The Study of Rhodium Complexes for C-H Bond Activation and Functionalization

WEBSTER-GARDINER, MICHAEL S. The study of rhodium complexes for C–H Bond Activation and Functionalization (Under the direction of Professor T. Brent Gunnoe).
The production of alkyl arenes from arenes and olefins is a significant sector of the petrochemical industry. Typically, alkyl arenes are synthesized through acid-based methodologies such as Friedel-Crafts or zeolite catalysis. As a result of the mechanism for these processes, polyalkylation occurs, which necessitates a trans-alkylation step to yield the desired monoalkylated species, and there is poor control of the regioselectivity for substituted arenes. Further, acid-based catalysis cannot produce anti-Markovnikov addition products or vinyl arenes. Transition metal catalysts for the selective oxidative vinylation of arenes with olefins offer an alternate mechanism which circumvents the flaws of traditional methods. These catalysts operate through two fundamental steps: transition metal arene C–H activation and olefin insertion into an M–Ph bond. Current examples of transition metal catalysts for this transformation are typically based on Ru, Ir, Pt and offer marginal selectivity and efficiency. This Dissertation is focused on extension of oxidative vinylation of arenes with olefins to low valent rhodium species.
Initial studies focused on diimine rhodium complexes of the form (DAB)Rh(L)(TFA) (DAB = diazabutadiene, L = η2-COE or η2-C2H4, TFA = trifluoroacetate, COE = cyclooctene) for C–H activation. These catalysts were demonstrated to be active for the H/D exchange between benzene-d6 and HTFA. However, these rhodium species were found to operate through protic electrophilic aromatic substitution and therefore formation of Rh–Ph was not observed. In addition, we demonstrated that many common salt additives for in situ generated catalysts are efficient for the H/D exchange of arenes and acids.
These (DAB)Rh(η2-C2H4)(TFA) species were demonstrated to be efficient for the oxidative vinylation of benzene with ethylene. The most efficient catalyst, (FlDAB)Rh(η2-C2H4)(TFA) (FlDAB = N,N'-bis-(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene), produces styrene in a single-step from benzene, ethylene, and Cu(OAc)2 with > 95% selectivity and > 800 TOs. The oxidative vinylation of benzene with propylene revealed the formation of alkenyl products allylbenzene, trans-β-methylstryene, cis-β-methylstryene, and α-methylstyrene in linear to branched (L:B) ratios of > 8:1. Further studies revealed that the oxidative vinylation of both electron deficient and electron rich arenes with α-olefins is efficient when using commercially available [Rh(η2-C2H4)2(µ-OAc)]2 and Cu(II) carboxylate. [Rh(η2-C2H4)2(µ-OAc)]2 can successfully produce alkenyl arenes with over 1400 TOs and L:B ratios of 16:1. The similarities to the commercialized Wacker process highlight the potential of this rhodium based oxidative vinylation catalysis for further study.