CC Triple Bond Activation by Heterocyclic Aluminum Phosphinides

Treatment of aryl-di(tert-butylethynyl)phosphines Aryl-P(CC-CMe3)2 [Aryl = C6H5 (4) 2,4,6-Me3C6H2 (5)] with dialkylaluminum hydrides, R2Al−H (R = CMe3, CH2CMe3), afforded heterocyclic, zwitterionic compounds 6−9 by hydroalumination and release of the corresponding dimeric tert-butylethynyl-dialkylaluminum derivatives, [R2Al-CC-CMe3]2 (10 and 11). The molecular structures of 6−8 reveal a P2C2Al ring with a tetracoordinated aluminum atom, two exocyclic CC double bonds, a tricoordinated phosphorus atom, and a phosphonium cation carrying an aryl and an alkynyl group. The CC triple bond of 7 was inserted into the Al−P bonds of the heterocycles upon warming to give the bicyclic compound 13. A similar compound (12) was directly formed when the bisalkyne 4 was treated with dineopentylaluminum hydride. Their molecular structures show the annulated four- (AlC2P) and five-membered heterocycles (P2C3) to contain three different CC double bonds. The sterically most shielded monocyclic compound (8; Aryl = 2,4,5-Me3C6H2; R = CMe3) decomposed upon warming. DFT calculations were used to examine these reactions.