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The 1,4-cycloaddition reactions of the singlet (¹Δ_g) 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 C_2v 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 …