[4+2] dimerization and cycloaddition reactions of alpha,beta-unsaturated selenoaldehydes and selenoketones

The reactions of alpha,beta-unsaturated aldehydes and ketones with bis(dime thylaluminum) selenide, (Me2Al)(2)Se, yield the corresponding alpha,beta-un saturated selenoaldehydes and selenoketones. They are too unstable to be is olated in the monomeric form, but they undergo regioselective [4 + 2] dimer ization via a "head-to-head" oriented transition state to afford diselenin derivatives (trans and cis isomers). Theoretical calculations at the densit y functional theory level show that this selectivity occurs because the "he ad-to-head" dimerization is thermodynamically favored over the "head-to-tai l" by about 14 kcal/mol. Both dimerization reactions have low energy barrie rs: 1.5 and 2.8 kcal/mol for the former and 0.9 and 1.3 kcal/mol for the la tter. In the presence of norbornadiene, these compounds function as 4 pi he terodienes (C=C-C=Se) to give the respective cycloadduct products. On the o ther hand, they act as 2 pi dienophiles (C=Se) in the reactions with cyclop entadiene except for selenoacrolein which serves as a 4 pi diene and only o ne C=C bond (2 pi) in cyclopentadiene is involved in the reaction. Theoreti cal calculations have been carried out in order to better understand these observations.