A Kinetic Scale for Dialkylaminyl Radical Reactions

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.