Understanding the 1,3-Dipolar Cycloadditions of Allenes.

We have quantum chemically studied the reactivity, site-, and regioselectivity of the 1,3-dipolar cycloaddition reaction between methyl azide and various allenes, including the archetypal allene propadiene, heteroallenes, and cyclic allenes, using density functional theory (DFT). The  1,3-dipolar  cycloaddition reactivity of linear (hetero)allenes decreases as the number of heteroatoms in the allene increases and the formation of the 1,5-adduct is, in all cases, favored over the 1,4-adduct, both effects find their origin from the strength of the primary orbital interactions . The cycloaddition reactivity of cyclic allenes was also investigated, and the increased predistortion of allenes,  that results upon cyclization, leads to systematically lower activation barriers not due to the expected variations in the strain energy, but instead from the differences in the interaction energy . The geometrical predistortion of cyclic allenes enhances the reactivity compared to linear allenes  through a unique mechanism that involves a smaller HOMO-LUMO gap, which manifests into more stabilizing orbital interactions .