title: [4 + 2] Dimerization and Cycloaddition Reactions of alpha,beta-Unsaturated Selenoaldehydes and Selenoketones.
abstract: The reactions of alpha,beta-unsaturated aldehydes and ketones with bis(dimethylaluminum) selenide, (Me(2)Al)(2)Se, yield the corresponding alpha,beta-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 4pi heterodienes (C=C-C=Se) to give the respective cycloadduct products. On the other hand, they act as 2pi dienophiles (C=Se) in the reactions with cyclopentadiene except for selenoacrolein which serves as a 4pi diene and only one C=C bond (2pi) in cyclopentadiene is involved in the reaction. Theoretical calculations have been carried out in order to better understand these observations.
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, and Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kodatsuno, Kanazawa, Ishikawa 920-8667, Japan.
authors.forename: Guang Ming
authors.initials: GM
authors.lastname: Li
authors.forename: Shuqiang
authors.initials: S
authors.lastname: Niu
authors.forename: Masahito
authors.initials: M
authors.lastname: Segi
authors.forename: Ralph A.
authors.initials: RA
authors.lastname: Zingaro
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Yamamoto
authors.forename: Kentaro
authors.initials: K
authors.lastname: Watanabe
authors.forename: Tadashi
authors.initials: T
authors.lastname: Nakajima
authors.forename: Michael B.
authors.initials: MB
authors.lastname: Hall