Page 1028 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1012
Computed Transition Structure Energy (kcal/mol)
CHAPTER 11
5-exo(chair) 5-exo(boat) 6-endo(chair) 6-endo(boat) endo:exo ratio
Free Radical Reactions
A 6.4 8.1 9.1 11.6 1:99
B 7.0 8.7 9.6 12.2 1:99
C 9.1 10.3 8.4 10.7 75:25
D 6.5 8.1 9.8 12.5 1:99
E 13.3 12.6 10.0 16.5 98:2
F 15.9 14.4 12.6 19.1 95:5
G 15.8 14.7 8.6 15.2 > 99 1
H 12.7 12.1 10.9 17.2 84:16

Competition between 5-exo and 6-endo has also been examined for the 2-thia and
2-sulfonyl analogs of the 5-hexenyl radicals. 115 As in the case of the parent radical,
a 5-methyl substituent favors the 6-endo mode and this is reinforced in the case of
the sulfonyl substituent, where the electrophilic radical prefers the more electron-rich
alkene position.

R R CH 3 R R
Bu SnH
3
CH 3
X SePh + + X
80° C reduction X
R = H, CH 3 5-exo 6-endo
X = S, SO 2

R X Product composition
Reduction 5-exo 6-endo
H S 17 1 70 1 12 8
H SO 2 3 8 73 1 23 1
S 38 6 7 1 54 3
CH 3
3 9 2 5 93 6
CH 3 SO 2

The 4-pentenyl radical can undergo 4-exo or 5-endo cyclization. UB3LYP/6-31G*
calculations find a preference of 1.8 kcal/mol for the 5-endo TS. The angle to approach
116
to the double bond is found to be about 89 .
16.3
89°

18.1

115 E. W. Della and S. D. Graney, Org. Lett., 4, 4065 (2002); E. W. Della and S. D. Graney, J. Org. Chem.,
69, 3824 (2004).
116
P. S. Engel, S. L. He, and W. B. Smith, J. Am. Chem. Soc., 119, 6059 (1997); C. Chatgilialoglu,
C. Ferreri, M. Guerra, V. Timokhin, G. Froudakis, and T. Gimisis, J. Am. Chem. Soc., 124, 10765
(2002).