Romero NA, Nicewicz DA. Mechanistic insight into the photoredox catalysis of anti-markovnikov alkene hydrofunctionalization reactions.
J Am Chem Soc 2014;
136:17024-35. [PMID:
25390821 PMCID:
PMC4277776 DOI:
10.1021/ja506228u]
[Citation(s) in RCA: 234] [Impact Index Per Article: 23.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
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We describe our efforts to understand
the key mechanistic aspects
of the previously reported alkene hydrofunctionalization reactions
using 9-mesityl-10-methylacridinium (Mes-Acr+) as a photoredox catalyst. Importantly, we are able
to detect alkene cation radical intermediates, and confirm that phenylthiyl
radical is capable of oxidizing the persistent acridinyl radical in
a fast process that unites the catalytic activity of the photoredox
and hydrogen atom transfer (HAT) manifolds. Additionally, we present
evidence that diphenyl disulfide ((PhS)2)
operates on a common catalytic cycle with thiophenol (PhSH) by way of photolytic cleaveage of the disulfide bond. Transition
structure analysis of the HAT step using DFT reveals that the activation
barrier for H atom donation from PhSH is significantly
lower than 2-phenylmalononitrile (PMN) due to structural
reorganization. In the early stages of the reaction, Mes-Acr+ is observed to engage in off-cycle adduct
formation, presumably as buildup of PhS− becomes significant. The kinetic differences between PhSH and (PhS)2 as HAT catalysts indicate that
the proton transfer step may have significant rate limiting influence.
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