CAS MCSCF/CAS MCQDPT2 Study of the Mechanism of Singlet Oxygen Addition to 1,3-Butadiene and Benzene

Abstract

The 1,4-cycloaddition reactions of the singlet (1Δg) oxygen with s-cis-1,3-butadiene and benzene, with the formation of 3,6-dihydro[1,2]dioxin and 2,3-dioxabicyclo[2.2.2]octa-5,7-diene, respectively, were studied by means of the CAS MCSCF/CAS MCQDPT2 ab initio method with the 6-31G* basis set. In the case of butadiene the reaction was found to be exoenergetic and the product was found to have C2 symmetry, with the peroxide moiety in the gauche configuration. In the case of benzene the reaction was found to be endoenergetic and the bicyclic product formed was found to have C2v symmetry, with the peroxide moiety in the syn configuration. Three possible reaction routes were studied: (i) concerted cycloaddition, (ii) stepwise cheletropic cycloaddition with the formation of zwitterionic 2,5-dihydrofuran l-oxide as an intermediate, and (iii) stepwise cycloaddition with the formation of a linear intermediate. In the case of butadiene routes (i) and (ii) were excluded, because only second-order saddle points were found on the corresponding reaction pathways. The linear intermediate (I1) found in route (iii) has a biradical character, and its energy relative to that of the separate reactants is 4.1 kcal/mol. The dominant activation barrier corresponds to the transition structure T1 leading to I1 and amounts to 9.9 kcal/mol. The rearrangement of I1 to the product (P) involves only a minor activation barrier of 7.5 kcal/mol (relative to I1). In the case of benzene the reaction occurs in a concerted manner with a single transition structure having C2v symmetry; the activation barrier is 25.3 kcal/mol. This difference in binding mechanism can be explained in terms of the configuration of the peroxide moiety in the adduct.