Stabilization of the cyano(dimethylamino)methyl radical - Synergistic effect due to interaction between alpha-amino and alpha-cyano groups on the radical stabilization energy

The thermolysis reactions of 3a and 2a, b were studied over a temperature range of 40 degrees C and the activation parameters were determined. They were compared with the activation parameters of structurally comparable hydrocarbons of similar strain in order to obtain the radical stabilization enthalpies RSEs of the cyano(dimethylamino)methyl radical 1a. For this comparison the geminal interaction enthalpies of the cyano and the dimethylamino groups in the ground state had to be determined for the series of amino nitriles 4a-4c and 4j-4n by thermochemical methods. The geminal interaction in the ground state varies between -0.6 kcal/mol stabilization in 4a and a destabilization of +10.7 and +7.0 kJ mol(-1) for secondary and tertiary alpha-dialkylaminonitriles, respectively. A synergistic (i.e, more than additive) stabilization enthalpy of 26 kJ mol(-1) of 1a was found in contrast to predictions in the literature. This stabilization is interpreted by conjugation between the substituents, which are separated by the radical center. [GRAPHICS] ]