Journal of the American Chemical Society, Vol.131, No.42, 15387-15393, 2009
Toolbox Approach to the Search for Effective Ligands for Catalytic Asymmetric Cr-Mediated Coupling Reactions
Chromium catalysts derived from chiral sulfonamides represented by A effect the couplings of aldehydes with vinyl, allyl, or alkyl halides. With three distinct sites for structural modification, A affords access to a structurally diverse pool of chiral sulfonamides. The Cr catalysts derived from these sulfonamides exhibit a broad range of catalyst-substrate matching profiles. A strategy is presented to search for a satisfactory chiral sulfonamide for a given substrate. In order to demonstrate the generality and effectiveness of this approach, five diverse C-C bond-forming cases have been selected from the halichondrin synthesis. For each of the cases, two ligands have been deliberately searched for, to induce the formation of (R)and (S)-alcohols, respectively, at the arbitrarily chosen efficiency level of ">= 80% yield with >= 20:1 stereoselectivity in the presence of <= 20 mol % of a Cr catalyst". For 9 out of the 10 cases studied, a satisfactory catalyst has been found within this pool of sulfonamides. Even for the remaining case, a Cr catalyst inducing stereoselectivity up to 8:1 has been identified.