Theoretical study on rotational barriers of 1,3-dipoles and mechanisms of 1,3-dipolar reactions

SUMMARY The different orbitals for different spins molecular orbital and complete active space (CAS) SCF MO descriptions of azomethyne ylides, carbonyl ylides and related oxygenated dipoles were performed as a continuation of earlier papers. Rotational energy barriers for the C‐C double bond of ethylene at the equilibrium and dissociated geometries, and the 1,3-dipolar species were calculated by UHF, APUHF, UMP, APUMPn, CASSCF and CASSCF MP2 procedures, and were discussed in relation to their biradical characters and stereospecificity of 1,3-dipolar reactions. It was found that the biradical characters of carbonyl ylides and related species are quenched by the symmetry-allowed orbital interactions with olefins, in accord with the symmetry-allowed concerted property. The rotational energy barriers for 1,5 biradicals were also examined in relation to the non-concerted mechanism of 1,3-dipolar reactions. The implications of these results are discussed in relation to the concerted and biradical mechanisms of 1,3-dipolar additions such as ozonolysis reactions. ©1997 by John Wiley & Sons, Ltd.