Page 1001 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Conclusions about radical structure can also be drawn from analysis of ESR 985
spectra. The ESR spectra of the bridgehead radicals A and B are consistent with
pyramidal geometry at the bridgehead carbon atoms. 58 SECTION 11.1
Generation and
Characterization of Free
Radicals
.
.
A B

The ESR spectra of a number of bridgehead radicals have been determined and the
hyperfine couplings measured (see Section 11.1.3). Both the H and 13 C couplings

are sensitive to the pyramidal geometry of the radical. 59 The reactivity of bridgehead
radicals increases with increased pyramidal character. 60

Radical H 13 C a
Adamantyl 6 58 132 113 6
Bicyclo[2.2.2]octyl 6 64 143 113 2
Bicyclo[2.2.1]heptyl 2 35 151 112 9
Bicyclo[2.1.1]hexyl 0 174 111 9
Bicyclo[1.1.1]pentyl −1 2 223 110 3
a. = the C−C−C bond angle at the bridgedhead radical.

The broad conclusion of all these studies is that alkyl radicals except methyl are
pyramidal, but the barrier to inversion is low. Radicals also are able to tolerate some
geometric distortion associated with strained ring systems.
The allyl radical would be expected to be planar in order to maximize delocal-
ization. Structure parameters have been obtained from ESR, IR, and electron diffraction

measurements and confirm that the radical is planar. 61 The vinyl radical, CH = CH ,
2
is found by both experiment and theory to be bent with a C−C−H bond angle of
62
about 137 . Substituents affect the preferred geometry of vinyl radicals. Conjugation
with -acceptor substituents favors a linear geometry, whereas -donor substituents
favor a bent geometry. 63 For -donors the barriers for isomerization are in the order
CH 3 1
< OH 13 3
< F 19 5
kcal/mol, according to BLYP/6-311G(2d,2p
calcu-
3
lations. Although these barriers have not been measured experimentally, reaction
stereoselectivity is in agreement with the results. For the -acceptor substituents, the
preferred geometry is one in which the substituent is aligned with the singly occupied
p orbital, not the bond.

58 P. J. Krusic, T. A. Rettig, and P. v. R. Schleyer, J. Am. Chem. Soc., 94, 995 (1972).
59 C. J. Rhodes, J. C. Walton, and E. W. Della, J. Chem. Soc., Perkin Trans. 2, 2125 (1993); G. T. Binmore,
J. C. Walton, W. Adcock, C. I. Clark, and A. R. Krstic, Mag. Resonance Chem., 33, Supplement S53
(1995).
60
F. Recupero, A. Bravo, H. R. Bjorsvik, F. Fontana, F. Minisci, and M. Piredda, J. Chem. Soc., Perkin
Trans. 2, 2399 (1997); K. P. Dockery and W. G. Bentrude, J. Am. Chem. Soc., 119, 1388 (1997).
61 R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 39, 2147 (1963); A. K. Maltsev, V. A. Korolev,
and O. M. Nefedov, Izv. Akad. Nauk SSSR, Ser. Khim., 555 (1984); E. Vajda, J. Tremmel, B. Rozandai,
I. Hargittai, A. K. Maltsev, N. D. Kagramanov, and O. M. Nefedov, J. Am. Chem. Soc., 108, 4352
(1986).
62 J. H. Wang, H.-C. Chang, and Y.-T. Chen, Chem. Phys., 206, 43 (1996).
63
C. Galli, A. Guarnieri, H. Koch, P. Mencarelli, and Z. Rappoport, J.Org. Chem., 62, 4072 (1997).