TOWARD AN UNDERSTANDING OF THE HIGH ENANTIOSELECTIVITY IN THE OSMIUM-CATALYZED ASYMMETRIC DIHYDROXYLATION .4. ELECTRONIC EFFECTS IN AMINE-ACCELERATED OSMYLATIONS

Electronic effects in osmylation reactions accelerated by pyridine and quinuclidine derivatives were investigated by varying the substituent s on the amine ligand as well as on the alkene substrate. Ligand subst ituent effects were gauged by determination of the equilibrium constan ts for coordination of the amines to OsO4, evaluation of structural pr operties and reduction potentials of the amine-OsO4 complexes, and ana lysis of the kinetics of osmylations in the presence of the amines. Su bstrate substituent effects were gauged by kinetic Hammett studies usi ng several different amine/alkene combinations. Nonlinear Hammett rela tionships resulting from alkene substituent effects were observed, and the deviation from a linear free energy relationship was found to dep end on the structure, binding capacity, and concentration of the amine . The results were evaluated in terms of the contending ''[3 + 2]'' an d ''[2 + 2]'' mechanisms currently under consideration. A change in me chanism that depends on the structural and electronic properties of bo th alkene and amine is proposed.