Stereochemistry and mechanism of catalytic hydrogenation of substituted methylenecyclohexanes

Substituted alkylmethylenecyclohexanes were hydrogenated over several transition metal catalysts. The ratios of the epimeric products were almost unity over freshly prepared Raney Ni but the axial Me counterparts were favoured over aged catalyst. The axial Me products were also preferred on Pt or Rh catalysts. Pd catalysed hydrogenation gave predominantly the equatorial Me isomers at high catalyst ratio, while the axial Me counterparts were favoured at the early stage of the reaction at a catalyst-substrate ratio of 1:20. The change of the ratio of epimeric products depending on the amount of catalyst is due to the rapid migration of exo-cyclic double bond to the inside of the cyclohexane ring at high catalyst ratio. The mechanism of hydrogenation is discussed in terms of the modified Horiuti-Polanyi mechanism.