Reaction of an Al/P-based frustrated Lewis pair with benzophenone: formation of adducts and aluminium alcoholates via β-hydride elimination

Treatment of the Al/P-based frustrated Lewis pair (FLP) Mes(2)P-C((AlBu2)-Bu-t)=C(H)-Ph (1) with benzophenone afforded the simple 1:1 adduct (4) with a O=CPh2 molecule coordinated to the aluminum atom by an Al <- O donor-acceptor bond. Steric repulsion may prevent an interaction between the electrophilic carbonyl carbon atom and the Lewis-basic phosphorus atom. 4 is unstable in solution at room temperature, the coordination to aluminium increases the polarisation of the carbonyl group and favours its reduction. As suggested by quantum chemical calculations, a C-H bond of a Bu-t group approaches the electrophilic center and facilitates beta-hydride elimination with the release of isobutene and the formation of an Al-OCHPh2 ligation. An intact O=CPh2 molecule completes the coordination sphere of the metal atom (5). The second tBu group at aluminium reacts similarly by the selective formation of an Al(OCHPh2)(2) moiety (6). The thermodynamics of adduct formation and the mechanism of the hydride shift have been evaluated by quantum chemical DFT calculations.