Photochemical Electron Transfer Initiated Oxidative Fragmentation of Aminopinacols. Factors Governing Reaction Rates and Quantum Efficiencies of C-C Bond Cleavage

Abstract

The fragmentation reactions of the cation radicals of a series of remote diamine-substituted pinacols have been investigated. The cation radicals were generated upon photooxidation with excited 2,6,9,10-tetracyanoanthracene in acetonitrile. The products are consistent with cleavage of the central C-C bond. The rate constants for fragmentation were determined both from steady-state quantum yield studies and from time-resolved measurements. In general, the rate constants for fragmentation increase with increasing stability of the radical and cation products. However, the results of temperature dependence studies clearly demonstrate that conformational effects play an important role in the transition state. In some cases, these conformational effects can result in changes in the reactivity order expected from purely thermodynamic considerations.

Diffusional distortion of the free-energy gap law

“…18 Against all the odds, the model is still used by some experimentalists, in particular, for justification of Marcus' freeenergy gap law. 24,25 We have shown here that the model is not valid for k e /k sep ӷ1 when the separation quantum yield Ӷ1. This is exactly the region where all experimental data were obtained.…”

“…This is exactly the region where all experimental data were obtained. 24,25 These are fast reactions controlled by diffusion that were described within the exponential model appropriate only for kinetic control reactions. One should expect a complete change in (⌬G r ) dependence for the same reactions, but in solution of different viscosity.…”

Unified Theory of Photochemical Charge Separation

“…If there were a true FEG law obtained from kinetically controlled geminate recombination, (p would be much closer to 1. When (p << 1 as in [94], this is a straight indication of diffusioncontrolled recombination to which the exponential model is inapplicable. Parabolic interpolation of similar data found in [95] leads only to imitation of the free-energy-gap (FEG) law.…”

Photoinduced electron transfer retropinacol reaction of 4-(N,N-dimethylamino)phenyl pinacols in chloroform

“…The oxidative fragmentation of pinacols (retropinacol reaction) can be accomplished either thermally 1,2 or photochemically, [3][4][5][6][7] giving aldehydes or ketones as the main products. In general, the fragmentation does not occur by direct photolysis, 5a and a strong electron-withdrawing sensitizer, such as 2,6,9,10-tetracyanoanthracene, 3 chloranil, 4a 2,3,5,6-tetracyanobenzene 4b or 1,4-dicyanonapthalene, 5,6 must be used to initiate the photoinduced electron transfer and to generate the pinacol radical cation which has been proposed to be the primary key intermediate for the fragmentation.…”