Page 1000 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

984 expected to be in the direction favoring axial attack. 52 Structural evidence suggests
that the cyclohexyl radical is somewhat pyramidal with an equatorial hydrogen. 53
CHAPTER 11
Equatorial attack leading to trans product causes the hydrogen at the radical site to
Free Radical Reactions become eclipsed with the two neighboring equatorial hydrogens. Axial attack does not
suffer from this strain, since the hydrogen at the radical site moves away from the
equatorial hydrogens toward the staggered conformation that is present in the chair
conformation of the ring.

H
.
R R X
X-Y
. X
H
R R

The inversion of the cyclohexyl radical can occur by a conformational process.
This is expected to have a higher barrier than the radical inversion, since it involves
bond rotations very similar to the ring inversion in cyclohexane. An E of 5.6 kcal/mol
a
54
has been measured for the cyclohexyl radical. A measurement of the rate of inversion
8

of a tetrahydropyranyl radical (k = 5 7×10 s −1 at 22 C) has been reported. 55
.
O Ph
. CH 2 O CH Ph
2
It can be concluded from these data that radical inversion is also fast in cyclic systems.
Another approach to obtaining information about the geometric requirements of
free radicals has been to examine bridgehead systems. Recall that small bicyclic rings
strongly resist formation of carbocations at bridgehead centers because the skeletal
geometry prevents attainment of the preferred planar geometry. There is significant
rate retardation for reactions in which the norbornyl radical is generated in a rate-
determining step. 56 Typically, such reactions proceed 500 to 1000 times slower than
the corresponding reaction generating the t-butyl radical. This is a much smaller rate
retardation than the 10 −14 found in S 1 solvolysis (see p. 435). Rate retardation is
N
still smaller for less strained bicyclic systems. The decarbonylation of less strained
bridgehead aldehydes was found to proceed without special difficulty. 57

[(CH ) CO] 2
3 3
.
C . C H
H O + C O
O
52 W. Damm, B. Giese, J. Hartung, T. Hasskerl, K. N. Houk, O. Huter, and H. Zipse, J. Am. Chem. Soc.,
114, 4067 (1992).
53 J. E. Freitas, H. J. Wang, A. B. Ticknor, and M. A. El-Sayed, Chem. Phys. Lett., 183, 165 (1991);
A. Hudson, H. A. Hussain, and J. N. Murrell, J. Chem. Soc., A, 2336 (1968).
54
B. P. Roberts and A. J. Steel, J. Chem. Soc., Perkin Trans. 2, 2025 (1992).
55 A. J. Buckmelter, A. I. Kim, and S. D. Rychnovsky, J. Am. Chem. Soc., 122, 9386 (2000).
56 A. Oberlinner and C. Rüchardt, Tetrahedron Lett., 4685 (1969); L. B. Humphrey, B. Hodgson, and
R. E. Pincock, Can. J. Chem., 46, 3099 (1968); D. E. Applequist and L. Kaplan, J. Am. Chem. Soc.,
87, 2194 (1965).
57
W. v. E. Doering, M. Farber, M. Sprecher, and K. B. Wiberg, J. Am. Chem. Soc., 74, 3000 (1952).