Nitrous oxide as a 1,3-dipole: a theoretical study of its cycloaddition mechanism

The 1,3-dipolar cycloadditions of nitrous oxide and substituted alkynes have been studied at the B3LYP/6-31G(d,p) level. The reaction is controlled by LUMO (dipole)--HOMO (dipolarofile) and involves aromatic transition structures. The shape of the potential energy surface and the regioselectivity are not affected by the polarity of the solvents, except in the case of N2O + HC triple bond CSiH3. Different reactivity criteria including FMO coefficients product C, local softness differences Delta, magnetic susceptibility anisotropy chi(anis), and nucleus-independent chemical shifts NICS were used to predict the regioselectivity in all studied cases; the C, Delta criteria turn out to give the best results among them. The aromaticity of the transition structure is not a factor in determining the regiochemistry of the cycloaddtition reactions.