[4 + 2] Dimerization and Cycloaddition Reactions of α,β-Unsaturated Selenoaldehydes and Selenoketones

The 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 (CC−CSe) to give the respective cycloadduct products. On the other hand, they act as 2π dienophiles (CSe) in the reactions with cyclopentadiene except for selenoacrolein which serves as a 4π diene and only one CC bond (2π) in cyclopentadiene is involved in the reaction. Theoretical calculations have been carried out in order to better understand these observations.