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 Ph$_3$PCH(CHvCSiMe$_3$)AltBu$_2$(3) was synthesized from an alkynyl-substituted phosphonium salt [Ph$_3$PCH$_2$CuCSiMe$_3$][Br] (2) and a diorganoalanate Li[tBu$_2$AlH$_2$]·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 Ph$_3$PCH(CHvCSiMe$_3$){(Ph)C(H)O}AltBu$_2$ (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 Ph$_3$PC(CHvCHSiMe$_3$) {(Ph)NCO}AltBu$_2$(6). The proton shuttling initially proceeds from the ylidic carbon atom to the isocyanate’s nitrogen atom providing Ph$_3$PC(CHvCSiMe$_3$){(PhNH)CO}AltBu$_2$ (5), and finally to the carbon atom originally bonded to the aluminum atom. ... mehrThe experimental findings were rationalized through DFT calculations including an analysis of the rearrangement mechanism.