Palladium-catalysed C-F alumination of fluorobenzenes: Mechanistic diversity and origin of selectivity

Abstract

A palladium pre-catalyst, [Pd(PCy3)2] is reported for the efficient and selective C-F alumination of fluorobenzenes with the aluminium(i) reagent [{(ArNCMe)2CH}Al] (1, Ar = 2,6-di-iso-propylphenyl). The catalytic protocol results in the transformation of sp2 C-F bonds to sp2 C-Al bonds and provides a route to reactive organoaluminium complexes (2a-h) from fluorocarbons. The catalyst is highly active. Reactions proceed within 5 minutes at 25 °C (and at appreciable rates at even -50 °C) and the scope includes low-fluorine-content substrates such as fluorobenzene, difluorobenzenes and trifluorobenzenes. The reaction proceeds with complete chemoselectivity (C-F vs. C-H) and high regioselectivities (>90% for C-F bonds adjacent to the most acidic C-H sites). The heterometallic complex [Pd(PCy3)(1)2] was shown to be catalytically competent. Catalytic C-F alumination proceeds with a KIE of 1.1-1.3. DFT calculations have been used to model potential mechanisms for C-F bond activation. These calculations suggest that two competing mechanisms may be in operation. Pathway 1 involves a ligand-assisted oxidative addition to [Pd(1)2] and leads directly to the product. Pathway 2 involves a stepwise C-H → C-F functionalisation mechanism in which the C-H bond is broken and reformed along the reaction coordinate, guiding the catalyst to an adjacent C-F site. This second mechanism explains the experimentally observed regioselectivity. Experimental support for this C-H activation playing a key role in C-F alumination was obtained by employing [{(MesNCMe)2CH}AlH2] (3, Mes = 2,4,6-tri-methylphenyl) as a reagent in place of 1. In this instance, the kinetic C-H alumination intermediate could be isolated. Under catalytic conditions this intermediate converts to the thermodynamic C-F alumination product.