Origins of the Unfavorable Activation and Reaction Energies of 1-Azadiene Heterocycles Compared to 2-Azadiene Heterocycles in Diels-Alder Reactions.
The reactivities of butadiene, cyclopentadiene, furan, thiophene, pyrrole, and their 1-aza- and 2-aza-derivatives in Diels-Alder reactions with ethylene and fumaronitrile were investigated with density functional theory (M06-2X/6-311G(d,p)). The activation free energies for the Diels-Alder reactions of cyclic 1-azadienes are 10-14 kcal mol-1higher than those of cyclic 2-azadienes, and the reaction free energies are 17-20 kcal mol-1more endergonic. The distortion/interaction model shows that the increased activation energies of cyclic 1-azadienes originate from increased transition state distortion energies and unfavorable interaction energies, arising from addition to the nitrogen terminus of the C═N bond.
Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095-1569, United States.
Jason S
0000-0001-6680-2936
JS
Fell
Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095-1569, United States.
Blanton N
BN
Martin
Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095-1569, United States.
K N
0000-0002-8387-5261
KN
Houk