The selective catalytic functionalization of C–H bonds of hydrocarbons remains one of the foremost challenges facing synthetic chemists. Processes that convert C–H bonds of simple hydrocarbons into new C–C bonds are particularly important. For example, alkyl and alkenyl arenes are currently produced on a scale of billions of pounds per year, and the addition of aromatic C–H bonds across olefin C=C bonds, olefin hydroarylation, provides an atom economical reaction with broad potential including applications in both commodity scale processes as well as fine chemical synthesis. Current industrial catalysts (e.g., Friedel-Crafts catalysis or zeolites) for arene alkylation are based on acid-mediated olefin activation. New catalysts that operate by an entirely different pathway that involves transition metal-mediated C–H activation followed by olefin insertion into metal-aryl bonds offer new opportunities.
The Gunnoe group has been studying olefin hydroarylation (to produce alkyl aromatics) and oxidative olefin hydroarylation (to produce alkenyl aromatics) catalyzed by well-defined homogeneous catalysts based on Ru, Rh and Pt. The goal is to combine understanding of transition metal mediated C–H activation and controlled olefin insertion to design novel catalytic routes for important classes of chemicals. For TpRu(L)(NCMe)Ph (Tp = hydridotris(pyrazolyl)borate; L = CO, PMe3, P(OCH2)3CEt, P(N–pyrrolyl)3, etc.) catalyst precursors, which provide a range steric and electronic profiles, the impact of the donor ability of the ligand “L” on the rate of stoichiometric benzene C–H activation has been elucidated. Importantly, these studies have led to an understanding of the primary catalyst deactivation pathway and a prediction that replacing anionic Tp ligands with charge-neutral tris(pyrazolyl)alkane ligands would provide increased catalyst longevity.
In fact, using [(HC(pz’)3)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr’4] [HC(pz’)3 = tris(3,5-dimethylpyrazolyl)methane] as catalyst precursor gives > 500 turnover numbers (TONs) of ethylbenzene formation (~95% yield) while the corresponding TpRu(P(OCH2)3CEt)(NCMe)Ph complex gives 20 TONs under the same conditions.
In an effort directed toward alkenyl arene synthesis, catalysts based on d8 transition metals have been pursued. Detailed studies of Pt(II) complexes supported by chelating bipyridyl ligands revealed a strategy for the direct formation of vinyl arenes; however, catalyst decomposition to Pt(s) is problematic. It was hypothesized that Rh(I) complexes could be effective catalysts. Recently, it was reported that (FlDAB)Rh(TFA)(η2-C2H4) [FlDAB = N,N’-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene; TFA = trifluoroacetate] converts benzene, ethylene and Cu(II) acetate to styrene, Cu(I) acetate, and acetic acid with high selectivity and yields ≥ 95%. Turnover numbers > 800 have been demonstrated with catalyst stability up to 96 hours.