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Highly Regioselective Anaerobic Photocyclization of 3-Styrylpyridines
journal contribution
posted on 2001-04-06, 00:00 authored by Frederick D. Lewis, Rajdeep S. Kalgutkar, Jye-Shane YangThe photochemical behavior of the cis isomers of the three isomeric styrylpyridines and two
(aminostyryl)pyridines has been investigated under aerobic and anaerobic conditions. Both 3-styrylpyridine
and its 3‘-amino derivative undergo highly regioselective formation of 2-azaphenanthrene products under
anaerobic conditions. In the presence of oxygen, mixtures of 4- and 2-azaphenanthrene products are obtained.
The formation of 2-azaphenanthrenes in the absence of oxygen is attributed to conversion of the
4a,4b-dihydroazaphenanthrene primary photoproduct to a 1,4-dihydropyridine intermediate by means of a formal
1,7-hydrogen shift. This intermediate is moderately stable in the absence of oxygen and has been characterized
by comparison of its 1H NMR and electronic absorption spectra with calculated spectra. This intermediate is
converted to the 2-azaphenanthrene in both the absence and presence of oxygen. The regioselectivity of
photocyclization of 3-substituted stilbenes and related diarylethylenes is suggested to depend on the relative
rate constants for ring opening and sigmatropic rearrangements of the dihydrophenanthrene intermediates as
well as their rates of reaction with oxygen or other oxidants.