Products and stereochemistry of thermolysis of some diazenes containing the 2-phenyl-2-butyl group

Abstract

Thermolysis of (−)-(R)-1-benzyl-2-(2-phenyl-2-butyl)diazene, (−)-R-4, produces (+)-(S)-2-methyl-1,2-diphenylbutane with 12% net retention of configuration at 110 °C in benzene or pentane containing 1 M butanethiol. The ratio of the rate constant for coupling of the initial radical pair to that of rotation of the 2-phenyl-2-butyl radical, kc/kr, is calculated to be ∼0.04–0.08. Thermolysis of (+)-(S-1-(2-phenyl-2-butyl)-2-(2-propyl)diazene at 100 °C in benzene containing 1 M butanethiol produces 2,3-dimethyl-3-phenylpentane with no observable rotation. The product must be less than 15% optically pure. The reaction produces (−)-(R)-2-phenylbutane, (−)-(R)-16, with 0.6–1.3% retention of configuration. The ratio of the rate constant for disproportionation to (−)-(R)-16 to kr is calculated to be 0.02–0.04. Thermolysis of (−)-(R)-1-(4-nitrophenyl)-2-(2-phenyl-2-butyl-diazene, (−)-(R)-12, at 175 °C in diphenylether produced no 2-(4-nitrophenyl)-2-phenylbutane. This product was produced on photolysis of (+)-(S)-12 in hexadecane at 15 °C but had no rotation. Neither (+)-(S)-7 nor (−)-(R)-12 was racemized during thermolysis and (+)-(S)-12 was not racemized during photolysis. Activation parameters for thermolysis of (±)-4 are ΔH≠ = 33 kcal/mol, ΔS≠ = 15 eu and for (±)-12, ΔH≠ = 37.3 kcal/mol, ΔS≠ = 5.5 eu.