| Résumé | (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. |
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