Page 1027 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

were included in the study. There is a change to a preference from 5-exo to 6-endo 1011
with the introduction of the 2-oxo group.
SECTION 11.2
O Characteristics of
Reactions Involving
R 1 R 1 Radical Intermediates
R 6 R 6
R 5 R 5
R 1 R 5 R 6 R 1 R 5 R 6
A H H H E H H H
H H F CH H H
B CH 3 3
C H CH 3 H G H CH 3 H
D H H CH 3 H H H CH 3

Experimentally, it is found that the hexenyl radicals prefer the 5-exo route, whereas
the -keto radicals prefer the 6-endo path. This effect is mirrored in the calculations,
which find the 5-exo route strongly favored in the 5-hexenyl system (except for the
5-methyl derivative C), whereas for the -keto radicals the 6-endo TS is preferred,
although again the steric effect of a 6-methyl group somewhat disfavors this mode.
Figure 11.9 shows various TSs. The conjugation present in the -keto radicals imparts
a planarity to the radical that favors the endo structure. 114

2.19 Å 2.20 Å

6-endo chair 6-endo boat

2.15 Å
2.16 Å
2.15 Å

5-exo chair 5-exo boat 6-exo twist boat
Fig. 11.9. Structures for various conformations of the radical cyclization transition structures
for 2-oxo-5-hexenyl radical. Reproduced from J. Am. Chem. Soc., 125, 4271 (2003), by
permission of the American Chemical Society.

114
J. L. Broeker and K. N. Houk, J. Org. Chem., 56, 3651 (1991).