title: Diels-Alder reactions of allene with benzene and butadiene: concerted, stepwise, and ambimodal transition states.
abstract: Multiconfigurational complete active space methods (CASSCF and CASPT2) have been used to investigate the (4 + 2) cycloadditions of allene with butadiene and with benzene. Both concerted and stepwise radical pathways were examined to determine the mechanism of the Diels-Alder reactions with an allene dienophile. Reaction with butadiene occurs via a single ambimodal transition state that can lead to either the concerted or stepwise trajectories along the potential energy surface, while reaction with benzene involves two separate transition states and favors the concerted mechanism relative to the stepwise mechanism via a diradical intermediate.
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095-1569, United States.
authors.forename: Hung V
authors.initials: HV
authors.lastname: Pham
authors.forename: K N
authors.initials: KN
authors.lastname: Houk
mesh_terms: D000466:Alkadienes / Q000737:chemistry*; D001554:Benzene / Q000737:chemistry*; D002070:Butadienes / Q000737:chemistry*; D003198:Computer Simulation; D061565:Cycloaddition Reaction
chemical_list: D000466:Alkadienes; D002070:Butadienes; C025947:propadiene; D001554:Benzene; C031763:1,3-butadiene