Advances in Rhodium-Catalyzed Oxidative Arene Alkenylation

Abstract

Conspectus Alkyl and alkenyl arenes are of substantial value in both large-scale and fine chemical processes. Billions of pounds of alkyl and alkenyl arenes are produced annually. Historically, the dominant method for synthesis of alkyl arenes is acid-catalyzed arene alkylation, and alkenyl arenes are often synthesized in a subsequent dehydrogenation step. But these methods have limitations that result from the catalytic mechanism including (1) common polyalkylation, which requires an energy intensive transalkylation process, (2) quantitative selectivity for Markovnikov products for arene alkylation using α-olefins, (3) for substituted arenes, regioselectivity that is dictated by the electronic character of the arene substituents, (4) inability to form alkenyl arenes in a single process, and (5) commonly observed slow reactivity with electron-deficient arenes. Transition-metal-catalyzed aryl–carbon coupling reactions can produce alkyl or alkenyl arenes from aryl halides. However, these reactions often generate halogenated waste and typically require a stoichiometric amount of metal-containing transmetalation reagent. Transition-metal-catalyzed arene alkylation or alkenylation that involves arene C–H activation and olefin insertion into metal–aryl bonds provides a potential alternative method to prepare alkyl or alkenylation arenes. Such reactions can circumvent carbocationic intermediates and, as a result, can overcome some of the limitations mentioned above. In particular, controlling the regioselectivity of the insertion of α-olefins into metal–aryl bonds provides a strategy to selectively synthesize anti-Markovnikov products. But, previously reported catalysts often show limited longevity and low selectivity for anti-Markovnikov products. In this Account, we present recent developments in single-step arene alkenylation using Rh catalyst precursors. The reactions are successful for unactivated hydrocarbons and exhibit unique selectivity. The catalytic production of alkenyl arenes operates via Rh-mediated aromatic C–H activation, which likely occurs by a concerted metalation–deprotonation mechanism, olefin insertion into a Rh–aryl bond, β-hydride elimination from the resulting Rh–hydrocarbon product, and net dissociation of alkenyl arene with formation of a Rh hydride. Reaction of the Rh hydride with Cu­(II) oxidant completes the catalytic cycle. Although Rh nanoparticles can be formed under some conditions, mechanistic studies have revealed that soluble Rh species are likely responsible for the catalysis. These Rh catalyst precursors achieve high turnovers with >10,000 catalytic turnovers observed in some cases. Under anaerobic conditions, Cu­(II) carboxylates are used as the oxidant. In some cases, aerobic recycling of Cu­(II) oxidant has been demonstrated. Hence, the Rh arene alkenylation catalysis bears some similarities to Pd-catalyzed olefin oxidation (i.e., the Wacker–Hoechst process). The Rh-catalyzed arene alkenylation is compatible with some electron-deficient arenes, and they are selective for anti-Ma...

Oxidative Alkenylation of Arenes Using Supported Rh Materials: Evidence that Active Catalysts are Formed by Rh Leaching

“…Catalytic Arene Alkenylation using Rh/SiO 2 as Catalyst Precursor . We began our studies of catalysis using supported Rh materials with the conversion of benzene and ethylene to styrene using conditions that are similar to our previously reported catalysis using molecular Rh catalyst precursors [13,14,36–38] . All reactions were performed at least in triplicate with results reported with standard deviations.…”

“… [3,9–11] Although EB/SM and PO/SM processes have been commercialized, existing industrial processes for styrene and alkyl arene syntheses have some disadvantages, including: 1) multi‐step processes; for example, the EB/SM process includes benzene alkylation, transalkylation, and ethylbenzene dehydrogenation, while the PO/SM process involves benzene alkylation, ethylbenzene oxidation, oxygen transfer, and dehydration; 2) energy‐intensive operations such as trans‐alkylation and distillation; and 3) the inability to produce anti‐Markovnikov products ( i.e ., 1‐aryl alkenes or alkanes) [12] . Thus, there is increased interest in developing new catalytic processes for direct arene alkenylation at a low operating temperature that offer anti‐Markovnikov selectivity [13–15] …”

“…Transition metal‐catalyzed arene alkenylation ( i.e ., oxidative olefin hydroarylation) that functions by a pathway involving arene C−H activation and olefin insertion offers possible advantages over traditional acid‐catalyzed arene alkylation (Scheme 2). [14,16–20] These potential advantages include: a) direct arene alkenylation (rather than alkylation) via β‐hydride elimination after the olefin insertion step, b) selective production of 1‐aryl alkane/alkene by circumventing carbocationic intermediates that lead to Markovnikov selectivity, c) conversion of electron‐deficient arenes, d) new regioselectivity for alkenylation or alkylation of substituted arenes, and e) inhibition of polyalkylation since alkylated or alkenylated products can be less reactive than starting arenes [14] . Molecular catalysts involving Ni, Ir, Ru, Pt, Pd and Rh have been studied for catalytic C−H alkylation or alkenylation of arenes with α‐olefins [15,16,21–35] …”

“…Recently, we reported Rh‐catalyzed arene alkenylation to directly synthesize styrene and 1‐aryl alkenes using molecular catalysts in homogeneous processes [13,14,36–41] . Based on the mechanistic studies, our initial proposed catalytic cycle involves (Scheme 3): a) Rh‐carboxylate assisted arene C−H activation, b) ethylene coordination and insertion into a Rh‐aryl bond, c) β‐hydride elimination, and d) alkenyl arene dissociation and oxidation of a Rh−H intermediate with CuX 2 (X=carboxylate) to regenerate the starting catalyst.…”

Cp*Co(III)-Catalyzed C–H Hydroarylation of Alkynes and Alkenes and Beyond: A Versatile Synthetic Tool

“…This approach has enabled the synthesis of many compounds of interest, such as natural products, materials, and pharmaceuticals. 1 During the last two decades, palladium(II) has unarguably been in the limelight when accomplishing C–H bond-activation reactions, 2 although other transition metals, such as rhodium(III), 3 ruthenium(II), 4 and iridium(III), 5 have also been capable of efficiently carrying out these kinds of transformations. Nonetheless, the use of the aforementioned second- and third-row-transition metals comes with the disadvantage of toxicity and high cost.…”