Web Release Date: January 16,
Absolute Rate Constants for Reactions of Tributylstannyl Radicals
with Bromoalkanes, Episulfides, and 
-Halomethyl-Episulfides,
-Cyclopropanes, and -Oxiranes: New Rate Expressions for Sulfur
and Bromine Atom Abstraction
and
Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, and Department of Chemistry, North Carolina A&T, 1601 East Market Street, Greensboro, North Carolina 27411
Received October 6, 2003
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)/
, = 2.3RT kcal/mol, errors are 2
) 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)/). 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 -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 
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.
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