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Distortion/interaction energy control of 1,3-dipolar cycloaddition reactivity.

Computations of activation barriers and reaction energies for 1,3-dipolar cycloadditions by a high-accuracy quantum mechanical method (CBS-QB3) now reveal previously unrecognized quantitative trends in activation barriers. The distortion/interaction theory explains why (1) there is a monotonic decrease of ∼6 kcal/mol in the barrier height along the series oxides, imine, and ylide, for each class of 1,3-dipoles; (2) the corresponding nitrilium and azomethine betaines have almost identical cycloaddition barrier heights; (3) cycloadditions of a given 1,3-dipole with ethylene and acetylene have the same activation energies, in spite of very different reaction thermodynamics and frontier orbital gaps. There is a linear correlation between distortion energies (ΔEd⧧) and the activation barrier (ΔE⧧ = 0.75ΔEd⧧ − 2.9 kcal/mol) that is general for substituted and unsubstituted 1,3-dipoles in these cycloadditions. The energy to distort the 1,3-dipole to the geometry favorable for interaction with the dipolarophile, ...