Hydrometallation of amino-trialkynylsilanes – intramolecular M–N interactions (M = Al, Ga) and potential activation of Si–N bonds
Hydrometallation of a pyrrolidyl functionalised trialkynylsilane, H8C4N–Si(CC–CMe3)3, with equimolar quantities of H–M(CMe3)2 (M = Al, Ga) resulted in the formation of mixed alkenyl–dialkynylsilanes (3a, 3b) which have a Lewis-acidic Al or Ga atom in the geminal position to the Si atom and form four-membered M–N–Si–C heterocycles by a strong interaction of the amine N atoms with the Lewis-acidic metal atoms. This interaction results in a concomitant lengthening and weakening of the Si–N bonds. Dual hydrometallation afforded alkynyl–dialkenylsilanes (4a, 4b) with two Lewis-acidic metal atoms. Al–N and Ga–N interactions to one of the Lewis-acidic centres led again to the formation of M–N–Si–C heterocycles. The second Al atom of 4a interacted with C–H bonds of the vinylic tert-butyl group, while the Ga atom of 4b was coordinated to the α-C atom of the remaining alkynyl substituent. Dual hydrometallation of the corresponding pyrrolyl-trialkynylsilane resulted in compounds with different structures (5a, 5b) due to the delocalisation of the lone pair of electrons at nitrogen in the aromatic ring. One metal atom is coordinated to the α-C atom of the alkynyl group, and the other has close contact to a C–H bond of the pyrrole ring. The synthesis of 4b gave an unprecedented bicyclic by-product (6) which has a Ga–H–Ga 3c-2e bond. It was formally formed by hydrogallation of the trialkyne with the sesquihydride [H2Ga–CMe3]2[H–Ga(CMe3)2]2 and was also obtained by the selective reaction with this starting material.