Stereochemistry and Mechanism of Catalytic Hydrogenation and Hydrogenolysis. III. Catalytic Hydrogenolysis of Benzyl-type Alcohols and Their Derivatives

Abstract

Abstract In order to obtain evidence for the mechanism we proposed of the catalytic hydrogenolysis of benzyl-type alcohols and their methyl ethers, optically active 2-phenyl-2-butanol (Ia), 2-phenyl-2-methoxybutane (IIa), ethyl atrolactate (Ib), and ethyl 2-phenyl-2-methoxypropionate (IIb) were hydrogenolyzed over Raney nickel and Pd(H) catalysts under hydrogen or helium atmosphere in an ethanol solution. In all cases, the hydrogenolysis proceeded stereoselectively with retention of configuration over Raney nickel catalyst but with inversion of configuration over palladium catalyst. A reverse stereoselectivity was observed when the benzyl alcohols (Ia and 2-phenyl-1,2-dihydroxypropane (Ic)) and methyl ethers (IIa) and (IIb) were hydrogenolyzed in the presence or absence of various additives over nickel catalysts. It was also found that the hydrogenolysis of benzyl alcohols always proceeds with retention of configuration over nickel catalyst containing sodium hydroxide. Various binary mixtures of Ia, IIa, Ib and IIb were hydrogenolyzed competitively over Raney nickel and Pd(H) catalysts. The order of relative rates was found to be Ia\gtrsimIb\gtrsimIIa\gtrsimIIb over Raney nickel, and IIa\gtrsimIa>>IIb\gtrsimIb over Pd(H) catalyst. The results support our mechanism. The catalytic hydrogenolysis is a competitive reaction of two courses (SNi type and SN2 type reactions), and the stereoselectivity reflects the difference of the free energy levels of the transition states to form π-benzylic complexes. The free energy levels of the corresponding transition states depend on stereoelectronic factor, affinity of substituents for catalyst metal, catalyst hindrance and electronic effects of substituents. The acetates (IVa) and (IVb) of Ia and IIa were hydrogenolyzed with inversion of configuration over nickel and palladium catalysts, whereas the stereoselectivity of the hydrogenolysis of the benzoate (V) of IIa depended on the kind of catalyst. The results are also explained on the basis of our mechanism.