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”.