Investigations into the mechanism of the dissociative electron transfer reduction of organic molecules containing oxygen-oxygen or sulfur-oxygen bonds

The electron transfer (ET) reduction of two series of small to medium sized organic compounds containing a single X1-X2 bond (where X1 = S or O and X2 = O) was performed in aprotic media. Homogeneous and heterogeneous electrochemical techniques were employed in concert with digital simulation and semi-empirical PM3 and density functional theory (DFT) calculations to delineate the mechanism of the initial ET. Controlled potential electrolysis (CPE) along with standard analytical techniques were used to determine the products of the ET reduction reactions.;A small series of substituted analogues of 1a, 3,3,6,6-tetrakis(4-methoxyphenyl)-1,2-dioxane (1b), trans-2,2,5,5-tetrakis(4-methoxyphenyl)-3,4-dioxabicyclco[4.3.0]nonane (1c) and 3,6-bis(4-methoxyphenyl)-3,6-bis(4-nitrophenyl)-1,2-dioxane (1d) were studied to investigate the effect of substituent on the mechanisms involved in the ET initiated chemistry of 1a. CV's and CPE experiments of all of these analogues indicated that a similar homogeneous ET-fragmentation chain process was operative. Results from digital simulation and DFT calculations indicate the differences observed in the CV's of 1a, 1b and 1c are attributed to (a) the flexibility of the molecular structure, (b) the propensity for electronic interaction between the aryl rings and the O-O bond and (c) the degree of favourable interactions between alkoxyl radical and alkoxide anion centres of the DRA. These factors are reflected in the heterogeneous rate constant and result in their distinct CV characteristics. CV's of 1d indicated that the mechanism of ET reduction was stepwise dissociative and a distinct radical anion intermediate was formed in this case.;Evidence from the CV experiments of benzyl benzenesulfenate (2a ) and t-butyl benzenesulfenate (2b) indicated that ET reduction proceeds through formation of distinct radical anion intermediates. CPE, experiments revealed that ET to 2a resulted in S-O bond cleavage while ET to 2b led to C-O bond cleavage. The difference in the mode of cleavage and CV features are related to the characteristics of the singly occupied molecular orbital (SOMO) of the radical anion intermediate.;Cyclic voltammetry (CV) and CPE experiments on 3,3,6,6-tetraphenyl-1,2-dioxane (1a) provided evidence that 1a underwent O-O bond cleavage upon ET reduction without formation of an intermediate radical anion. It was observed that the species formed upon O-O bond cleavage, a distonic radical anion (DRA) underwent a rapid fragmentation leading to the formation of ethylene, benzophenone and the benzophenone radical anion. The benzophenone radical anion could then homogeneously transfer an electron to 1a generating the DRA which fragments to give another equivalent of the benzophenone radical anion, thus initiating a homogeneous ET-fragmentation chain process. The efficiency of the chain process was found to be dependent on the potential of the electrode when reduction of 1a is performed heterogeneously.