Dimeric alumatranes as catalysts for trimethylsilylcyanation reaction†
The solid-state structures of dimeric alumatranes with three five-membered rings chelated by [(OCMe2CH2)nN(CH2CH2O)3−n]3− (n = 1, L1; n = 2, L2; n = 3, L3), which vary by the number of CMe2 groups adjacent to the OH functionality [1 (L1H3), 2 (L2H3), and 3 (L3H3)], were determined by single-crystal X-ray diffraction. The X-ray structures revealed that the aluminum geometries were slightly distorted trigonal bipyramids. The obtained aluminum complexes are the first structurally characterized dimeric alumatranes with tricyclic five-membered rings. Quite unexpectedly, the sterically bulky side arms with dimethyl substituents were always located in the bridging sites, as determined by density functional theory calculations. Their solution-state structures were analyzed by 1H, 13C, and 27Al NMR techniques, and their gas-phase structures were determined by mass spectrometry. Unlike Al(OCH2CH2)3N, complexes 1–3 were all dimeric in the solid state, solution phase, and gas phase. In addition, they were found to promote the reaction of aryl, heteroaryl, and alkyl aldehydes with trimethylsilylcyanide to provide the corresponding products in excellent yields under mild conditions of room temperature, a short reaction time of 1 h, and a very low catalyst loading of 0.5 mol%.