Page 872 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

856
2 3 1.426
CHAPTER 10 2 3
1.389 1.363
Concerted Pericyclic 1 4
Reactions 1 4
2.000 2.668
2.09
0 2.60 1.375
0 1.486 1.400
6
5 1.291
1.449
0
0
Fig. 10.9. Asynchronous transition structures for Diels-Alder reactions of butadiene
with maleic acid and 1,2,4-triazoline using B3LYP/6-31G* calculations. Reproduced
from Tetrahedron, 57, 5149 (2001) and J. Am. Chem. Soc., 120, 12303 (1998), by
permission of Elsevier and the American Chemical Society, respectively.

The asynchronous TS results from an increase in the extent of charge transfer, leading
to partial ionic character in the TS.
δ +
1
δ –
EWG
2 1’
3
2’ EWG
4
There seems to be another element of asynchronicity associated with bond
formation in D-A reactions. The formation of the new double bond and the lengthening
of the reacting dienophile bond seem to run ahead of the formation of the new
bonds. For example, in the MP4SDTQ/6-31G* TS for the reaction of butadiene and
ethene, the new bonds are only 22% formed at the TS. The same picture emerges
by following the transformations of the orbitals during the course of the reaction. 56
The transfer of -electronic characteristics from the dienophile bond to the product
bond seems to occur ahead of the reorganization of electrons to form the two
new bonds.

1.37 2.2
1.38
1.40

Visual models, additional information and exercises on the Diels-Alder
Reaction can be found in the Digital Resource available at: Springer.com/carey-
sundberg.

A wide variety of diene substituents were surveyed using B3LYP/6-31G(d,p)
calculations to determine the effect on the E for D-A addition with ethene. 57 There
a
was stabilization of the TS by EWG substituents, which was accompanied by a small
positive charge (NPA) on ethene. This indicates that the electronic interaction involves
56 C. Spino, M. Pesant, and Y. Dory, Angew. Chem. Int. Ed. Engl., 37, 3262 (1998).
57
R. Robiette, J. Marchand-Brynaert, and D. Peeters, J. Org. Chem., 67, 6823 (2002).