Journal of Organic Chemistry p. 19 - 25 (1986)
Update date:2022-08-30
Topics:
Franz, James A.
Suleman, Naushadali K.
Alnajjar, Mikhail S.
Absolute rate constants for the abstraction of hydrogen atom from thiophenol, tributylstannane, and tributylstannane-d in hexane and tirbutylstannane and dicyclohexylphosphine (DCPH) in cyclohexane were determined by competition of the abstraction reaction of benzyl radical, produced by the photolysis of dibenzyl ketone, with self-termination using termination rate constants of Fischer et al. in cyclohexane.For tributylstannane in cyclohexane, log(kabs/M-1s-1) = (8.647+/-0.172) - (5.584+/-0.238)/θ, and for DCPH, log (kabs/M-1s-1) = (8.001+/-0.184) - (6.278+/-0.26)/θ, θ = 2.303RT kcal/mol.A rate expression for benzyl radical termination in hexane was developed by combining values of kabs/(kt)1/2 from the competition of benzyl abstraction from Bu3SnH vs. termination using the values of kabs obtained in cyclohexane.The resulting expression for self-termination of benzyl in hexane, ln (2kt/M-1s-1) = (25.98+/-0.36) - (1803.58+/-199.4)/RT, is in satisfactory agreement with the termination rates predicted by using the Smoluchowski equation for rate control by translational diffusion (ln (2kt) = 26.53-2065.6/RT).For benzyl in hexane, values of kabs/(kt)1/2 combined with the experimental expression for termination gave, for reaction with thiophenol, log (kabs/M-1s-1) = (8.273+/-0.180) - (3.79+/-0.24)/θ and for the reaction of benzyl and tributylstannane-d, log (kabs/M-1s-1) = (8.712+/-0.410) - (6.155+/-0.56)/θ.Relative rate expressions (kre/kabs) for the cyclization of the 2-allylbenzyl radical vs. abstraction from tributylstannane, thiophenol, and DCPH were combined with the absolute rate expressions for abstraction from these donors to give, for the cyclization of 2-allylbenzyl radical to 2-indanylmethyl radical, the expression log (kre/s-1) = (11.14+/-0.30) - (16.28+/-0.53)/θ.
View MoreDoi:10.1039/c7tc00417f
(2017)Doi:10.1039/c5cc00697j
(2015)Doi:10.1016/S0040-4039(02)00115-6
(2002)Doi:10.1002/cssc.201501093
(2015)Doi:10.1007/s10600-014-0827-x
(2014)Doi:10.1021/ja00889a027
(1963)