Page 940 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

924 advanced MO and DFT calculations support the idea of an aromatic TS. 243 The TS can
range in character from a 1,4-cyclohexadiyl diradical to two nearly independent allyl
CHAPTER 10 244
radicals, depending on whether bond making or bond breaking is more advanced.
Concerted Pericyclic The electrons remain paired in either case, however, and the two representations are
Reactions
best considered to be resonance structures. The energy surface in the transition region
seems to be quite flat; that is, there does not seem to be a strong difference in the
energy over the range from 1.64 to 2.19 Å. 245
.
.

Substituent effects provide other insights into the nature of the TS for the Cope
rearrangement. Conjugated substituents at C(2), C(3), C(4), or C(5) accelerate the
reaction. 246 Donor substituents at C(2) and C(3) have an accelerating effect. 247 The net
effect on the reaction rate of any substituent is determined by the relative stabilization
of the TS and ground state. 248 The effect of substituents on the stabilization of the
TS can be analyzed by considering their effect on two interacting allyl systems. We
consider the case of phenyl substituents in detail.
X X X X
2
Y 1 Y Y Y
3 .
4 6 .
5
‡
As shown in Table 10.7, phenyl substituents at positions 2 and 3 reduce the H .On
the other hand, a 1-substituent, which is conjugated in the reactant but not the product,
‡
increases the H .
The first step in interpreting these substituent effects is to recognize how they
affect the reactant and product energy. Substituents that are conjugated, such as
cyano and phenyl, are more stabilizing on a double bond than at a saturated carbon.
For example, the rearrangement of 3,4-diphenyl-1,5-hexadiene to 1,6-diphenyl-1,5-
hexadiene is exothermic by 10.2 kcal/mol, indicating about 5 kcal/mol of stabilization
at each conjugated double bond.

Ph Ph

Ph Ph
243
D. A. Hrovat, W. T. Borden, R. L. Vance, N. G. Rondan, K. N. Houk, and K. Morokuma, J. Am. Chem.
Soc., 112, 2018 (1990); D. A. Hrovat, K Morokuma, and W. T. Borden, J. Am. Chem. Soc., 116, 1072
(1994); O. Wiest, K. A. Black and K. N. Houk, J. Am. Chem. Soc., 116, 10336 (1994); M. D. Davidson,
I. H. Hillier, and M. A.Vincent, Chem. Phys. Lett., 246, 536 (1995); S. Yamada, S. Okumoto, and
T. Hayashi, J. Org. Chem., 61, 6218 (1996); W. T. Borden and E. R. Davidson, Acc. Chem. Res., 29,
57 (1995); P. M. Kozlowski, M. Dupuis, and E. R. Davidson, J. Am. Chem. Soc., 117, 774 (1995);
K. N. Houk, B. R. Beno, M. Nendel, K. Block, H.-Y. Yoo, S. Wilsey, and J. K. Lee, Theochem, 398,
169 (1997); E. R. Davidson, Chem. Phys. Lett., 284, 301 (1998).
244 J. J. Gajewski and N. D. Conrad, J. Am. Chem. Soc., 100, 6268, 6269 (1978); J. J. Gajewski and
K. E. Gilbert, J. Org. Chem., 49, 11 (1984).
245
P. M. Kozlowski, M. Dupuis, and E. R. Davidson, J. Am. Chem. Soc., 117, 774 (1995); E. R. Davidson,
J. Phys. Chem., 100, 6161 (1996).
246
M. J. S. Dewar and L. E. Wade, J. Am. Chem. Soc., 95, 290 (1972); J. Am. Chem. Soc., 99, 4417
(1977); R. Wehrli, H. Schmid, D. E. Bellus, and H. J. Hansen, Helv. Chim. Acta, 60, 1325 (1977).
247 M. Dollinger, W. Henning, and W. Kirmse, Chem. Ber., 115, 2309 (1982).
248
For analysis of substituent effects in molecular orbital terminology, see B. K. Carpenter, Tetrahedron,
34, 1877 (1978); F. Delbecq and N. T. Anh, Nouv. J. Chim., 7, 505 (1983).