Potential energy surfaces of cycloaddition reactions

Cycloadditions reactions can be used to obtain compounds of various ring sizes. Although they have been largely investigated experimentally, considerable controversy still surrounds their mechanism. To improve the understanding of these reactions the potential energy surfaces of the following series of cycloadditions have been computed: [2+2] cycloadditions (H2CCH2 + H2CCH2, H2CO + H2CO, H2CCH2 + O2); Diels-Alder reaction (H2CCHCHCH2 + H2CCH2); 1,3 dipolar cycloadditions (HCNO + HCCH, HCNO + H2CCH2, H2CNHO + H2CCH2). The computations were performed using the ab initio MC-SCF method, which allows treatment of both concerted and non-concerted paths on an equal footing. All the critical points of the surface were optimized using analytic gradient methods and characterized by computing the Hessian matrix by finite differences of the gradients. All surfaces are investigated at minimal (STO-3G) and extended (4-31G) levels. The results of these computations indicate that: (i) for a [2+2] cycloaddition only step-wise reaction paths exist, involving diradicaloid transition states and intermediates; (ii) for the Diels-Alder reaction, the concerted synchronous pathway appears to be preferred over non-synchronous concerted or non-conccerted pathways; (iii) for the 1,3 dipolar cycloadditions the concerted pathway is preferred over the non-concerted pathway involving a diradical intermediate. © 1988.