Page 1030 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1014

CHAPTER 11
Free Radical Reactions 106.6

158.3
kcal/mol (d)
(c)

(b)
16.3
(a) 13.5
13.2
8.7 8.7
4.0

–8.6 –8.5
–10.5
16.5
Fig. 11.10. Comparison of energies of (a) 8-endo-s-cis, (b) 8-endo-s-trans, (c) 7-exo-s-cis,
and (d) 7-exo-s-trans transition structures. Reproduced from J. Am. Chem. Soc., 120, 7469
(1998), by permission of the American Chemical Society.

6
The rate of decarboxylation of aroyloxy radicals is about 10 s −1 near room
temperature. 119 Decarboxylation of alkanoyloxy radicals is even faster. Thus only very
rapid reactions can compete with decarboxylation. Hydrogen abstraction from very
reactive hydrogen atom donors, such as triethylsilane, can compete with decarboxy-
lation at moderate temperatures.
These radical stability effects can be observed in the rates of formation of radicals
as well as their lifetimes. It has already been indicated that radical structure and
stability determines the temperature at which azo compounds undergo decomposition
with elimination of nitrogen (see Section 11.1.4). Similar trends have been estab-
lished in other radical-forming reactions. Rates of thermal decomposition of t-butyl
peroxyesters, for example, vary over a wide range, depending on the structure of the
carbonyl substituent. 120 These data clearly indicate that the bonding changes involved
in the rate-determining step are not localized in the O−O bond. Radical character must
also be developing at the alkyl group by partial cleavage of the alkyl-carbonyl bond.

O O
)
R C O O C(CH 3 3 R C O O C(CH 3 3 R + CO 2 + OC(CH )
)
3 3

119 J. Chateauneuf, J. Lusztyk, and K. U. Ingold, J. Am. Chem. Soc., 110, 2886 (1988); H. Misawa,
K. Sawabe, S. Takahara, H. Sakuragi, and K. Tokumaru, Chem. Lett., 357 (1988).
120
P. D. Bartlett and R. R. Hiatt, J. Am. Chem. Soc., 80, 1398 (1958).