The Generation of Carbene Anion Radicals from Diazo Compounds. Factors Influencing the Unimolecular Decomposition of the Anion Radicals of Diazo Compounds in Solution

Article abstract of DOI:10.1021/ja00283a009

Anion radicals of two diazo compounds, azibenzil and diethyl diazomalonate, had previously been reported to undergo unimolecular decomposition in aprotic solvents to generate the corresponding carbene anion radicals.These processes have been examinated in detail by transient electrochemical techniques.The rate constants for the loss of dinitrogen in DMF at 273.2 K are 407 and 125 s^-1 for the anion radicals of diethyl diazomalonate and azibenzil, respectively.There is little difference in the rate constants in the two solvents.The activation enthalpies fall in the range 10.6-12.7 kcal/mol, and the entropies of activation are close to -6 cal/(K*mol).Hydrogen bonding to water (i) (K_i = 0.8 M^-1) facilitates unimolecular decomposition. The hydrogen bonded complex, Ph(PhCO)C=N₂^.-/HOH, undergoes first-order decomposition with a rate constant in acetonitrile at 273.2 K equal to 1700 s^-1.This rate enhancement is attributed to the localization of charge away from nitrogen.The carbonyl group adjacent to the diazo function in both substrates also facilitates loss of dinitrogen.It is concluded that unimolecular loss of nitrogen is a more favorable process than previously thought, especially at higher temperatures and when the charge is delocalized.