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, PhCHCHCOPh,
and PhCN at room temperature. These furnish a series of unprecedented
main group heterocycles 5–10 with
the CP unsaturated bonds remaining intact. The mechanisms
are considered in light of thorough density functional theory (DFT)
calculations.