Carreaux et al.
SCHEME 2
alkyl boronic moieties, we decided to offer a rationaliza-
tion of this phenomenon.
Aside from the preparative utility of this reaction, the
nature of its mechanism is very interesting in a wider
context. Thus, the well-known dimerization of acrolein
3 has been used as a textbook case to show the impor-
tance of second-order orbital interactions.6 A recent paper
from Quadrelli et al.7 on the 2 3a f 4a transformation
has shown the importance of nonclassical (bridging) and
classical (Woodward-Hoffmann8) secondary orbital in-
teractions in the formation of the 2-substituted dimer 4a
as indicated in Scheme 1. In addition, a detailed study
by Evanseck and Kong9 on the Diels-Alder reaction
between acrolein and butadiene (Scheme 1, 3a + 3b f
4b) has shown that the nonclassical [4+3] term is the
most important one among the possible secondary orbital
interactions, and that this effect is increased when
solvent effects are taken into account.
The different regioselectivities observed in the reaction
between dienes 5 and vinyl boranes 6 (Scheme 1) have
been widely studied by Singleton.10 This author and,
recently, Goodman et al.11 have found that, although the
[4+2] cycloadducts are formed in the 5 + 6 f 7 trans-
formation, the geometries of the corresponding transition
structures are compatible with [4+3] interactions, favor-
ing the formation of the meta-cycloadducts 7 indicated
in Scheme 1. In this latter case these nonclassical
(bridging) secondary orbital interactions involving B‚‚‚C
terms have been detected computationally. However, the
reasons underlying the final exclusive formation of [4+2]
cycloadducts are not clear at present. In addition, the
validity (or the necessity) of the secondary orbital inter-
actions model has been questioned12 and direct measure-
ment of the magnitude of this kind of interaction is
required9,13 to assess the validity of the model.
Com p u ta tion a l Meth od s
All calculations included in this paper have been
obtained by using the GAUSSIAN 9814 series of pro-
grams, with the standard 6-31+G* basis set.15 To include
electron correlation at a reasonable computational cost,
Density Functional Theory (DFT)16 has been used. In this
study, these calculations have been carried out by means
of the three-parameter functional developed by Becke,17
which is usually denoted as B3LYP. This method has
been shown to produce reliable results in pericyclic
rections and, in particular, in [4+2] cycloadditions.18 Zero-
point vibrational energies (ZPVEs) were computed at the
B3LYP/6-31+G* level and were not scaled. All transition
structures and minima were fully characterized by
harmonic analysis. For each located transition structure,
only one imaginary frequency was obtained in the diago-
nalized Hessian matrix, and the corresponding vibration
was found to be associated with nuclear motion along the
reaction coordinate under study. For the parent model
reaction 1a + 1a f p-2a (see Scheme 2), intrinsic
reaction coordinate (IRC)19 calculations were performed
Within this context, the present work aims to explore
the scope of the 1 + 1 f 2 transformation and subsequent
reactions to yield cyclic structures not readily accessible
by conventional hydroboration techniques. In addition,
in this paper we report for the first time a computational
study of the mechanism of this particular transformation,
as well as a direct evaluation of the nonclassical second-
ary orbital interactions. Finally, an explanation for the
evolution from [4+3] to [4+2] structures is presented.
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J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
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