Correlation between Optical Activity and the Helical Molecular Orbitals of Allene and Cumulenes

Helical frontier
molecular orbitals (MOs) appear in disubstituted
allenes and even-n cumulenes. Chiral molecules are
optically active, but while these molecules are single-handed chiral,
π-orbitals of both helicities are present. Here we computationally
examine whether the optical activity of chiral cumulenes is controlled
by the axial chirality or the helicity of the electronic structure.
We exploit hyperconjugation with alkyl, silaalkyl, and germaalkyl
substituents to adjust the MO helicity without altering the axial
chirality. For the same axial chirality, we observe an inversion of
the helical MOs contribution to the electronic transitions and a change
of sign in the electronic circular dichroism and optical rotation
dispersion spectra. While the magnitude of the chiroptical response
also increases, it is similar to that of chiral cumulenes without
helical π-orbitals. Overall, helical π-orbitals correlate
with the big chiroptical response in cumulenes, but are not a prerequisite
for it.