A Kinetic Scale for Dialkylaminyl Radical Reactions
journal contributionposted on 24.04.1996 by Osama M. Musa, John H. Horner, Haifa Shahin, Martin Newcomb
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A kinetic scale for dialkylaminyl radicals was established by measuring unimolecular rate constants for a series of dialkylaminyl radical clocks that spans eight orders of magnitude and using clock reactions to measure the second order rate constants for reactions of several hydrogen atom donors. N-Hydroxypyridine-2-thione derivatives of carbamic acids (so-called PTOC carbamates) were used as radical precursors in direct, laser-flash kinetic measurements and in indirect, radical chain kinetic studies. The calibrated radical clocks are N-methyl-6,6-diphenyl-5-hexenaminyl, N-methyl-trans-5-phenyl-4-pentenaminyl, N-methyl-5,5-diphenyl-4-pentenaminyl, N-methyl-trans-2-phenylcyclobutanaminyl, and N-methyl-trans-2-phenylcyclopropanaminyl. Calibrated hydrogen atom donors are Bu3SnH, t-BuSH, PhSH, and PhSeH. Whereas the tin hydride reactions with dialkylaminyl radicals are slower than reactions with alkyl radicals, the polarity-matched reactions of the electron-rich dialkylaminyl radicals with the electron-poor hydrogen donors t-BuSH, PhSH, and PhSeH have rate constants nearly equal to those for reactions of alkyl radicals with the same donor.