A new class of chiral organogermanes derived from C2-symmetric dithiols: synthesis,characterization and stereoselective free radical reactions

A new class of dithiostannanes and dithiogermanes have been prepared from 1,1'-binaphthyl-2,2'-dithiol and 3,3'-bis(trimethylsilyl)-1,1'-binaphtho-2,2'-dithiol. While reduction of 4-butyl-4-chloro-3,5-dithia-4-stanna-cyclohepta[2,1-a;3,4-a']dinaphthalene to the corresponding tin hydride was unsuccessful, 4-tert-butyl-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene and 4-tert-butyl-2,6-bis(trimethylsilyl)-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene were obtained by reduction of the parent germanium chlorides with NaBH(4) and LiBH(4), respectively. Kinetic constants for hydrogen transfer to a primary alkyl radical were measured for both germanium hydrides. Reduction of alpha-halo carbonyl compounds by these germanium hydrides occurs with moderate ee values (up to 42%), while hydrogermylation of methyl methacrylate occurs with low selectivity (<3/1) for the former hydride but high selectivity (>10/1) for the latter.