A KINETIC-STUDY OF THE MIGRATION OF A PHENYL GROUP FROM SULFUR TO A CARBON RADICAL CENTER - REARRANGEMENT OF THE ALPHA-(PHENYLTHIO)BENZYL RADICAL

The first rate constants for the neophyl-like 1,2-phenyl migration from sulfur to a carbon-centered radical site are reported. An Arrhenius expression was determined for the neophyl-like 1,2-phenyl migration reaction of the alpha-(phenylthio)benzyl radical (2) to form the diphenylmethylthio radical (6) in nonane at 160-215-degrees-C: log (k/s-1) = (10.65 +/- 0.74) - (21.4 +/-1.55)/theta, theta = 2.3 RT kcal/mol (errors are 1-sigma). The rate constants were measured in a competition between the rearrangement of 2 versus hydrogen abstraction from thiophenol in nonane. The basis rate expression for hydrogen abstraction, log (k(abs)/M-1 s-1) = (8.60 +/- 0.02) - (6.64 +/- 0.03)/theta, was determined by a competition between self-termination (k(t)) of 2 and abstraction (k(abs)) from thiophenol from 27 to 220-degrees-C in nonane. The relative rate expression, log (k(abs)/k(t)1/2) = 2.86 - 5.24/theta, was converted to the absolute rate expression for abstraction by use of the von Smoluchowski expression for the self-reaction of 2, log (k(t)1/2) = 5.74 - 1.40/theta. Experimental diffusion coefficients of benzyl phenyl sulfide in nonane were determined for comparison with calculated values for modeling the diffusion of 2. MNDO-PM3(RHF) calculations predict that the rearrangement 2 --> 6 proceeds through a transitional structure, 10, rather than an intermediate. Substituted thiaspiro[2.5]octadienyl and thiiranylcarbinyl radicals are predicted to exhibit no or low (0.5 kcal/mol) barriers to C-S cleavage, respectively, suggesting that thiiranylcarbinyl radicals are candidates for picosecond radical clocks.