Page 1046 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1030 atom
abstraction
X CCH CHR
2
3
CHAPTER 11
X
Free Radical Reactions
2
X 3 CCH CHR
.
R R
addition
to alkene X 3 CCH CH(CH CH) CH CHR telomer
2 .
2
2
n
The addition of bromotrichloromethane to cyclohexene gives a nearly 1:1 mixture
of the two possible stereoisomers. 160
Br
.
CCl 3 Br
. BrCCl 3 CCl
CCl 3 + CCl 3 CCl 3 + 3
. trans 55% cis 45%
This result indicates that the initially added trichloromethyl group has little influence
on the stereochemistry of the subsequent bromine atom abstraction. The inter-
mediate 2-(trichloromethyl)cyclohexyl radical presumably relaxes to the equatorial
conformation faster than the occurrence of bromine atom abstraction, and there
2 3
is little preference for axial or equatorial approach. In contrast, with -
octahydronaphthalene, the addition is exclusively trans-diaxial.
Br

BrCCl 3 +
CCl 3

The trans-fused decalin system is conformationally rigid and the stereochemistry of
the product indicates that the initial addition of the trichloromethyl radical is from an
axial direction. This is expected on stereoelectronic grounds because the radical should
initially interact with the orbital. The axial trichloromethyl group then shields
∗
the adjacent radical position and directs the bromine abstraction in the trans sense.
Addition of bromotrichloromethane to norbornene is also anti. This is again the result
of steric shielding by the trichloromethyl group, which causes the bromine atom to be
abstracted from the endo face of the intermediate radical.

CCl 3
hv
+ BrCCl 3
Br
The addition of bromotrichloromethane to terminal alkenes occurs in reasonable
yield in THF without the addition of a specific initiator. 161 Bromotrichloromethane
additions are effectively catalyzed by Fe(0), which acts as an initiator by one-electron
reduction. 162

160 J. G. Traynham, A. G. Lane, and N. S. Bhacca, J. Org. Chem., 34, 1302 (1969).
161 M. Heintz, G. L. Ny, and J. Y. Nedelec, Tetrahedron Lett., 25, 5767 (1984).
162
F. Bellesia, L. Forti, F. Ghelfi, and U. M. Pagnoni, Synth. Commun., 27, 961 (1997).