Catalytic synthesis of "super" linear alkenyl arenes using a Rh(I) catalyst supported by a "capping arene" ligand: Access to aerobic catalysis

Abstract

Alkyl and alkenyl arenes are used in a wide range of industrial products including plastics, detergents, fuels and fine chems. However, the synthesis of 1-phenylalkanes (or their unsatd. variants) is not accessible with current industrial routes. Alkyl benzene surfactant precursors are produced from benzene and α-olefins using acid-based catalysis, which primarily results in a distribution of 2- and 3-Ph alkanes. Transition metal mediated hydroarylation of olefins that circumvents the formation of carbocation intermediates is an alternative to acid-based catalysis for alkenyl or alkyl arene synthesis. This process couples olefin insertion into metal-aryl bonds and metal mediated C-H activation. The simple rhodium complex [Rh(µ-OAc)(η^2-C_2H_4)_2]_2 selectively catalyzes oxidative hydroarylation of α-olefins using Cu(II) as the oxidant to generate alkenyl arenes that produce straight-chain 1-aryl alkanes upon hydrogenation. The development of oxidatively resistant ligands that cap and protect the Rh center has overcome the instability of this initial Rh catalyst towards O_2 or reactive Cu-peroxo intermediates. Not only has the longevity of the catalyst been enhanced, but the Cu(II) oxidant can now be recycled in situ. These ligands provide Rh catalysts that can maintain activity for >1 mo with multiple Cu(II) regeneration steps using O_2 and acid, while retaining linear product selectivity and high turnover nos. Also, air can be used as in situ oxidant to give approx. 10,000 catalytic turnovers. Computational modeling has suggested an oxidative addn./reductive elimination pathway for the overall catalytic process.