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, CH₂CMe ₃), 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 P ₂C₂Al 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-Me ₃C₆H₂; R = CMe₃) decomposed upon warming. DFT calculations were used to examine these reactions.