Model calculations of chemical interactions. Part 7.—Role of vicinial delocalization in the regiochemical control of the cycloaddition of diazomethane and formonitrile oxide to methyl vinyl ether

摘要: In strict accordance with the preceding theoretical and experimental studies on diastereofacial selectivity, we undertook a study of regioselectivity in 1,3-dipolar cycloadditions; examples chosen included reactions alkyl vinyl ethers diazomethane, most famous difficult regiochemical experiment literature cycloadditions one that has yet to find well grounded explanation, formonitrile oxide, small 1,3-dipole fairly high dipole moment is capable enhancing any eventual role electrostatic effects.The cycloaddition methyl ether (MVE) diazomethane was checked experimentally found afford 3-methoxy-1-pyrazoline as only characterized highly abundant adduct, although formation minor amounts other regioisomer could not be ruled out definitely.Concerted transitions structures (TS) were calculated at different levels theory: 5-methoxy-2-isoxazoline found, agreement experiment, favoured regioadducts respectively. both cases transition ‘earlier’ than unfavoured one, had anti-conformations O—Me substituent, spite lower stability anti conformation free MVE.According our analysis, MVE main features can explained follows: largest vicinal stabilization traceable four-electron three-centre π conjugation between lone pair bond allylic fragment O—CC, which also largely responsible for rotameric potential-energy profile MVE. TSs these interactions are differently perturbed, regioisomeric approaches, more favourable perturbation dictating regiochemistry; examples, approach occurs when C⋯X (X = N, O) formed substituted carbon atom. This very same origin earliness TS consequent weakness its incipinet stabilization. The conformational significantly affected by effects.