Unprecedented "off-pathway" [2+2] Cycloaddition-Retroelectrocyclization Reaction Between an Unsymmetric Alkyne and Tetracyanoquinodimethane

In recent years, the [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction between electron-rich alkynes and electron-deficient alkenes has emerged as one of the most effective synthetic routes to prepare a large variety of molecular and polymeric electron donor-acceptor systems. Besides its simplicity, fast rate, and high yield, this reaction may also display complete and predictable regioselectivity, as in the case when tetracyanoquinodimethane (TCNQ) is used in combination with unsymmetric, activated alkynes. Here, we report the first example of a [2+2] CA-RE reaction between TCNQ and an aniline-activated alkyne following an "inverted" regiochemistry, thus leading to the exclusive formation of an unexpected regioisomer in contrast to the expected one. A combined experimental and theoretical study helped us to unravel the peculiar reaction mechanism underlying the regioselectivity switching.