III.1.2
Geometries.
The geometries of the TSs involved in
these DA reactions are given in Fig. 2. Only the DA reaction
between Cp and ethylene (2), TS2, presents a synchronous bond-
formation process. The synchronicity of the process is broken by
the unsymmetric substitution of ethylene, with the shorter lengths
corresponding to the bond-formation at the
b-conjugated position
of the electrophilically activated ethylenes.
The extent of the synchronicity of the bond-formation can be
measured as the difference between the lengths of the two
s bonds
Fig. 2
Geometries of the transition structures involved in the Diels–Alder
reactions of Cp (4) and the substituted ethylene series. The distances are
given in A
˚ .
formed in the reaction, i.e., Dd
= dist1 - dist2. The asynchronicity
ranges from Dd
= 0.0 for the DA reaction of Cp (4) with ethylene
(2) up to Dd
= 1.17 for the DA reaction of Cp (4) with the iminium
cation 19. Although there is not a strict linear relationship between
asynchronicity and the polar character of the reaction, an increase
in the polar character of the reaction seems to be accompanied by
an increase in its asynchronicity.
At the highly asynchronous TSs, e.g. Dd
> 0.5, the analysis
of the IRC from the TSs to the cycloadducts indicates that
such cycloadditions have a two-stage mechanism.
10
At the first
stage of the reaction, a C–C bond is formed through a nucle-
ophilic/electrophilic interaction. These TSs are associated with a
two-center interaction. After the first C–C bond is formed, the
formation of the second C–C bond begins, thus initiating the
second stage of the reaction. Stabilizing factors for a feasible
zwitterionic intermediate, including charge delocalization
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