Page 1026 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1010 S CO 2 CH 3 CH
CO 2 CH 3 3 CO CH 3
2
O Bu SnH
CHAPTER 11 CH 2 N 3 +
O AIBN
Free Radical Reactions
85% 15%

The relatively low rate and high activation energy noted for Entry 37 of Table 11.3
also reflects a stereoelectronic effect. The preference for delocalization at the radical
center requires coplanarity of the substituents at the radical site, which results in poor
alignment. In view of the restrictions on the mode of approach of the radical to the
double bond, significant strain develops in the TS and requires rotation of the benzylic
methylene group out of its preferred coplanar alignment. 110

poor orbital alignment optimal alignment
in planar conformation disrupts benzyl
conjugation

Several computational studies have explored the cyclization of the 5-hexenyl
radical. CBS-RAD(B3LYP) calculations provided thermochemical and kinetic param-
eters that are in good agreement with experiment. 111 Similar results were obtained with
UB3LYP/6-31G(d
calculations. 112

5-exo 6-endo
G −13 7 kcal/mol −16 5 kcal/mol
G ‡ 9.3 kcal/mol 12.0 kcal/mol
6.3 kcal/mol 8.7 kcal/mol
E a
7 −1
5 −1
1 2×10 s 2 7×10 s
k gas
6 −1
5 −1
8 2×10 s 1 6×10 s
k benzene
Cyclization of 5-hexenyl radicals has been compared computationally
(BLYP/31+G**) with 2-oxo-5-hexenyl radicals. 113 Several methyl-substituted analogs

110 J. A. Franz, N. K. Suleman, and M. S. Alnajjar, J. Org. Chem., 51, 19 (1986).
111
B. J. Maxwell, B. J. Smith, and J. Tsanaktsidis, J. Chem. Soc., Perkin Trans. 2, 425 (2000).
112 B. S. Jursic, Theochem, 492, 285 (1999).
113
A. G. Leach, R. Wang, G. E. Wohlhieter, S. I. Khan, M. E. Jung, and K. N. Houk, J. Am. Chem. Soc.,
125, 4271 (2003).