Early Main Group Metal Catalysts for Imine Hydrosilylation

Abstract

The efficient catalytic reduction of imines with phenylsilane is achieved by using the potassium, calcium and strontium based catalysts [(DMAT)K·(THF)]∞, (DMAT)2Ca·(THF)2 and (DMAT)2Sr·(THF)2 (DMAT = 2‐dimethylamino‐α‐trimethylsilylbenzyl). Eight different aldimines and the ketimine Ph2C=NPh could be successfully reduced by PhSiH3 at temperatures between 25‐60 °C with catalysts loadings down to 2.5 mol%. Also, simple amides like KN(SiMe3)2 or Ae[N(SiMe3)2]2 (Ae = Ca, Sr, Ba) catalyze this reaction. Activities increase with metal size. For most substrates the activity increases along the row K < Ca < Sr < Ba. Fastest conversion was found for imines with alkyl substituents at N and aryl rings at C, e.g. PhC(H)=NtBu, while tBuC(H)=NtBu or PhC(H)=NPh react much slower. Reasonable functional group tolerance is observed. The proposed metal hydride mechanism is supported by stoichiometric reactions using a catalyst model system, isolation of intermediates and DFT calculations.