Understanding the 1,3‐Dipolar Cycloadditions of Allenes

Abstract

We have quantum chemically studied the reactivity, site‐, and regioselectivity of the 1,3‐dipolar cycloaddition between methyl azide and various allenes, including the archetypal allene propadiene, heteroallenes, and cyclic allenes, by 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 formation of the 1,5‐adduct is, in all cases, favored over the 1,4‐adduct. Both effects find their origin in 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 geometric predistortion of cyclic allenes enhances the reactivity compared to linear allenes through a unique mechanism that involves a smaller HOMO–LUMO gap, which manifests as more stabilizing orbital interactions.

Quantification of the Helicality of Helical Molecular Orbitals

“…Finally, we examine cyclic allenes where the allenic moiety is constrained, thus perturbing the helical π-system. Cycloallenes are synthetically interesting compounds that, given their reduced symmetry due to the ring structure, exhibit helical MOs. ,,− Most recently, Ramirez et al demonstrated that secondary orbital interactions due to the helical frontier MO topology are responsible for the endo selectivity in Diels–Alder reactions. Four molecular structures of simple cycloallenes are shown in the top of Figure , from left to right: 1,2-cyclo­hexadiene, 1,2-cyclo­heptadiene, 1,2-cyclo­octadiene, and 1,2-cyclo­nonadiene.…”

Chemical reactivity from an activation strain perspective

“…Thereafter, we showcase some recent advances of applying the ASM to various chemical problems, such as the competition between S N 2 and E2 reaction, 12 how metallylenes activate small molecules, 13 and the cycloaddition reactivity of cycloalkenes, cycloalkynes, and cycloallenes. 14 …”

Quantification of the Helicality of Helical Molecular Orbitals

“…Cycloallenes are synthetically interesting compounds that, given their reduced symmetry due to the ring structure, exhibit helical MOs. 29,30,[71][72][73][74][75] Most recently, Ramirez et al 30 demonstrated that secondary orbital interactions due to the helical frontier MO topology are responsible for the endo selectivity in Diels-Alder reactions. Four molecular structures of simple cycloallenes are shown in the top of Figure 7, from left to right, 1,2cyclohexadiene, 1,2-cycloheptadiene, 1,2-cyclooctadiene and 1,2-cyclononadiene.…”

“…29,30,[71][72][73][74][75] Most recently, Ramirez et al30 demonstrated that secondary orbital interactions due to the helical frontier MO topology are responsible for the endo selectivity in Diels-Alder reactions. Four molecular structures of simple cycloallenes are shown in the top of Figure7, from left to right, 1,2-…”