Solvent and Aryl-Substituent Effects on the Rates of Thermal Decomposition of α-Azidostyrenes

Article abstract of DOI:10.1021/jo00276a017

The effect on the rates of first-order thermolysis of α-azidostyrenes by aryl substitution have been measured in benzyl alcohol and 1-butanol in the temperature range 60-100 deg C.Values for ΔH(excit) are in the range 24-28 kcal/mol, and for ΔS(excit), +1 to -9 eu.The rate constants of the meta and para substituents are correlated with ?⁺ substituent constants where ρ = -0.7 (r = 0.98) in both solvents.Relative to α-azidostyrene, 2-methyl-α-azidostyrene and 2,4,6-trimethyl-α-azidostyrene experience retardation of thermolysis by factor of 0.97 and 0.58, respectively, in 1-butanol at 80 deg C.The activation parameters, ρ value, ?⁺ rate correlation, and the steric retardation support a concerted mechanism where, in the transition state, positive charge is delocalized to the aryl ring, coplanarity of the developing azirine ring and the aryl ring being required.Thermolysis rates of α-azidostyrene at 80.2 deg C are little affected by changes in solvent polarity, but are markedly accelerated in methyl acrylate, in which ΔH(excit) = 20 kcal/mol and ΔS(excit) = -17 eu, supporting a hypothesis by others that this reaction does not produce a nitrene intermediate, as was first suggested, but is likely a 1,3-dipolar cycloaddition mechanism of the azide function of α-azidostyrene with methyl acrylate to give a triazoline, which rapidly loses nitrogen.