Finally, a representation of the activation barriers (D
E
#
) with re-
spect to the electrophilicity values of the corresponding substituted
ethylenes is shown in Fig. 4. There is a good correlation between
the increase of the electrophilicity of the ethylene derivative and
the reduction of the activation barrier associated with these DA
reactions (
R
2
= 0.92). Therefore, the proposed classification can be
related to the electrophilic character of the reagents, expressed as
D
w. In the case of the series of DA reactions of Cp examined here,
the classification is related to the electrophilicity of the substituted
ethylene. The subgroup of
N-DA reactions includes molecules
with an electrophilicity lower than 1.2 eV while the subgroup of
P-DA reactions includes molecules with an electrophilicity be-
tween 1.7 and 5.0 eV. Finally, the subgroup of
I-DA reactions
includes molecules with an electrophilicity higher than 5.0 eV.
Note that the subgroups of
P- and
I-DA reactions include
molecules classified as moderate and strong electrophiles within
the electrophilicity scale.
13
Fig. 4
Plot of the activation barriers (D
E
#
)
vs. the electrophilicity index
(
w) of the ethylenes,
R
2
= 0.92, for the Diels–Alder reactions of Cp (
4)
with the substituted ethylene series.
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