Mechanism of the Intramolecular Hexadehydro-Diels-Alder Reaction.
Theoretical analysis of the mechanism of the intramolecular hexadehydro-Diels-Alder (HDDA) reaction, validated against prior and newly measured kinetic data for a number of different tethered yne-diynes, indicates that the reaction proceeds in a highly asynchronous fashion. The rate-determining step is bond formation at the alkyne termini nearest the tether, which involves a transition-state structure exhibiting substantial diradical character. Whether the reaction then continues to close the remaining bond in a concerted fashion or in a stepwise fashion (i.e., with an intervening intermediate) depends on the substituents at the remaining terminal alkyne positions. Computational modeling of the HDDA reaction is complicated by the significant diradical character that arises along the reaction coordinate, which leads to instabilities in both restricted singlet Kohn-Sham density functional theory (DFT) and coupled cluster theory based on a Hartree-Fock reference wave function. A consistent picture emerges, however, from comparison of broken-symmetry DFT calculations and second-order perturbation theory based on complete-active-space self-consistent-field (CASPT2) calculations.
Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota , Minneapolis, Minnesota 55455-0431, United States.
Daniel J
DJ
Marell
Department of Chemistry, Macalester College , Saint Paul, Minnesota 55105-1899, United States.
Lawrence R
LR
Furan
Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota , Minneapolis, Minnesota 55455-0431, United States.
Brian P
BP
Woods
Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota , Minneapolis, Minnesota 55455-0431, United States.
Xiangyun
X
Lei
Department of Chemistry, Macalester College , Saint Paul, Minnesota 55105-1899, United States.
Andrew J
AJ
Bendelsmith
Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota , Minneapolis, Minnesota 55455-0431, United States.
Christopher J
CJ
Cramer
Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota , Minneapolis, Minnesota 55455-0431, United States.
Thomas R
TR
Hoye
Department of Chemistry, Macalester College , Saint Paul, Minnesota 55105-1899, United States.
Keith T
KT
Kuwata
D000480:Alkynes / Q000737:chemistry*; D003198:Computer Simulation; D061565:Cycloaddition Reaction; D007700:Kinetics; D015394:Molecular Structure
D000480:Alkynes