reduction temperature as 400°C.¹²¹

Imaizumi et al. studied the hydrogenation of 1,4-dialkyl-1,3-cyclohexadienes over the nine group VIII (groups 8–10) metals and copper in ethanol at room temperature and atmospheric pressure.¹²² The selectivity for monoenes formation at 50% conver-sion increased in the order: Os–C, Ir–C < Ru–C, Rh–C, Pt < Pd–C, Raney Fe, Raney Co, Raney Ni, Raney Cu (= 100%). The selectivity for 1,4-addition product increased in the order Os–C, Ir–C < Ru–C, Rh–C, Raney Cu, Raney Fe, Raney Ni < Raney Co, Pd–C, Pt. Extensive formation of 1,4-dialkylbenzenes (more than 50% with the 1,3-dimethyl derivative) was observed over Raney Ni and Pd–C, while they were not formed over Raney Cu, Os–C, and Ir–C. In the hydrogenation of 4-methyl-1,3-pen-tadiene (39) (Scheme 3.15) over group VIII metals in cyclohexane at room tempera-ture and atmospheric pressure, high selectivity to monoenes was obtained with iron, nickel, cobalt, and palladium catalysts where the amounts of the saturate

2-methylpen-+ 1,4 addition

anti-1,3-butadiene trans-2-butene

H₂

Scheme 3.14 Formation of trans-2-butene via 1,4 addition of hydrogen to adsorbed s-trans-1,3-butadiene.

3.6 CONJUGATED DOUBLE BONDS 95

tane (40) in the product at 50% conversion was less than 4%, while over the platinum metals other than palladium 40 was formed in as much amounts as 18–46% (Table 3.11).¹²³ Among the monoenes 41–44 formed, the 3,4-addition product 41 increased in the order Os, Ir < Ru, Rh, Pt < Pd, Fe, Ni < Co. The results on cobalt catalysts that the monoene 41 was formed in more than 80% selectivity appear rather unusual, since it indicates that the more hindered double bond in 39 was hydrogenated predomi-nantly. On the other hand, over osmium the 1,2 addition to give 43 took place in 82%

selectivity, compared to only a few percents over cobalt catalysts.

Bell et al. studied the hydrogenation of trans-1-methoxy-1,3-butadiene (45) over Adams platinum, Lindlar palladium, Raney Ni (W-6), and nickel boride (P-2) as cata-lysts (Scheme 3.16).¹²⁴ Table 3.12 compares the products at the hydrogen uptake of approximately one molar equivalent of hydrogen in the hydrogenation of 45 at 30°C and initial hydrogen pressure of 0.36 MPa. Over Adams platinum formation of 1-methoxybutane was significant from the beginning of hydrogenation, while Raney Ni and Lindlar catalyst gave only small amounts of the saturated ether and no

hydro-39 40 41 42 43 44

+ + + +

1 2 3 4