Page 886 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

870 60 1
50
CHAPTER 10
40 2 2
Concerted Pericyclic 1
Reactions 30
1
4
20 4
3 3
10
0
–10 1 2
–20 2
–30
–40 3 3
4 4
–50
cis trans
∗
Fig. 10.12. Activation and reaction energies (B3LYP/6-31G ) in kcal/mol for
intramolecular Diels-Alder reactions of CH 2 =CH CH 2 n CH =CHCH =CH 2 for
n = 1 to 4. Adapted from Theochem, 618, 201 (2002).

In several IMDA reactions involving substituted systems, the direction of the
asynchronicity appears to be controlled by steric and torsional, rather than electronic,
factors. Often the internal bond formation is more advanced than the peripheral bond,
regardless of the location and electronic characteristics of the substituents. 105 This is
presumably due to the proximity (entropic) factor and is in contrast to intermolecular
D-A reactions, where electronic effects are dominant.

internal
bond

peripheral
bond

Lewis acid catalysis usually substantially improves the stereoselectivity of IMDA
reactions, just as it does in intermolecular cases. For example, the thermal cyclization
of 1 at 160 C gives a 50:50 mixture of two stereoisomers, but the use of C H AlCl

2
5 2
as a catalyst permits the reaction to proceed at room temperature, and endo addition
is favored by 7:1. 106

105 C. I. Turner, R. M. Williamson, M. N. Paddon-Row, and M. S. Sherburn, J. Org. Chem., 66, 3963
(2001); T. N. Cayzer, L. S. M. Wong, P. Turner, M. N. Paddon-Row, and M. S. Sherburn, Chem. Eur.
J., 8, 739 (2002).
106
W. R. Roush and H. R. Gillis, J. Org. Chem., 47, 4825 (1982).