CARBON-CARBON BOND FRAGMENTATION IN AMINOALCOHOL RADICAL CATIONS - KINETICS, THERMODYNAMIC CORRELATIONS, AND MECHANISM

A detailed study of the kinetics, thermodynamics and mechansim of carb on-carbon bond fragmentation in a series of aminoalcohol radical catio ns is presented. The compounds that provide the basis for this investi gation are derived from the parent structure, erythro-2-(phenylamino)- 1,2-diphenylethanol, by substitution at the para position of the N-phe nyl with methoxy, methyl, (hydrogen), chloro, and cyano groups (compou nds 1a-e, respectively). The rates for C-C bond fragmentation for radi cal cations 1a-e(.+) in CH3CN solution were determined by laser flash photolysis and vary from 3.9 x 10(4) (1a) to 7.4 x 10(6) s(-1) (1e). T he activation parameters for bond fragmentation in Ic-e(.+) are charac terized by low activation enthalpies and relatively large, negative ac tivation entropies. The bond fragmentation rates increase with the pea k potential for anodic oxidation of the neutral aminoalcohols, E(p)(1) . Correlation of the free energy of activation for bond fragmentation (Delta G(BF)double dagger) with FE(p)(1) (F is the Faraday constant) i mplies that the dependence of Delta G double dagger(BF) on Delta G deg rees(BF) is relatively weak, consistent with bond fragmentation in 1a- e(.+) being weakly endothermic or exothermic. The transient absorption spectra of the reactive intermediates produced by fragmentation of 1a -e(.+) are consistent with a mechansim involving heterolytic fragmenta tion of the C-1-C-2 bond with concomitant loss of the hydroxyl proton. By contrast, FT-ICR studies of 1a-e(.+) indicate that in the gas phas e homolytic fragmentation of the C-1-C-2 bond predominates. Semiempiri cal calculations using the AM1 Hamiltonian demonstrate that in the gas phase homolysis is the thermodynamically preferred pathway, consisten t with the FT-ICR results.