Bonded to Carbon or Nitrogen? This is a Question on the Regioselectivity in Hyperconjugative Aromaticity

In chemistry, regioselectivity
is the preference of one direction
of chemical bond making or breaking over all other possible directions.
Although it has been extensively investigated in various reactions,
the regioselectivity of hyperconjugative aromaticity on either main
group systems or transition metal ones remains elusive due to the
challenge of synthesizing the target products. Here we report a joint
theoretical and experimental study on this issue. Theoretical calculations
predicted that electron-withdrawing groups prefer an attachment to
the sp3-hybridized carbon atom rather
than the nitrogen atom in indoliums. For the electron-donating groups,
the two isomers bonded to the sp3-hybridized
carbon or nitrogen atom are almost isoenergetic. When both sp2- and sp3-hybridized
carbon and nitrogen atoms in the five-membered ring of indoliums are
considered, the isomer with the polyaurated substituents bonded to
the sp3-hybridized carbon atom is thermodynamically
more stable than that with the polyaurated substituents bonded to
the sp3-hybridized nitrogen atom. This
prediction is reasonably verified by experimental observation. Bond
dissociation energy is found to be more important than aromaticity
in rationalizing such a preference. Our findings could help experimentalists
to design and realize more novel hyperconjugative aromatics.