Rhodium-Catalyzed Alkenylation of Arenes with Multi-Substituted Olefins: Comparison of Selectivity and Reaction Rate as a Function of Olefin Identity

Rhodium-catalyzed arene alkenylation using Cu­(II) carboxylates
as the in situ oxidant and mono-substituted olefins has been previously
reported (e.g., J. Am. Chem. Soc. 2019, 139, 5474; J. Am. Chem. Soc. 2018, 140, 17007; Organometallics 2019, 38, 3860; J. Am. Chem.
Soc. 2020, 142, 10534). Herein,
studies are extended to multi-substituted olefins with the goal of
evaluating the effect of olefin substitution pattern and substituent
identity on selectivity and turnover frequency. The influence of olefin
substitution is probed by comparing the conversion of benzene to alkenyl
arenes with ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene, isobutene, 2-methyl-2-butene, and tetramethylethylene
as well as the phenyl-substituted olefins and isomers of propenylbenzene.
The rate of oxidative hydrophenylation for multi-substituted olefins
follows the trend monosubstituted > disubstituted > trisubstituted,
and tetrasubstituted olefins are unreactive. To probe the effect of
substituent size on Markovnikov/anti-Markovnikov regioselectivity,
cyclohexyl, tert-butyl, isopropyl, ethyl, and methyl
substituted α-olefins are compared. Selectivity for anti-Markovnikov
products generally increases as substituent steric bulk is increased.
Tolerance for some functionalized olefins is demonstrated. The ortho/meta/para regioselectivity
with mono-substituted arenes reveals that arene and olefin identity
influences selectivity. Further mechanistic studies provide evidence
for Curtin–Hammet control of ortho/meta/para regioselectivity with monosubstituted
arenes.