2.4- Hexadiene selective reduction

Another methodology that shows great promise for selective reductions is Phase Transfer Catalysis (PTC) under one atmosphere of H2. Under very gentle conditions, with a quaternary ammonium salt as the PT agent, 1 mol % of [RhCl(l,5-hexadiene]2 catalyzes the reduction of aromatic rings yields vary to some extent with the phase transfer agent [31]. 

Conjugated Dienes. General Observations, The catalytic hydrogenation of conjugated dienes by pentacyanocobaltate(II) is completely selective, yielding only monoolefin, which cannot be reduced further (19, 20, 21), Lack of reduction of 2,5-dimethyl-2,4-hexadiene indicates that dienes in which the 5-cis conformation is sterically hindered might not be catalytically hydrogenated (21). However, reduction of 3-methylene cyclohexene, a diene with a fixed s-trans conformation, disproves this and indicates that other steric factors are probably involved (24). 

The 1,4-hexadiene is made by codimerization of ethylene and butadiene, with a RhCls/EtOH catalyst (Eq. 12.16). The catalyst is about 80% selective for the rnms-i,4-hexadiene, a remarkable figure considering all the different dimers Chat could have been formed. The catalyst is believed to be a rhodium hydride formed by reduction of the RhCls with the ethanol solvent (Section 3.2). This must react with the butadiene to give mostly the ann -methylallyl (crotyl) intennediate, which selectively inserts an ethylene at the unsubstituted end. The ds/trans ratio of the product Hobably depends on the ratio of the two isomers of the crotyl... 

The potential of asymmetric catalytic hydrocyanation in the selective synthesis of the drug (S)-naproxen is shown by reaction 5.6.2.1. The aromatic substituents on the phosphorus atoms of 5.68 have a pronounced effect on the enantioselectivity of this reaction. Instead of CFj groups, if the aromatic rings are substituted in the same positions by CHj groups, the ee value drops by 70%. Based on the results of Ni-catalyzed hydrocyanation of 1,3 hexadiene with DCN, reductive elimination appears to be the rate-determining step. 

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