Treatment of diphenyldiethynylsilanes, (H5C6)2Si(C CR')2 (R' = C6H5, 4-MeC6H4), with two equivalents of dialkylaluminium hydrides, R2AlH (R = CMe3, CH2CMe3), afforded the corresponding dialkenylsilanes (35) by hydroalumination. The mixed alkenylalkynyl compounds resulting from the reduction of only one C C triple bond occurred as intermediates. Two of these (1 and 2) were isolated and characterized by crystal structure determinations. They show close interactions of the alpha-carbon atoms of the ethynyl groups with the coordinatively unsaturated aluminium atoms and a cis arrangement of H and Al atoms across the C = C double bonds. cis/trans Rearrangement took place upon the formation of the dialkenyl species 35 with an all-trans configuration of the alkenyl groups. These compounds have two tricoordinate aluminium atoms and are ideally preorganized to be applied as chelating Lewis acids, as was shown with the chelating coordination of chloride ions (compound 6). Rearrangement and 1,1-carbalumination was observed upon heating of a mixed alkenylalkynylsilane (7). A silacyclobutene derivative (8) was isolated, which has a SiC3 heterocycle and an endo- and an exocyclic C = C double bond. Quantum chemical calculations give insight into the reaction mechanism.
