An ab initio analysis of the Diels-Alder reaction between two isoprenes

- Concerted mechanism of isoprene cycloaddition is more favorable than stepwise one.
- Asynchronous characters of 30 concerted transition structures are analyzed.
- Conformation transition is important to the closure of stepwise intermediate.
- Influence of configuration and conformation of the transition states are discussed.
- Calculation support the experimental result that sylvestrene is favorable.

The concerted and stepwise mechanisms of the isoprene dimerization via Diels-Alder reaction have been studied with MP2 method. The frontier orbital densities can explain the asynchronous character of the concerted transition structures. The first transition states and intermediates of the stepwise reactions were identified and the conformational rotations of the intermediates were investigated for their stability and possibility to form monocyclic adducts. All the stepwise activation energies were about 15-40 kJ/mol higher than the concerted pathways and the meta-adduct (sylvestene) of the endo concerted reaction with cis-dienophile had the lowest activation energy, ∼47 kJ/mol, in all reaction systems. A novel synchronous transition state was also predicted which suggests an interesting intramolecular rearrangement of the monoterpenoids.

Asynchronous concerted and stepwise mechanisms of the [4 + 2] cycloaddition between two isoprenes have been investigated. The concerted pathways are little more favorable than the stepwise ones and sylvestrene may be the most abundant product because of its relative lower activation energy with respect to other dimers. The rotation barriers of the stepwise intermediate are very close to the energy difference between the intermediate and the first transition state of the stepwise pathway, which implies that the rotation transition structures (eclipsed:in or cis-) may be the second transition states of the stepwise paths.