The Concerted Reaction of 1,3 -Butadiene with Ethylene

The reaction of 1,3-butadiene with ethylene to give cyclohexene is an allowed reaction if the ethylene fragment approaches the butadiene fragment from one face, preserving a plane of symmetry as indicated in 4. The Hiickel-type arrangement of the p-orbitals shown in 4 involves six electrons and so should express some aromatic stabilization. 

We first take up the issue of computing the activation barrier for this parent Diels-Alder reaction. Rowley and Steiner experimentally determined an activation barrier of 27.5 0.5 kcal mol for Reaction 4.1. Using thermal corrections computed at B3LYP/6-31G, Houk and coworkers estimate that Alfi(0 K) = 

3 2 kcal mol A large number of computational estimates of the activation energy have been reported, many of which are listed in Table 4.1. 

A number of important trends can be drawn from Table 4.1, which are trends that have influenced how computational chemists approach related (and sometimes even largely unrelated) problems. Hartree-Fock (HF) self-consistent field (SCF) computations vastly overestimate the barrier, predicting a barrier twice as large as experiment. The omission of any electron correlation more seriously affects the transition state, where partial bonds require correlation for proper description, than the ground-state reactants. Inclusion of nondynamical correlation is also insufficient to describe this reaction complete active space self-consistent field (CASSCF) computations also overestimate the barrier by some 20 kcal 

Clearly, dynamical correlation must be included. The first study to obtain a reasonable value for the barrier was the work of Bach, which employed perturbation theory. MP2 lowers the barrier substantially from the HF result unfortunately, it predicts a barrier that is too low, in the range 16-20 kcal mol . MP3 raises the barrio- but predicts now too high a value (27-28 kcal mol ). Finally, MP4 provides very good agreement with experiment, but triples configurations must be included to get this best fit (E values of 22.4-22.8 kcal mol ). Very similar trends were obtained by Jorgensen in his examination of the Diels-Alder reaction of cyclopentadiene with ethylene and related dienophiles. 

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