Rotation-Triggered Transmetalation on a Heterobimetallic Cu/Al -Phosphine-Oxide-Substituted Imidazolylidene Complex.

A novel strategy for the preparation of heterobimetallic -heterocyclic carbene (NHC) complexes is demonstrated using -phosphine-oxide-substituted imidazolylidenes (PoxIms). In these heterobimetallic Cu/Al complexes, the Cu and Al centers can be either completely separated or brought near each other via the rotation of the -phosphinoyl group in the PoxIm ligands. Triggered by this rotation, transmetalation to exchange the Cu-OBu and Al-CF bonds occurs on -generated Cu/Al PoxIm complexes, and the Cu-CF and Al-OBu bonds are formed in excellent yield. On the basis of the results of mechanistic studies, including the isolation/ observation of key complexes and theoretical calculations, a plausible reaction mechanism for an intramolecular transmetalation is proposed to proceed via an activation complex that includes the simultaneous coordination of the phosphinoyl oxygen atom to the Cu as well as the Al centers. Furthermore, the formation of carbon-carbon bonds between Al(CF) and allyl bromide mediated/catalyzed by Cu/Al PoxIm complexes is demonstrated. Upon the consecutive transfer of three CF groups from a single molecule of Al(CF), allyl pentafluorobenzene derivatives were obtained. The present results demonstrate the role of phosphine oxide in the activation of organoaluminum reagents for the transmetalation between Cu(I) complexes bearing NHCs as well as the benefit of constructing an intramolecular system based on a heterobimetallic complex to achieve efficient transmetalation by programming the encounter of two organometallic fragments.