Synthesis of enantiopure C2-symmetric VERDI disulfonamides and their application to the catalytic enantioselective addition of diethylzinc to aromatic and aliphatic aldehydes

The enantiopure disulfonamides 7a-c have been prepared from the Ct-symmetric diketone 2, a starting material conveniently accessible from the "dimerization" of (+)-verbenone. These ligands, when treated with titanium isopropoxide and diethylzinc, function as catalysts for the enantioselective alkylation of aldehydes. Stereoselectivity levels ranging from 72 to 98% ee are seen depending on the structural characteristics of the aldehyde. In all cases, the absolute configuration of the carbinol product is R. A working mechanistic model is advanced for the purpose of rationalizing the high levels and direction of asymmetric induction exhibited by these VERDI catalysts.