Abstract
An indirect method for studying the kinetics of fast radical rearrangements has been developed. The method employs N-hydroxypyridine-2-thione ("PTOC") esters as radical precursors and hydrogen atom transfer from thiophenol as the basis reaction. The utility of this method was demonstrated in the calibration of the rate constant for cyclopropylcarbinyl radical (3) ring opening over a wide range of temperatures, to give a rate constant for ring opening; k3 = 1.0 x 10^8 s^-1 at 25 °C. This value compares well with values for k3 determined from other direct and indirect methods. PTOC-esters were also employed as radical precursors in the study of the rearrangement of the 2 ,2 -dimethyl-3-butenyl radical (8) to the 1,1-dimethyl-3-butenyl radical (10) via the intermediate (2,2- dimethylcyclopropyl)carbinyl radical (9). Hydrogen atom abstraction from n-Bu3SnH and thiophenol were used as the competition reactions. The rate constants for the cyclization (kg) and ring opening (k8 and k10) of the radicals 8 and 9 were found to be accelerated compared to those of the unsubstituted 3-butenyl (4) and cyclopropylcarbinyl (3) radials. The PTOC-ester precursors to the cyclopropylcarbinyl (3), 5-hexenyl (21), and 2,2-dimethyl-3-butenyl (8) radicals were used in the calibration of the rate constant for hydrogen atom abstraction t-butyl thiol by primary radicals. Arrhenius functions for (kH) determined using the primary radical clocks 3 and 21 were found to be in quite good agreement, giving a rate constant kH = 8.0 x 10^6 M^-1 s^-1 The PTOC-thiol method was therefore shown to be an effective technique for the calibration of rate constants of radical reactions. A PTOC-ester was also used as a precursor to the bicyclo[2.1.0]pent-2-yl (29) radical. This precursor (33) was employed in kinetic studies with thiophenol, t-butyl thiol, and 2 ,6 -dimethylthiophenol radical traps. Deuterium NMR experiments where 29 was generated from the PTOC-ester in the presence of thiophenol-S-d or 2,6- dimethylthiophenol-S-d as the radical trap showed the bicyclo[2.1.0]pent-2-yl radical to be a highly unusual species. In these experiments, the deuterated bicyclo[2.1.0]pentane 40 was obtained in a 16:1 ratio of the endo:exo deuterated hydrocarbons regardless of the thiol, precursor or temperature employed...
Glenn, Anne Gardner (1989). A new method for kinetic studies of fast radical rearrangements : development and applications. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1108944.