Phosphaaluminirenes: Synthons for Main Group Heterocycles

The phosphaaluminirenes HC­[(CMe)­(NDipp)]2Al­[C­(R)P] (Dipp = 2,6-i-Pr2C6H3, R = tBu or adamantyl) 2 and 3, featuring an unsaturated three-membered AlCP ring, have been synthesized as crystalline solids via a [1 + 2] cycloaddition reaction of the aluminum­(I) complex HC­[(CMe)­(NDipp)]2Al (1) with phosphaalkynes. Computational investigations infer three-centered 2π-electron aromaticity of the AlCP rings. Compound 3 is readily protonated by tBuOH to induce a ring-opening σ-bond metathesis, giving an alumina-substituted P-hydrogeno phosphaalkene 4. Remarkably, the high strain of the AlCP ring of 3 allows for facile ring enlargement in reactions with CyNC, bis­(diisopropylamino) cyclopropenylidene (BAC), elemental Se, Ph2CO, PhCHCHCOPh, and PhCN at room temperature. These furnish a series of unprecedented main group heterocycles 5–10 with the CP unsaturated bonds remaining intact. The mechanisms are considered in light of thorough density functional theory (DFT) calculations.