Abstract
Vinyl arenes are produced on a multi-million ton scale annually and are valuable industrial precursors to plastics, elastomers, surfactants, and detergents. Current industrial methods for vinyl arene production involve acid-mediated arene alkylation via Friedel-Crafts or zeolite catalysis, trans-alkylation to optimize yield of mono-alkyl arene, and subsequent dehydrogenation to afford the desired vinyl arene product. While this type of process has been operative in industry for decades, there are a number of deficiencies that suggest that an alternative method for vinyl arene production could be beneficial. Transition metal-mediated oxidative arene vinylation, in which a transition metal catalyst and oxidant mediate the direct and single-step conversion of arenes and olefins to vinyl arenes, offers a potential alternative to traditional acid-mediated mechanisms. Examples of this type of process have been reported previously for catalyst systems based on Pd, Ru, and Ir; however, all of these processes suffer from low selectivity, low yield, or both.
Pd(OAc)2 has been reported to catalyze the conversion of arenes and olefins to vinyl arenes, although with low selectivity. It was hypothesized that the addition of ligands to Pd(OAc)2 could offer the opportunity to tune the selectivity of oxidative arene vinylation reactions. A variety of bidentate and tridentate nitrogen and phosphine ligands were screened for activity and selectivity in oxidative benzene vinylation experiments to determine which could bias the selectivity towards styrene. The Pd(II) complex (dippDAB)Pd(OAc)2 [dippDAB = N,N’-bis(2,6-di-isopropylphenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene] was the most selective for the formation of styrene over stilbene or biphenyl, two common byproducts in these reactions. Catalytic reactions with (dippDAB)Pd(OAc)2 using a Cu(OAc)2 oxidant afforded reasonable yields of styrene with high selectivity for stilbene or biphenyl.
Performing catalysis under aerobic conditions, which allows for aerobic regeneration of the Cu oxidant in a manner akin to the Wacker process, afforded styrene in excess of 3000 turnovers. Unfortunately, under aerobic conditions, significant production of vinyl acetate (~700 turnovers) was also observed. This prompted us to re-examine control reactions with Pd(OAc)2 alone under optimized aerobic conditions, which showed that while reported reactions with Pd(OAc)2 alone afforded ~34% selectivity for styrene, selectivity under our optimized conditions was ~84% for both Pd(OAc)2 and (dippDAB)Pd(OAc)2.
The lack of selectivity observed for catalysis with Pd(II) complexes prompted us to shift our focus to isoelectronic Rh(I) complexes. The rhodium catalyst (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 air-recyclable Cu(II) oxidants to styrene with yields ≥ 95% (based on Cu(II) as the limiting reagent) and with quantitative selectivity. Turnover numbers > 800 have been demonstrated with catalyst stability up to 96 hours.
Examining
catalysis with the complex (FlDAB)Rh(OAc)(η2-C2H4) shows that the reaction
rate has a dependence on catalyst concentration between first- and half-order
that varies with both temperature and ethylene concentration, a first-order
dependence on ethylene concentration with saturation at higher concentrations of ethylene, and a zero-order dependence on the concentration of
Cu(II) oxidant. The kinetic isotope effect was found to vary linearly with the
order in (FlDAB)Rh(OAc)(η2-C2H4), exhibiting no KIE when [Rh] was in the
half-order regime, and a kH/kD value of 6.7(6) when [Rh] was in the first-order regime. From these combined experimental and computational studies, competing pathways, which involve all monomeric Rh intermediates and a binuclear Rh intermediate in the other case, were proposed.
Finally, a number of promising new applications for this research are discussed. In addition to summarizing other promising developments based on this research, preliminary results investigating the impact of the carboxylate moiety on the selectivity of oxidative arene vinylation reactions using α-olefins are described. Work on the development of aerobically-stable Rh(I) complexes for oxidative arene vinylation is also discussed, as well as efforts to transition to inexpensive Ni(II) catalysts for arene vinylation. Results from the projects discussed herein are then summarized to provide insight into new catalyst design.
