Mechanistic studies of 1,3-dipolar cycloadditions of bicyclic thioisomunchnones with alkenes. A computational rationale focused on donor-acceptor interactions

This paper describes a mechanistic study, with the interplay of experiment and theory, on the cycloadditions of a bicyclic 1,3-dipole versus a series of representative symmetrical (1-phenyl-1H-pyrrole-2,5-dione and dimethyl maleate) and asymmetrical [(E)-(2-nitrovinyl)benzene, acrylonitrile, and but-3-en-2-one] olefinic dipolarophiles. These results allow a comparative analysis with monocyclic dipoles and open further avenues to structurally diversified heteroatom-rich rings. The unichiral version of the bicyclic dipole leads to adducts containing up to five chiral centers, whose formation proceeds with high levels of facial stereoinduction in reactions involving bulky dipolarophiles. The second and largest part of this study provides a theoretical interrogation on the pericyclic mechanism with DFT-methods [M06-2X/6-31++G(d,p)]. In order to get further mechanistic insights, we have also explored charge transfers between reaction partners using NBO analysis, which satisfactorily justifies the stereochemical outcome.