First Spectroscopic and Absolute Kinetic Studies on (Alkoxycarbonyl)oxyl Radicals and an Unsuccessful Attempt To Observe Carbamoyloxyl Radicals

Article abstract of DOI:10.1021/ja00228a020

(Alkoxycarbonyl)oxyl radicals, ROCO2., 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 ROCO2. radicals.In CCl4 and in the absence of a reactive substrate it is clear that decarboxylation is a slow process (k 5 s^-1 at room temperature), and the radicals probably decay mainly by an intramoleular hydrogen atom transfer. (Alkoxycarbonyl)oxyl radicals are extremely reactive both in intermolecular hydrogen abstractions (e. g., k ca. 1 x 10⁷ and 9 x 10⁷ M^-1 s^-1 for cyclohexane and triethylsilane, respectively, at room temperature) and in additions (e. g., k ca. 2 x 10⁹ M^-1 s^-1 for styrene).Toward most substrates ROCO2. radicals are more reactive than C6H5CO2. radicals.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'NCO2., still remain undetected by direct methods, just as do acyclic radicals, RCO2..It is known that the photolysis of (XCO2)2 and XCO3C(CH3)3 can yield X. + CO2 as well as XCO2..It is suggested that the relative importance of the direct photodecomposition to X. + CO2 increases as the strength of the XCO2. bond decreases and that this is the reason for our failure to detect carbamoyloxyl (and acyloxyl) radicals.