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Use of N-hydroxypyridine-2-thioneoxycarbonyl derivatives in free radical kinetics measurements and synthetic applications
Rate constants for radical reactions were determined using Barton's N-hydroxypyridine-2-thione esters as the source of radicals. Arrhenius functions of the rate constants (k[T]) for the self-propagating reaction were determined for the octyl and undecyl derivatives of the Barton esters 45 and 48. The Arrhenius functions for the self-propagating reactions allowed the use of the esters as "radical clocks" in competition experiments over a range of temperatures. The second order rate constants for halogen atom abstraction (k[X]) from 1°, 2°, and 3° alkyl halides were measured. As expected, among a class of halides, the order of k[X] was 3° > 2° > 1°; for halides of the same carbon frame, the order of k[X] was I > Br > Cl. Values for the pseudo first order rate constants for hydrogen atom abstraction (k[H],) from tetrahydrofuran (THF), ether, and methylene chloride were measured. These rate constants were used to analyze the reactions in the proposed single electron transfer mechanism for the reduction of alkyl halides with metal hydrides. Second order rate constants were also measured for two synthetically useful processes. Rate constants for bromine and iodine atom abstraction by alkyl radicals from ethyl haloacetates were measured as models for the halides used in Curran's atom transfer reactions. Rate constants for nitrile abstraction k[CN] by an alkyl radical from isonitriles, as used in one carbon homologating reactions, were also measured. Intermolecular aminium radical additions to olefins were also investigated. The aminium radical was formed by protonating the aminyl radical generated from the Newcomb carbamate analogs 73 and 74 to the Barton esters. The aminium radicals generated from 73 and 74 were added to ethyl vinyl ether with some success. Also, reactions where intramolecular cyclizations followed the intermolecular aminium radical addition where a pyrrolidine ring was formed are discussed.