On the Mechanism of Electrochemical Generation and Decomposition of Phthalimide N-oxyl (PINO)
2021
Phthalimide N-oxyl (PINO)
is a potent hydrogen atom transfer (HAT) catalyst that can be generated
electrochemically from N-hydroxyphthalimide (NHPI). However, catalyst
decomposition has limited its application. This paper details mechanistic
studies of the generation and decomposition of PINO under electrochemical
conditions. Voltammetric data, observations from bulk electrolysis, and computational studies suggest two primary aspects.
First, base-promoted formation of PINO from NHPI occurs via multiple-site
concerted proton-electron transfer (MS-CPET). Second, PINO decomposition occurs
by at least two second-order paths, one of which is greatly enhanced by base. Optimal
catalytic efficiency in PINO-catalyzed oxidations occurs in the presence of
bases whose corresponding conjugate acids have pKas in the range of 12-15,
which strike a balance between promoting PINO formation and minimizing its decay.
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