A Kinetic Scale for Dialkylaminyl Radical Reactions

Abstract

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 Bu 3 SnH, 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 electronpoor hydrogen donors t-BuSH, PhSH, and PhSeH have rate constants nearly equal to those for reactions of alkyl radicals with the same donor.

Efficiencies of fragmentation of glycosaminoglycan chloramides of the extracellular matrix by oxidizing and reducing radicals: potential site-specific targets in inflammation?

“…It is thus difficult at present to reconcile the kinetic data [42], the SOTS-1 studies [23,27] and the fragmentation data presented here. It may be that superoxide reduces the chloramide directly, as confirmed for other reducing radicals in the previous kinetic study [41] to produce the amidyl radical which may react rapidly with oxygen to produce a nitroxide species as found in previous studies of sterically-hindered amidyl and aminyl radicals [43,44]. Ultimately a stable glycosaminoglycan nitroxide may be formed, as shown tentatively in Scheme 4.…”

“…Thus, to ensure a good competition to form a glycosaminoglycan NO 2 species, as in shown that these radicals react rapidly with oxygen, in one case a rate constant of 2 x 10 8 M -1 s -1 was measured [44]. It is therefore possible that fast reaction of the glycosaminoglycan amidyl radical with oxygen may also occur in the superoxide…”

Steric trends and kinetic parameters for radical reductions involving alkyldiphenyltin hydrides

“…1 As a chain-carrying reagent, tributyltin hydride is cheap, readily available and has favourable rate constants for delivery of hydrogen atom to carbon-centred and other radicals [2][3][4][5][6] while the corresponding stannyl radical reacts readily with a variety of free-radical precursors. [6][7][8][9][10][11][12][13][14][15] The ability to establish free-radical chain reactions of synthetic utility is a direct result of recent significant advances made in our understanding of the factors which govern radical reactivity [16][17][18][19][20] together with a knowledge of the important rate constants involved in the overall chain process. [1][2][3][4][5][6] Formation of carbon-carbon bonds through the use of inter-and intramolecular homolytic addition chemistry 1,21 and carbon-heteroatom bonds through the use homolytic substitution chemistry 19 are key chemical reactions of synthetic significance which directly compete with hydrogen abstraction processes from chain carriers such as tributyltin hydride.…”

Long-range oxidative damage to cytosines in duplex DNA

“…Our experimental strategy to probe for hole density on pyrimidines exploits the rapid (nanoseconds to picoseconds) ring opening of cyclopropylamine radical cations (21). Saito and coworkers have developed kinetic traps for holes residing on G (22) and A (23) based on the rapid ring opening of N 2 -cyclopropylguanosine ( CP G) and N 6 -cyclopropyladenine upon oxidation through DNA-mediated CT.…”