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.
Treatment of the new alkynyl chlorophosphine, Mes P(Cl) C C CMe3, with LiNR2 afforded various unprecedented aminophosphines, Mes P(NR2) C C CMe3, which showed a fascinating diversity in their reactivity towards H Al(t)Bu2. NMe2 and NEt2 derivatives yielded the hydroalumination products Mes P(NR2) C(Al(t)Bu2) 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 (i)Pr2N 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.
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