Origin of the Preferential Formation of Helicenes in Mallory Photocyclizations. Temperature as a Tool to Influence Reaction Regiochemistry

The
regiochemistry of four bis-Mallory photocyclization substrates
has been examined from experimental and computational perspectives.
Formation of all three possible regioisomers was only observed in
the reaction of one of the substrates. In the other three substrates,
only the two C2-symmetric products, but
not the C1 product, were formed. In the
three reactions that only formed two products, the photocyclization
temperature could be used to select for exclusive formation of one
or the other regioisomer. The use of temperature to select between
two regioisomers also worked in the photocyclization of the substrate
that formed three products. However, no temperature was located for
exclusive formation of the third component, one of the C2-symmetric products, which always formed alongside either
one or both of its regioisomers. B3LYP/6-311+G­(2d,p) calculations
were used to determine the energies of all of the dihydrophenanthrene
(DHP), tetrahydrophenanthrene (THP), and mono-Mallory photocyclization
intermediates. The oscillator strengths of the DHP precursors to the
helicene products were a factor of 4.8–9.2 smaller than those
of competitively formed DHPs. This observation suggests that establishment
of a photostationary state is responsible for the preferential formation
of helicenes that has been observed as a unique and useful feature
of many Mallory photocyclizations.