posted on 2003-04-12, 00:00authored byRoger W. Alder, Stephen P. East, Jeremy N. Harvey, Mark T. Oakley
Intramolecular and radical-promoted mechanisms for the rearrangement of azulene to naphthalene
are assessed with the aid of density functional calculations. All intramolecular mechanisms have very high
activation energies (≥350 kJ mol-1 from azulene) and so can only be competitive at temperatures above
1000 °C. Two radical-promoted mechanisms, the methylene walk and spiran pathways, dominate the
reaction below this temperature. The activation energy for an orbital symmetry-allowed mechanism via a
bicyclobutane intermediate is 382 kJ mol-1. The norcaradiene−vinylidene mechanism that has been
proposed in order to explain the formation of small amounts of 1-phenyl-1-buten-3-ynes from flash
thermolysis of azulene has an activation energy of 360 kJ mol-1; subtle features of the B3LYP/6-31G(d)
energy surface for this mechanism are discussed. All intermediates and transition states on the spiran and
methylene walk radical-promoted pathways have been located at the B3LYP/6-31G(d) level. Interconversion
of all n-H-azulyl radicals via hydrogen shifts was also examined, and hydrogen shifts around the five-membered ring are competitive with the mechanisms leading to rearrangement to naphthalene, but those
around the seven-membered ring are not. Conversion of a tricyclic radical to the 9-H-naphthyl radical is
the rate-limiting transition state on the spiran pathway, and lies 164.0 kJ mol-1 above that of the 1-H-azulyl
radical. The transition state for the degenerate hydrogen shift between the 9-H-azulyl and 10-H-azulyl radicals
is 7.4 kJ mol-1 lower. Partial equilibration of the intermediates in the spiran pathway via this shift may
therefore occur, and this can account for the surprising formation of 1-methylnaphthalene from 2-methylazulene. The rate-limiting transition state for the methylene walk pathway involves the concerted transfer
of a methylene group from one ring to the other and lies 182.3 kJ mol-1 above that of the 1-H-azulyl radical.
It is shown that rearrangement via a combination of 31% methylene walk and 69% spiran pathways can
account semiquantitatively for all the products from 1-13C-azulene, 9-13C-azulene, and 4,7-13C2-azulene, in
addition to accounting for the products from methylazulenes, and the formation of naphthalene-d0 and -d2
from azulene-4-d. It is also pointed out that a small extension to the spiran pathway could provide an
alternative explanation for the formation of 1-phenyl-1-buten-3-ynes.