Alkynyl functionalized Al/P-based frustrated Lewis pairs - aluminium alkynide elimination and evidence for the formation of 3H-phosphaallenes [R-P=C=C(H)-tBu]

Hydroalumination of dialkynylphosphines, aryl-P(C C-Bu-t)(2) (aryl = 2,4,6-Me3C6H2 (1), Ph (2)), with the bulky dialkylaluminium hydride H-Al[CH(SiMe3)(2)](2) afforded mixed alkenyl-alkynyl phosphines (3 and 4) with aluminium atoms in geminal positions to phosphorus. These compounds contain coordinatively unsaturated aluminium and phosphorus atoms and may be applicable as alkynyl functionalised frustrated Lewis pairs. Their unique constitution with dialkylaluminium and alkynyl groups in close proximity favours unusual secondary reactions with the elimination of an aluminium alkynide and formation of reactive 3H-phosphaallenes, aryl-P=C=C(H)-Bu-t (5 and 6), which depending on the steric shielding by the aryl groups, oligomerize at room temperature over days or weeks. The mesityl derivative 5 is formed in a very selective reaction, but the relatively unstable phenyl compound 6 was only detected in a mixture of several components. One of these (7) was isolated as orange-red crystals and identified as a heterocyclic zwitterionic compound with a central AlC2P2 ring. One of its phosphorus atoms is bonded to a terminal alkynyl group.