jo982028n_si_001.pdf (1.37 MB)
[4 + 2] Dimerization and Cycloaddition Reactions of α,β-Unsaturated Selenoaldehydes and Selenoketones
journal contribution
posted on 1999-02-17, 00:00 authored by Guang Ming Li, Shuqiang Niu, Masahito Segi, Ralph A. Zingaro, Hiroaki Yamamoto, Kentaro Watanabe, Tadashi Nakajima, Michael B. HallThe reactions of α,β-unsaturated aldehydes and ketones with bis(dimethylaluminum) selenide, (Me2Al)2Se, yield the corresponding α,β-unsaturated selenoaldehydes and selenoketones. They are too
unstable to be isolated in the monomeric form, but they undergo regioselective [4 + 2] dimerization
via a “head-to-head” oriented transition state to afford diselenin derivatives (trans and cis isomers).
Theoretical calculations at the density functional theory level show that this selectivity occurs
because the “head-to-head” dimerization is thermodynamically favored over the “head-to-tail” by
about 14 kcal/mol. Both dimerization reactions have low energy barriers: 1.5 and 2.8 kcal/mol for
the former and 0.9 and 1.3 kcal/mol for the latter. In the presence of norbornadiene, these compounds
function as 4π heterodienes (CC−CSe) to give the respective cycloadduct products. On the other
hand, they act as 2π dienophiles (CSe) in the reactions with cyclopentadiene except for
selenoacrolein which serves as a 4π diene and only one CC bond (2π) in cyclopentadiene is involved
in the reaction. Theoretical calculations have been carried out in order to better understand these
observations.