Disulfonimide Design Aids Organocatalysis

Chemists in Germany have designed a binaphthyl-based disulfonimide as a new motif in asymmetric organocatalysis (Angew. Chem. Int. Ed., DOI: anie.200901768). The catalyst created by Benjamin List and coworkers of the Max Planck Institute for Coal Research helps resolve some challenges for enantioselective organocatalysis, namely identifying new active functional groups, achieving higher catalyst turnover rates, and activating aldehydes. The team zeroed in on the binaphthyl design commonly used for chiral catalysts and selected the disulfonimide as a mimic for triflic-based compounds such as (F₃CSO₂)₂NSi(CH₃)₃ that are powerful but nonselective acid catalysts. List and coworkers propose that the active catalyst for their system is the N-silyl disulfonimide shown, which forms in situ. The catalyst provided high yields and enantioselectivities for the Mukaiyama aldol reaction in which an aldehyde is coupled with a silylated ketene acetal to form an aldol product. The N-silyl disulfonimide achieved turnover numbers of up to 8,800, which are rare for an organocatalyst and unprecedented for enantioselective Mukaiyama reactions, the researchers note. Their new motif should be useful for asymmetric silicon Lewis acid catalysis and opens the door wider for other enantioselective applications, List adds.