Determination of Addition Rates of Benzenethiyl Radicals to Alkynes by Flash Photolysis. Structures of Produced Vinyl-Type Radicals

Article abstract of DOI:10.1021/ja00378a025

The addition rate constants (k₁) of substituted benzenethiyl radicals to CH<*>CR (R = Ph, n-C3H7, and COOCH3) have been determined by flash photolysis.The Hammett reaction constants (ρ⁺) was estimated for each alkyne: ρ⁺(CH<*>CPh) = +1.38, ρ⁺(CH<*>CC3H7) = +1.34, and ρ⁺(CH<*>CCOOCH3) = +1.28.A small ρ⁺ value for CH<*>CCOOCH3 was attributed to a decrease in the polar effect in the transition state.Similar ρ⁺ values were estimated for the corresponding alkenes.A small difference in the ρ⁺ values suggests that the reactivities of the alkynes and alkenes toward a thiyl radical may be mainly determined by the thermodynamic stabilities of the produced radicals (PhSCH=C(*)R and PhSCH2C(*)HR).The rate constants (in M^-1 s^-1) for the benzenethiyl radical were as follows: k₁(CH2=CHPh) = 2.0E7, k₁(CH<*>CPh) = 7.9E5, k₁(CH2=CHCOOCH3) = 2.7E5, k₁(CH<*>CC3H7) = 1.4E4, k₁(CH2=CHC4H9) = 1.0E4, and k₁(CH<*>CCOOCH3) = 8.3E3.The unpaired electron of PhSCH=C(*)COOCH3 may be localized, whereas the unpaired electron of PhSCH2C(*)HCOOCH3 is stabilized by the interaction with the ester group.The stability of PhSCH=C(*)Ph is intermediate between the linear ? radical and the localized ? radical.