A P–H functionalized Al/P-based frustrated Lewis pair – hydrophosphination of nitriles, ring opening with cyclopropenones and evidence of PC double bond formation

Abstract

Hydroalumination of R-P(H)-CC-^tBu with bulky H-Al[CH(SiMe₃)₂]₂ afforded the new P–H functionalized Al/P-based frustrated Lewis pair R-P(H)-C[C(H)-^tBu]-AlR₂ [R = CH(SiMe₃)₂; FLP 7]. A weak adduct of 7 with benzonitrile (8) was detected by NMR spectroscopy, but could not be isolated. tert-Butyl isocyanide afforded a similar, but isolable adduct (9), in which the isocyanide C atom was coordinated to aluminium. The unique reactivity of 7 became evident from its reactions with the heteroatom substituted nitriles PhO-CN, PhCH₂S-CN and H₈C₄N-CN. Hydrophosphination of the CN triple bonds afforded imines at room temperature which were coordinated to the FLP by Al–N and P–C bonds to yield AlCPCN heterocycles (10 to 12). These processes depend on substrate activation by the FLP. Diphenylcyclopropenone and its sulphur derivative reacted with 7 by addition of the P–H bond to a C–C bond of the strained C₃ ring and ring opening to afford the fragment (Z)-Ph-C(H)C(Ph)-C-X-Al (X = O, S). The C–O or C–S groups were coordinated to the FLP to yield AlCPCX heterocycles (13 and 14). The thiocarbonyl derived compound 14 contains an internally stabilized phosphenium cation with a localized PC bond, a trigonal planar coordinated P atom and a short PC distance (168.9 pm). Insight into formation mechanisms, the structural and energetic properties of FLP 7 and compounds 13 and 14 was gained by quantum chemical DFT calculations.