Carbon-carbon bond formation is the central method by which synthetic chemists add complexity, which often represents value, to molecules. Uniting a carbon chain with an aromatic substrate to yield an alkyl arene product is thus a molecular means of creating value-added materials. A traditional method for generating alkyl arenes is Friedel-Crafts catalysis, in which an alkyl halide or olefin is activated to react with an aromatic substrate. Unfortunately, despite the development of new generations of solid-state catalysts, the reaction often requires relatively harsh conditions and frequently gives poor to moderate selectivity. Conversely, a halide can first be incorporated into the aromatic ring, and the aryl halide can subsequently be joined by a variety of catalytic coupling techniques. But generating the aryl halide itself can be problematic, and such methods typically are not atom-economical. The addition of aromatic C-H bonds across the C-C double bonds of olefins (olefin hydroarylation) is therefore an attractive alternative in the preparation of alkyl arenes. Despite the dominance and practical advantages of heterogeneous catalysts in industrial synthesis, homogeneous systems can offer an enhanced ability to fine-tune catalyst activity. As such, well-defined homogeneous catalysts for the hydroarylation of olefins provide a potentially promising avenue to address issues of selectivity, including the production of monoalkylated arene products and the control of linear-to-branched ratios for synthesis of long-chain alkyl arenes, and provide access to more ambient reaction conditions. However, examples of homogeneous catalysts that are active for the conversion of unactivated aromatic and olefin substrates to alkyl arene products that function via metal-mediated C-H activation pathways are limited. In this Account, we present results from research aimed at the development of Ru(II) catalysts supported by the hydridotris(pyrazolyl)borate (Tp) ligand for the addition of aromatic C-H bonds across olefins. On the basis of detailed mechanistic studies with TpRu(L)(NCMe)R catalysts, in which the neutral ancillary ligand L is varied, we have arrived at guidelines for the development of improved catalysts that are based on the octahedral-d6 motif.

Abstract:Complexes of the type TpRu(L)(NCMe)R [L ) CO or PMe3; R ) Ph or Me; Tp ) hydridotris-(pyrazolyl)borate] initiate C-H activation of benzene. Kinetic studies, isotopic labeling, and other experimental evidence suggest that the mechanism of benzene C-H activation involves reversible dissociation of acetonitrile, reversible benzene coordination, and rate-determining C-H activation of coordinated benzene. TpRu(PMe3)(NCMe)Ph initiates C-D activation of C6D6 at rates that are approximately 2-3 times more rapid than that for TpRu(CO)(NCMe)Ph (depending on substrate concentration); however, the catalytic hydrophenylation of ethylene using TpRu(PMe 3)(NCMe)Ph is substantially less efficient than catalysis with TpRu(CO)(NCMe)Ph. For TpRu(PMe3)(NCMe)Ph, C-H activation of ethylene, to ultimately produce TpRu-(PMe3)(η 3 -C4H7), is found to kinetically compete with catalytic ethylene hydrophenylation. In THF solutions containing ethylene, TpRu(PMe3)(NCMe)Ph and TpRu(CO)(NCMe)Ph separately convert to TpRu(L)(η 3 -C4H7) (L ) PMe3 or CO, respectively) via initial Ru-mediated ethylene C-H activation. Heating mesitylene solutions of TpRu(L)(η 3 -C4H7) under ethylene pressure results in the catalytic production of butenes (i.e., ethylene hydrovinylation) and hexenes.

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