Diastereoselective control in ketose hydrogenations with supported copper and nickel catalysts
A series of Cu and Ni catalysts has been studied in ketose hydrogenations. High diastereoselectivity to one of the two possible epimeric products occurs for each ketose in a series of pentulose and hexulose reactants. The preferred products of Cu catalysis fit a particular stereochemical trend and those from modified Ni systems the opposite trend, demonstrating control by the catalysts through 1,2-asymmetric induction.
In D-fructose reductions, the selectivity of Cu catalysts to D-mannitol can be increased by halide ions. The diastereoselectivity of Ni catalysts can be modified by sulphate doping from the D-mannitol selectivity of normal Ni systems to a high degree of control to D-glucitol. Although there is a definite trend in Ni catalysis, suggesting the involvement of hydrogen spillover, the selectivity in Cu systems is little influenced by change in support. In deuteration experiments, only one D-atom becomes attached to the carbon skeleton in the reduction steps.
The reasons for diastereoselectivity by 1,2-asymmetric induction are discussed in the light of these observations.