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.molecular dynamics | density functional theory | transition state theory | cycloadditions | concerted reaction T he Diels-Alder reaction is one of the most important reactions used in organic synthesis (1, 2), and its discovery was recognized by the Nobel Prize in Chemistry in 1950. The reaction is a paradigm for methods that efficiently increase structural complexity, because two single bonds, a six-membered ring, and up to four stereocenters are formed in a single step.Mechanisms of Diels-Alder reactions have been the subject of intensive scrutiny by experimental and theoretical methods (3-6). The timing of bond formation has been the focus of attention. Many experimental studies show that the Diels-Alder reaction is stereospecific with respect to both reactants (7), a result that is compatible with a concerted mechanism, traditionally defined as a reaction path involving no intermediates. DFT and ab initio calculations based on transition state theory predict that a concerted, one-step mechanism is the lowest energy pathway on the ground-state potential energy surface (PES) (8). However, at the time-resolved level, the two new carbon-carbon bonds may not form simultaneously along a dynamical trajectory. The time gap between formation of the two bonds is strongly connected to the expected degree of stereospecificity observed in the reaction, but the details of this relationship are uncertain due to several recent reports: (i) In studies of retro-Diels-Alder (retro-DA) dynamics at femtosecond time resolution of reaction R2 in Scheme 1, Zewail and coworkers found two sets of transients, one identified with concerted asynchronous trajectories, the other with diradicaloid stepwise trajectories (9, 10). Because reactants are photochemically activated, a full understanding of the experiments must include the role of conical intersections between ground and excited states (11). (ii) In thermal shock tube studies of the retro-DA reactions R1, R2, and R3 in Scheme 1, Lewis, Baldwin, and coworkers found that the retro-DA reaction of cis-4...
