First spectroscopic and absolute kinetic studies on (alkoxycarbonyl)oxyl radicals and an unsuccessful attempt to observe carbamoyloxyl radicals

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DOIResolve DOI: http://doi.org/10.1021/ja00228a020
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TypeArticle
Journal titleJournal of the American Chemical Society
ISSN0002-7863
Volume110
Issue20
Pages67276731; # of pages: 5
Abstract(Alkoxycarbonyl)oxyl radicals, ROCO 2 •, have been generated by laser flash photolysis (LFP) of dialkyl peroxydicarbonates and tert-butyl peresters of alkyl carbonic acids. These radicals possess a broad, structureless absorption band extending from ca. 400 to 800 nm. By monitoring the kinetics of decay of this absorption, absolute rate constants have been measured for the first time for some reactions of ROCO 2 • radicals. In CCl 4 and in the absence of a reactive substrate it is clear that decarboxylation is a slow process (k ≤ 10 5 s -1 at room temperature), and the radicals probably decay mainly by an intramolecular hydrogen atom transfer. (Alkoxycarbonyl)oxyl radicals are extremely reactive both in intermolecular hydrogen abstractions (e.g., k ∼ 1 × 10 7 and 9 × 10 7 M -1 s -1 for cyclohexane and triethylsilane, respectively, at room temperature) and in additions (e.g., k ∼ 2 × 10 9 M -1 s -1 for styrene). Toward most substrates ROCO 2 • radicals are more reactive than C 6H 5CO 2 • radicals. 5 Repeated attempts to detect an analogous transient absorption in the visible region of the spectrum by LFP of tert-butyl percarbamates were not successful. Thus, carbamoyloxyl radicals, RR′NCO 2 •, still remain undetected by direct methods, just as do acyloxyl radicals, RCO 2 •. It is known that the photolysis of (XCO 2) 2 and XCO 3C(CH 3) 3 can yield X • + CO 2 as well as XCO 2 •. It is suggested that the relative importance of the direct photodecomposition to X • + CO 2 increases as the strength of the XCO 2 • bond decreases and that this is the reason for our failure to detect carbamoyloxyl (and acyloxyl) radicals.
Publication date
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC)
Peer reviewedYes
NPARC number21276748
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Record identifier3c9b2460-9ee5-4d19-a504-cb1622d36742
Record created2015-10-13
Record modified2016-05-09
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