Page 867 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

10.2.4. Computational Characterization of Diels-Alder Transition Structures 851

The idea of complementary electronic interactions between the diene and SECTION 10.2
dienophile provides a reliable qualitative guide to the regio- and stereoselectivity of The Diels-Alder Reaction
the D-A reaction. Structural and substituent effects can be explored in more detail
by computational analysis of TS structure and energy. Comparison of the relative
energy of competing TSs allows prediction and interpretation of the course of the
reaction. Ab initio HF calculations often can be relied on to give the correct order of
isomeric TS structures. Accurate E estimates require a fairly high-level treatment of
a
electron correlation. Reliable results have been achieved with B3LYP/6-31G*, MP3/6-
38
31G*, and CCSD(T)/6-31G* computations. These calculations permit prediction and
interpretation of relative reactivity and regio- and stereoselectivity by comparison of
competing TSs. There are other aspects of TS character that can be explored, including
the degree of asynchronicity in bond formation and the nature of the electronic reorga-
nization within the TS. Kinetic isotope effects can be calculated from the TS and
provide a means of validation of TS characteristics by comparison with experimental
results. 39
A range of quantum chemical computations were applied to Diels-Alder reactions
as the methods were developed. The consensus that emerged is illustrated by typical
recent studies. 25 40 For symmetrical dienes and dienophiles without strong EWG
substituents, the reaction is synchronous, that is the degree of bond making of the

C(1)−C(1 ) and C(4)−C(2 ) bonds is the same. As we will see shortly, this does not

always seem to be the case for strongly electrophilic dienophiles, even when they are
symmetric. The TS displays aromaticity, as indicated by the computed NICS value
(see Section 8.1.3), 41 which implies that there is enhanced delocalization of the six
electrons that participate in bonding changes. Fradera and co-workers have used the
AIM localization and delocalization parameters
and to investigate the electron
42
distribution in the TS for ethene/butadiene cycloaddition. At the HF/6-31G* level, the
delocalization indices are about 0.4 for all the reacting bonds (plus 1.0 for the residual
bonds). There is stronger delocalization between the para than the meta positions.
43
Both of these parameters are very similar to those found for benzene. These similar-
ities support the idea that the electronic distribution in the TS for the D-A reaction
resembles that of the system of benzene, an idea that goes back to the 1930s. 44

38 T. C. Dinadayalane, R. Vijaya, A. Smitha, and G. N. Sastry, J. Phys. Chem. A, 106, 1627 (2002);
B. R. Beno, S. Wilsey, and K. N. Houk, J. Am. Chem. Soc., 121, 4816 (1999).
39
B. R. Beno, K. N. Houk, and D. A. Singleton, J. Am. Chem. Soc., 118, 9984 (1996); E. Goldstein,
B. Beno, and K. N. Houk, J. Am. Chem. Soc., 118, 6036 (1996).
40 S. Sakai, J. Phys. Chem. A, 104, 922 (2000); R. D. J. Froese, J. M. Coxon, S. C. West, and K. Morokuma,
J. Org. Chem., 62, 6991 (1997).
41
H. Jiao and P. v. R. Schleyer, J. Phys. Org. Chem., 11, 655 (1998).
42
J. Poater, M. Sola, M. Duran, and X. Fradera, J. Phys. Chem. A, 105, 2052 (2001).
43 X. Fradera, M. A. Austen, and R. F. W. Bader, J. Phys. Chem. A, 103, 304 (1999).
44
M. G. Evans, Trans. Faraday Soc., 35, 824 (1939).