Ring extension and rearrangement reactions of an ambiphilic aluminacyclobutene

The combination of aluminum and carbon as a Lewis acid and base, respectively, to form an intramolecular Frustrated Lewis Pair (FLP) has been less explored compared to other combinations. Herein, we present the synthesis of a metallacyclic aluminum/carbon-based ambiphile and its reactivity towards carbonyl and isocyanate compounds. The aluminacyclobutene title compound Ph₃PCH(CHvCSiMe₃)AltBu₂(3) was synthesized from an alkynyl-substituted phosphonium salt [Ph₃PCH₂CuCSiMe₃][Br] (2) and a diorganoalanate Li[tBu₂AlH₂]·2thf (1). The alanate 1 is structurally characterized and applied in a hydrometallation reaction. The obtained ylide-based ambiphile 3 shows typical FLP reactivity towards carbonyl compounds resulting in ring expansion, forming Ph₃PCH(CHvCSiMe₃){(Ph)C(H)O}AltBu₂ (4). In the reaction with phenyl isocyanate, a rearrangement reaction of the ambiphile was observed, which led to proton migration and the formation of an AlOCN heterocycle of the composition Ph₃PC(CHvCHSiMe₃) {(Ph)NCO}AltBu₂(6). The proton shuttling initially proceeds from the ylidic carbon atom to the isocyanate’s nitrogen atom providing Ph₃PC(CHvCSiMe₃){(PhNH)CO}AltBu₂ (5), and finally to the carbon atom originally bonded to the aluminum atom. The experimental findings were rationalized through DFT calculations including an analysis of the rearrangement mechanism.