PNNL

Abstract

Arrhenius rate expressions were determined for the abstraction of bromine atom from 2-phenethyl bromide by tri-n-butylstannyl radical (Bu3Sn¥) in benzene using transient absorption spectroscopy, (log(kabs,Br/M-1 s-1) ) (9.21 ( 0.20) - (2.23 ( 0.28)/õ, õ ) 2.3RT kcal/mol, errors are 2ó) and for the abstraction of sulfur atom from propylene sulfide to form propylene, (log(ks/M-1 s-1) ) (8.75 (0.91) - (2.35 (1.33)/õ). Rate constants for reactions of organic bromides, RBr, with Bu3Sn¥ were found to vary as R ) benzyl (15.6) > thiiranylmethyl (6.2) > oxiranylmethyl (3.1) > cyclopropylmethyl (1.3) > 2-phenethyl (1.0), with kabs,Br ) 6.8 * 107 M-1 s-1 at 353 K for 2-phenethyl bromide.Bromine abstraction from R bromomethylthiirane is about 7-fold faster than sulfur atom abstraction and is comparable to the reactivity of a secondary alkyl bromide. The potential surface for the vinylthiomethyl f allylthiyl radical rearrangement at UB3LYP/6-31G(d) and UB3LYP/6-311+G- (2d,2p) levels of theory suggests that the thiiranylmethyl radical is produced about 9 kcal/mol above the allylthiyl radical on the rearrangement surface, consistent with the observed enhancement of the Br atom abstraction from the thiirane and with synchronous C-S bond scission of the thiirane ring. The selectivities reported in this work for S vs Cl and Br abstraction provide applications for radical-based synthesis and new competition basis rate expressions for trialkylstannyl radicals.