Hydroalumination of alkynyl-aminophosphines as a promising tool for the synthesis of unusual phosphines: P-N bond activation, a transient phosphaallene, a zwitterionic AlP2C2 heterocycle and a masked Al/P-based frustrated Lewis pair

摘要

Treatment of the new alkynyl-chlorophosphine, Mes-P(Cl)-C equivalent to C-CMe3, with LiNR2 afforded various un-precedented aminophosphines, Mes-P(NR2)-C=C-CMe3, which showed a fascinating diversity in their reactivity towards H-(AlBu2)-Bu-t. NMe2 and NEt2 derivatives yielded the hydroalumination products Mes-P-( NR2)-C((AlBu2)-Bu-t)=C(H)-CMe3 which have an Al-N and an activated P-N bond. Elimination of aluminium amide yielded the transient 3H-phosphaallene, Mes-P=C=C(H)-CMe3, which finally afforded a five-membered AlP2C2 heterocycle with an Al-P bond and two exocyclic C=C bonds. This heterocycle is directly formed with the sterically shielded (Pr2N)-Pr-i- and dimethylpiperidinophosphines. A unique R2AlH adduct resulted from the NPh2 and N(SiMe3)(2) substituted phosphines. It may be viewed as an Al/P-based frustrated Lewis pair (FLP) which coordinates an R2AlH moiety. The heterocyclic AlP2C2 compound is formed in the final step of this reaction. Hydroalumination with Et2AlH yielded [Mes-P(H)-C(AlEt2)=C(H)-CMe3](2) which features an Al2P2C2 heterocycle and two Al-P bonds. This dimer resembles the class of hidden or masked FLPs which show a reactivity similar to uncoordinated FLPs. Its unique structural motif is a P-H bond which may result in a new type of FLP chemistry.