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Carbon-carbon bond cleavage in radical anions of strained diphenylethane derivatives

Fragmentation reactions of radical anions (mesolytic cleavages) of 1,1,2,2-tetraalkyl-1-(4'-nitrophenyl)-2-phenylethanes with varying degrees of steric strain have been studied in polar solvents. The rates and activation parameters obtained were compared to the corresponding values obtained for homolytic bond scissions in the neutrals. The free energies of activations for both types of reactions strongly depend on the strain present in the molecule. With the exception of the most crowded members of the series, nearly all of the strain is released in the transition state of homolytic reactions, and ca. 77% of it is relieved in the transition states of mesolysis. The mesolytic fragmentations have activation energies that are on average ca. 12.5 kcal/mol lower than the homolytic processes. The observed redox activation is shown to have thermodynamic origins.

Mesolytic scission of C-C bonds in radical cations of amino derivatives: Steric and solvent effects

The radical cations of 1,2-dialkyl-1-(4'-(dimethylamino)phenyl)-2-phenylethanes (2·+) have been observed to undergo unimolecular cleavage of central C-C bonds. The observed enthalpies of activation are significantly lower than those measured for the homolysis of the corresponding neutral substrates. The entropies of activation are small or negative, despite production of fragments and partial freeing of rotational degrees of freedom upon fragmentation. These activation parameters are solvent dependent, generally decreasing in solvents with higher polarity in a self-compensatory manner, leading to the free energies of activation (and rate constants) that remain constant in different solvents. The free energies of activation depend on strain present in the structures, with ca. 63% of the strain relatived in the transition state. The fragmentation reactions of 2·+ have activation energies that are on average 23 kcal/mol lower than those for homolysis of 2. The observed activation of single bonds for scission has thermodynamics origins. The fragmentation reactions of radical cations are also compared to the fragmentation reactions of the analogous radical anions.

Transition-State Polarization in Cleavage of C-C Bonds in Radical Anions

The substituent effect on the rate of C-C bond cleavage in radical anions of 1-(4-nitrophenyl)-2-(substituted-phenyl)-1,1,2,2-tetraethylethanes has been explored.The data provide evidence for two distinctive modes of bond scission.One mode is characterized by a significant negative charge transfer across the scissile bond in the transition state.Such polarization of the transition state is in contradiction to the prediction based on the fragments' stability.The second mode, dominant in cases where the charge shift leads to negative charge accumulation on an already electron-rich fragment, involves a ?* radical anion.Both modes point to a general kinetic preference for a cleavage of radical anions that allows for charge delocalization across the scissile bond.