Nitrile ylides frontier orbitals

Frontier molecular orbital theory correctly rationalizes the regioselectivity of most 1,3-dipolar cycloadditions (73JA7287). When nitrile ylides are used as 1,3-dipoles, the dipole... 

Fig. 17. Frontier molecular orbitals of bent and linear nitrile ylides. (Reprinted with permission from the Journal of the American Chemical Society. Copyright by the American Chemical Society.)...

The exclusive orientation observed in this reaction is unusual and cannot be adequately accounted for on the basis of frontier orbital theory. A similar inconsistency was also found on irradiation of azirine 233. The orientation of the cycloaddition reaction of aldehyde 233 proceeds in an alternate regiochemical sense from that observed with related bimolecular nitrile ylide-aldehyde cycloadditions where one obtains only d-oxazolines 235. In... 

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