Early Main Group Metal Catalysts for Imine Hydrosilylation.

The efficient catalytic reduction of imines with phenylsilane is achieved by using the potassium, calcium and strontium based catalysts [(DMAT)K (THF)]∞ , (DMAT)2 Ca⋅(THF)2 and (DMAT)2 Sr⋅(THF)2 (DMAT=2-dimethylamino-α-trimethylsilylbenzyl). Eight different aldimines and the ketimine Ph2 C=NPh could be successfully reduced by PhSiH3 at temperatures between 25-60 °C with catalyst 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, for example, 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.