1. Introduction
Earlier1, the interdependence of steric-electronic factors for spiroconjugated molecules2 was determined to be more complex than that discernable from molecular structures alone, thereby calling into question the use of the Cahn–Ingold–Prelog (CIP) priority rules for determining the chiral configuration (R/S)3,4. Cumulenes are a class of linearly conjugated π-systems terminated by tri-coordinate carbon atoms5. The cumulene molecules are labeled by n , which refers to the number of cumulated double bonds; where the total number of carbon atoms is specified by n + 1. For [n ]cumulenes with evenn to form closed-shell π-systems the terminal carbon atoms need to be mutually perpendicular. These molecules will be referred to as “even” [n ]cumulenes, see Scheme 1 . The frontier π molecular orbitals (MOs) in unsubstituted even [n ]cumulenes are all explicitly degenerate, a consequence of the ideal D2d symmetry of the molecule. The degeneracy of the orbitals is lifted by α ,ω -disubstitution, which reduces the symmetry of the molecule to C2 or lower, rendering the molecule chiral. An axial torsion toward the achiral planar transition state may also break the degeneracy for a mutual rotation of the end carbons6–11. These molecules have helical frontier MOs as a direct result of the perpendicular end-groups, as investigated by Hendon et al .,12 an effect they named “electrohelicity”.