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

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.