Local Softness as a Regioselectivity Indicator in [4+2] Cycloaddition Reactions

As part of our continuing interest in the connection between chemical reactivity and the HSAB principle, both globally and locally, we explored via 3-21 G calculations on local and global softness the statement made by Gazquez and Mendez. Their statement, claiming that two chemical species should react through atoms showing equal softness, has been precised to rationalize the regioselectivity of Normal Electron Demand Diels−Alder reactions between terminally monosubstituted 1,3-butadienes and monosubstituted ethenes. In the presence of this substitution pattern, a head-to-head mode of cyclization is largely experimentally observed, also known as “ortho rule”. The closest values of the condensed local softness were found for the unsubstituted termini, thereby suggesting that these cycloadditions would proceed through an asynchronous pathway, as supported by transition state geometries reported by Houk et al.