Mechanistic Insight into the Photoredox Catalysis of Anti-Markovnikov Alkene Hydrofunctionalization Reactions

2015-12-17T06:17:44Z (GMT) by Nathan A. Romero David A. Nicewicz
We describe our efforts to understand the key mechanistic aspects of the previously reported alkene hydrofunctionalization reactions using 9-mesityl-10-methylacridinium (<b>Mes-Acr</b><sup><b>+</b></sup>) 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 ((<b>PhS</b>)<sub>2</sub>) operates on a common catalytic cycle with thiophenol (<b>PhSH)</b> 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 <b>PhSH</b> is significantly lower than 2-phenylmalononitrile (<b>PMN)</b> due to structural reorganization. In the early stages of the reaction, <b>Mes-Acr</b><sup><b>+</b></sup> is observed to engage in off-cycle adduct formation, presumably as buildup of <b>PhS</b><sup>−</sup> becomes significant. The kinetic differences between <b>PhSH</b> and (<b>PhS</b>)<sub>2</sub> as HAT catalysts indicate that the proton transfer step may have significant rate limiting influence.