Dynamic kinetic transformation of sulfinyl chlorides:: Synthesis of enantiomerically pure C2-symmetric bis-sulfoxides

Two modular and highly convergent approaches for the synthesis of both isomers of a large number of optically pure C-2-symmetric bis-sulfoxides have been developed, and their scope and limitations have been assessed. The first one uses as intermediate diastereomerically pure C-2-symmetric bis-sulfinate esters 6(S-S,S-S) and 6(R-S,R-S), obtained by dynamic kinetic resolution of ethane-1,2-bis-sulfinyl chloride 5. A single inducer of chirality, the glucose-derived DAG (diacetone-D-glucose) 1 is used for the enantioselective synthesis of both diastereomerically pure C-2-symmetric bis-sulfinate esters, thanks to the opposite stereodirecting effect of pyridine and (Pr2NEt)-Pr-i used to catalyze the reaction. The second approach is based on the copper-catalyzed oxidative coupling of optically pure lithiomethyl sulfoxides. Both isomers of a large number of methyl sulfoxides can be obtained in a convergent manner using (R-S)- and (S-S)-DAG methanesulfinate esters 8R(S) and 8S(S). Methanesulfinates 8R(S) and 8S(S) are also obtained in an enantioselective way by a dynamic kinetic resolution of methane sulfinyl chloride 24. The final bis-sulfoxides are obtained with enhanced enantioselectivities compared to the corresponding monomers, as a result of the Horeau effect which is operating in both approaches. A model based on the formation of pentacoordinated sulfur intermediate is proposed. This model can explain the dynamic kinetic resolution observed via Berry pseudorotations, without the commonly accepted in situ racemization of the starting material. The usefulness of the approaches is demonstrated by the preparation of complexes of Pd(II) and Ru(II) bearing bidentated chiral sulfoxides as ligands.