52898-42-7Relevant articles and documents
Calibration of a new horologery of fast radical "clocks". Ring-opening rates for ring- and α-alkyl-substituted cyclopropylcarbinyl radicals and for the bicyclo[2.1.0]pent-2-yl radical
Bowry, Vincent W.,Lusztyk, Janusz,Ingold
, p. 5687 - 5698 (2007/10/02)
Rate constants have been determined at 37°C for the ring opening of a variety of alkyl-substituted cyclopropylcarbinyl radical "clocks" by nitroxide radical trapping (NRT) using TEMPO. Relative yields of unrearranged and rearranged trialkylhydroxylamines were measured at various TEMPO concentrations, and these data were then combined with absolute rate constants for the reactions of structurally appropriate alkyl radicals with TEMPO as determined by laser flash photolysis. Cyclopropylcarbinyl radicals, including the bicyclo[2.1.0]pent-2-yl radical, were generated by H-atom abstraction from the parent hydrocarbon and, in a few cases, also from the appropriate diacyl peroxide. Twelve substrates yielded sixteen clock reactions because some cyclopropylcarbinyls can undergo two different ring-opening reactions. For six methyl-substituted cyclopropylcarbinyls rate constants ranged from 0.8 × 108 s-1 for 1-methylcyclopropylcarbinyl to 47 × 108 s-1 for pentamethylcyclopropylcarbinyl. Rate constants for the ring opening of cyclopropylcarbinyl, α-methyl- and α,α-dimethylcyclopropylcarbinyl are 1.2, 0.70, and 0.88 × 108 s-1, respectively. Rate constants for H-atom abstraction by tert-butoxyl from various positions in the 12 cyclopropane substrates relative to the rate of H-atom abstraction from cyclopentane have also been determined by using competitive NRT.
Kinetics and Regioselectivity of Ring Opening of Substituted Cyclopropylmethyl Radicals
Beckwith, Athelstan L. J.,Bowry, Vincent W.
, p. 2681 - 2688 (2007/10/02)
Accurate analysis of the mixture of hydroxylamines 8 formed when suitable peroxides 4 undergo homolysis in the presence of the nitroxyl radical 1,1,3,3-tetramethylisoindolin-2-yloxy (T) has afforded rate constants for ring opening of the cyclopropylmethyl radicals 2a, 2c, 2dcis, 2dtrans, and 2e and cyclization of the but-3-enyl radicals 1a, 1c, and 1d.The presence of methyl substituents enhances the rates both of cyclization and of ring opening to give primary radicals.In the case of the trans-2-methylcyclopropyl radical, 2dtrans, this effect leads to preferential formation of the less thermodynamically stable possible product, 1d, below about 60 deg C.In general, the effects of substituents support the view that the transition structure for the cyclopropylmethyl-butenyl radical interconversion is dipolar.Combination of data from four sets of workers using three different kinetic techniques affords the following recommended Arrhenius equation for the cyclopropylmethyl radical clock reaction: log k1(2a) = 13.31 - 7.6/2.3RT.
