The Study of d8 Transition Metal Catalysts for Oxidative Arene Alkenylation

Alkyl and alkenyl arenes are important commodity chemicals that are produced on a large scale. Acid-catalyzed arene alkylation for the industrial production of alkyl and alkenyl arenes requires an energy intensive trans-alkylation process, which results from poly-alkylation due to the higher reactivity of alkylated arenes compared to starting arenes. In addition, acid-catalyzed methods have limitations that result from the reaction mechanism including exclusive selectivity for Markovnikov products for arene alkylation using ɑ-olefins, inability to form alkenyl arenes in a single process starting with arenes and alkenes, and commonly observed slow reactivity with electron-deficient arenes. Transition metal-catalyzed aryl-carbon coupling reactions can be used to produce alkyl or alkenyl arenes. However, these C–C coupling reactions, which usually involve late transition metals such as Pd, often require activated aryl halide substrates and metal-containing trans-metalation reagents, and they typically generate halogenated by-products. An alternative method for the production of alkenyl arenes is transition metal-catalyzed oxidative arene alkenylation, which converts an arene, alkene and oxidant to an alkenyl arene. This dissertation is focused on development of oxidative arene alkenylation using late transition metal catalysts.

The Gunnoe group recently reported a diimine Rh(I) complex that serves as a catalyst precursor for the direct oxidative conversion of benzene and ethylene to styrene with over 800 turnover numbers and nearly quantitative yield relative to air-recyclable Cu(II) oxidant. In an effort to determine the influence of the diimine ligand, a series of diimine ligated Rh(I) complexes was shown to yield statistically identical results in terms of activity and product selectivity in the catalytic benzene alkenylation. These ligated Rh complexes dissociate diimine ligands and are likely transformed into [Rh(μ-OAc)(η2-C2H4)]2 under catalytic conditions. In the absence of Cu(II) oxidants, [Rh(μ-OAc)(η2-C2H4)]2 undergoes thermolysis in benzene at 150 °C to form Rh(0) species. Furthermore, under the catalytic conditions with Cu(OAc)2, [Rh(μ-OAc)(η2-C2H4)]2 undergoes rapid decomposition to form catalytically inactive and insoluble Rh species. Thus, the observed induction period under some conditions is likely due to the formation of insoluble Rh (rapid) followed by re-dissolution of the Rh (slow). Using either Cu(OAc)2 treated to minimize particle size or more soluble Cu(II) oxidants such as copper(II) 2-ethylhexanoate [Cu(OHex)2], the decomposition of [Rh(μ-OAc)(η2-C2H4)]2 to form insoluble rhodium species can be mitigated and the catalytically active Rh species maintained. In such cases, an induction period is not observed.

An oxidative conversion of unactivated arenes and simple alkenes to alkenyl arenes using unpurified air or O2 as the sole oxidant has been developed. This method uses simple RhCl3 as catalyst precursor and does not require metal-containing co-oxidants and additional ligands. Conditions to achieve > 1,000 turnovers of alkenyl benzene products has been demonstrated. The catalyst is selective for anti-Markovnikov (linear) 1-phenylproylene products over Markovnikov (branched) 2-phenylpropylene products with linear:branched ratio in the range of 4-6:1. The previously reported anaerobic benzene propenylation catalyzed by [Rh(μ-OAc)(η2-C2H4)2]2 produces phenylpropylene products with ~8:1 ratio of linear 1-phenylpropylene products to branched 2-phenylpropylene when Cu(II) salts is used as the in situ oxidant. Based on kinetic studies, the different selectivities for catalytic reactions in the presence of Cu(II) (anaerobic catalysis) and absence of Cu(II) (using O2 as the sole oxidant) are proposed to result from a change of rate-limiting steps from olefin insertion (under anaerobic conditions) to the reaction of Rh with O2 after olefin insertion (under aerobic conditions).

An Ir(I) catalyst for arene alkenylation shows selectivity for linear 1-aryl propylene products using benzene and propylene. Conditions to accomplish 70:1 linear/branched ratio have been demonstrated. Compared to reported Rh-catalyzed arene alkenylation, the Ir-catalyzed process exhibits reduced reactivity and limited product yield relative to Cu(II) carboxylate oxidant.