Dynamics, transition states, and timing of bond formation in Diels–Alder reactions

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 109; no. 32; pp. 12860 - 12865
• Main Authors: Black, Kersey, Liu, Peng, Xu, Lai, Doubleday, Charles, Houk, Kendall N
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 07.08.2012; National Acad Sciences
• Series: Inaugural Article
• Subjects: Butadienes; Carbon - chemistry; chemical bonding; Chemical bonds; Chemical reactions; Diels Alder reactions; High temperature; Hydrocarbons - chemistry; Models, Molecular; Molecular Conformation; Molecular Dynamics Simulation; phase transition; Phase transitions; Physical Sciences; Potential energy; Reactants; Saddle points; Simulation; simulation models; stereochemistry; stereoisomerism; Symmetry; Temperature; Time Factors; Trajectories; Transition zones; Velocity
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1209316109

The time-resolved mechanisms for eight Diels–Alder reactions have been studied by quasiclassical trajectories at 298 K, with energies and derivatives computed by UB3LYP/6-31G(d). Three of these reactions were also simulated at high temperature to compare with experimental results. The reaction trajectories require 50–150 fs on average to transverse the region near the saddle point where bonding changes occur. Even with symmetrical reactants, the trajectories invariably involve unequal bond formation in the transition state. Nevertheless, the time gap between formation of the two new bonds is shorter than a C─C vibrational period. At 298 K, most Diels–Alder reactions are concerted and stereospecific, but at high temperatures (approximately 1,000 K) a small fraction of trajectories lead to diradicals. The simulations illustrate and affirm the bottleneck property of the transition state and the close connection between dynamics and the conventional analysis based on saddle point structure.