Page 868 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

852 1 1.39
1.438 0.397
2 1'
CHAPTER 10 1.347 1.455
3 2'
Concerted Pericyclic
Reactions 4
meta para meta para
1,3 0.073 1,4 0.103 1,3 0.07 1,4 0.10
2,1' 0.050 2,2' 0.086
1,2' 0.042

The TS of D-A reactions can also be characterized with respect to synchronicity.
If both new bonds are formed to the same extent the reaction is synchronous, but if
they differ it is asynchronous. Synchronicity has been numerically defined in terms of
Wiberg bond order indices. 45

n

B − B / B av
i
av
1 i=1
S = 1− (10.1)
y
2n−2
where n is the number of bonds directly involved in the reaction, B is the relative
i
variation in the B at the TS. The terms B and B av are defined as follows:
i
i
B i TS −B i R
B = (10.2)
i
P
B −B R
i i
n
B = n −1 B i (10.3)
av
i=1
Computations have also been applied to the analysis of exo:endo ratios. The
computed differences in energies of the exo and endo TS are often small and are
subject to adjustments when solution models are used. 46 Cyclopentadiene has been a
common subject, since there is more experimental data for this compound than for
any other. MP3/6-31G*-level computations were used to compare the exo and endo
TS E for the reactions with acrylonitrile and but-2-en-3-one (methyl vinyl ketone),
a
47
and ZPE and thermal corrections were included in the calculations Good qualitative
agreement was achieved with the experimental results, which is little stereoselectivity
for acrylonitrile and endo stereoselectivity for but-3-en-2-one.
Acrylonitrile But-3-en-2-one

G ‡ G ‡
E a E a
exo 18.49 31.72 16.16 29.86
endo 18.53 31.69 15.92 29.42
Difference −0 04 +0 03 +0 24 +0 44

45
A. Moyano, M. A. Pericas, and E. Valenti, J. Org. Chem., 54, 573 (1989); B. Lecea, A. Arrieta, G. Roa,
F. P. Ugalde, and F. P. Cossio, J. Am. Chem. Soc., 116, 9613 (1994).
46 M. F. Ruiz-Lopez, X. Assfeld, J. I. Garcia, J. A. Mayoral, and L. Salvatella, J. Am. Chem. Soc., 115,
8780 (1993).
47
W. L. Jorgensen, D. Lim, and J. F. Blake, J. Am. Chem. Soc., 115, 2936 (1993).