Vinylcyclohexene hydrogenation

Compounds 101 and 104 were formed by Diels-Alder reaction of 68 with vinylcyclohexene and vinylcyclopentene, respectively (69TL1523 72MI1). These adducts, after hydrogenation and subsequent acidic treatment with concentrated sulfuric acid, afforded spiro compounds 103 and 106 (Scheme 25). 

Tetrahydrobenzyl alcohol (( )3-cyclohexenene-l-methanol) and 30% aqueous hydrogen peroxide were purchased from Fluka, AG. 3-Cyclohexene-1-carboxylic acid and cis-4-cyclohexene-l,2-dicarboxylic acid were used as purchased from Lancaster Chemical Co. Methyl iodide, acetic anhydride, Oxone (potassium peroxymonosulfate), Aliquot 336 (methyl tri-n-octylammonium chloride), sodium tungstate dihydrate and N,N-dimethylaminopyridine (DMAP) were purchased from Aldrich Chemical Co. and used as received. 3,4-Epoxycyclohexylmethyl 3, 4 -epoxycyclohexane carboxylate (ERL 4221) and 4-vinylcyclohexene dioxide were used as purchased from the Union Carbide Corp. (4-n-Octyloxyphenyl)phenyliodonium hexafluoroantimonate used as a photoinitiator was prepared by a procedure described previously (4). 

P-1 nickel can also be used for the selective hydrogenation of dienes. For instance, 4-vinylcyclohexene was hydrogenated with high selectivity (98%) to 4-ethylcyclohexene (equation 18), whilst 2-methyl-1-hexene was obtained with 93% selectivity from 2-methyl-1,5-hexadiene over it (equation 19)66. 

Most researchers have found 1,2-addition of hydrogen in C=C hydrogenation of conjugated double bonds131-134, for example, in the reduction of 1-vinylcyclohexene, 4-methylene-l,2,3-trimethylcyclobutene-3-ol benzoate and some steroid derivatives. 

The monosubstituted double bond in 4-vinylcyclohexene is hydrogenated over P-1 nickel in preference to the disubstituted double bond in the ring, giving 98% of 4-ethylcyclohexane [73]. Similarly in limonene the double bond in the side chain is reduced while the double bond in the ring is left intact if the compound is treated with hydrogen over 5% platinum on carbon at 60° and 3.7 atm (yield 97.6%) [348]. 

In acetylenes containing double bonds the triple bond was selectively reduced by controlled treatment with hydrogen over special catalysts such as palladium deactivated with quinoline [565] or lead acetate [56], or with triethylam-monium formate in the presence of palladium [72]. 1-Ethynylcyclohexene was hydrogenated to 1-vinylcyclohexene over a special nickel catalyst (Nic) in 84% isolated yield [49]. 

Industrial processes were developed for the selective partial hydrogenation of 4-vinylcyclohexene with Ni catalysts exhibiting minimized isomerization activity in the presence of additives298,299. For example, supported nickel arsenides prepared by reducing nickel arsenate with NaBFLt display high selectivity in the formation of 4-ethylcyclohexene (96% selectivity at 96% conversion on Ni-As-Al2C>3, 398 K, 25 atm H2, acetone additive). 

Vinylcyclohexene (29) has been selectively hydrogenated to 4-ethylcyclohexene (30) in high yields of 97 and 98% over P-2 Ni19 and Nic,20 respectively, in ethanol at 25°C and 1 atm H2. Both the nickel catalysts are known to be of low isomerization activity and sensitive to the structure of substrates. The same selective hydrogenation was also achieved over a nickel catalyst in the presence of ammonia, which minimized the isomerization to a more highly substituted double bond.72 Similarly, over P-2 Ni,... 

The hydrogenation of 4-vinylcyclohexene (46) to 4-ethylcyclohexene (47) was also reported to take place over a supported nickel arsenide, Ni-As(B), which was prepared by the sodium borohydride reduction of nickel arsenate supported on either silica 2,83 or alumina. " These catalysts, however, fimction best in the presence of additives. When the reaction was run in pentane at 125 C and 25 atmospheres of hydrogen in the presence of a small amount of acetone, the product mixture at 96% conversion was 96% 4-ethylcyclohexene (47) and 4% ethylcyclohexane (48). No isomeric olefins were detected. " ... 

Besides this, other intermediates, e. g., 4-vinylcyclohexene, may be formed. Although this is a disadvantage in this case, too, the main problem with this system is that the Ni complex decomposes in the presence of hydrogen. 

Finally, the cycloisomerization of 4-vinylcyclohexene (a butadiene dimer) to bicyclo[3.3.0]-2-octene (6) was found to occur at 250° over a silicophosphoric acid catalyst (11), along with a very large amount of hydrogen transfer (ethylcyclohexenes, methylethylcyclopentenes, ethylbenzene) and polymerization, a reaction closely related to that of limonene (5). A much better yield of the same hydrocarbon (6) was obtained from 1,5-cyclooctadiene (72% at 200°) with the same catalyst (25). 

Notably, the labelling studies with the ionized vinylcyclohexenes enabled the distinction of the two moieties in the molecular ions that eventually yield the diene fragments. In fact, H atom shifts were found to be relatively slow in Ihe ions undergoing the RDA processes, in contrast to ionized cyclohexene which is known to suffer fast and extensive hydrogen scrambling after field ionization (FI) even al very short ion lifetimes . Obviously, once again, dissociation of the particularly weak bis-allylic C—C bond present in the 4-vinylcyclohexene-type radical cations is sufficienlly fast to largely suppress isomerization by 1,2- or 1,3-H shifts. 

Strained double bond. Thus selective hydrogenation of 4-vinylcyclohexene (equation II) and of l-octene-4-yne (equation III) can be effected. This method of protection has also been used to effect selective bromination of the aromatic ring of eugenol (equation IV). 

Tetrachloropalladate(II) ion catalyzes the interconversion of 1- and 2-butenes in aqueous solutions containing chloride and hydronium ions. Sodium tetrachloropalladate(II) catalyzes the conversion of allylbenzene to propenyl-benzene in acetic acid solutions. Tetrakis(ethylene))Lt,/x -dichlororhodium(l) catalyzes butene isomerization in methanolic hydrogen chloride solutions . Cyclooctadienes isomerize in benzene-methanol solutions of dichlorobis-(triphenylphosphine)platinum(11) and stannous chloride. Chloroplatinic acid-stannous chloride catalyzes the isomerization of pentenes. Coordination complexes of zero-valent nickel with tris(2-biphenylyl)phosphite or triphenyl-phosphine catalyze the isomerization of cis-1,2-divinylcyclobutane to a mixture of c/5,m-l, 5-cyclooctadiene and 4-vinylcyclohexene . Detailed discussions of reaction kinetics and mechanisms appear in the papers cited. 

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