Syntheses of Donor–Acceptor-Functionalized Dihydroazulenes

The
dihydroazulene (DHA)/vinylheptafulvene (VHF) photo/thermoswitch
has been of interest for use in molecular electronics and advanced
materials. The switching between the two isomers has previously been
found to depend strongly on the presence of donor and acceptor groups.
The fine-tuning of optical and switching properties relies on ready
access to new derivatives via efficient synthetic protocols. The central
DHA core is conveniently prepared in a four-step synthesis starting
from acetophenone and tropylium substrates. Here, the outcome of this
reaction as a function of the nature of the substituent group on the
phenyl unit of acetophenone is investigated in detail. A wide variety
of functional groups (nitro, cyano, halo, alkyl, amido, and thioether)
was tolerated, and the route provided access to a large selection
of substituted DHA derivatives (position 2 of DHA). These compounds
were investigated for their ability to undergo subsequent functionalization
in the seven-membered ring by a regioselective bromination–elimination
protocol, introducing a bromo substituent at position 7. Halo-substituted
DHAs were subjected to palladium-catalyzed cyanation, Sonogashira,
Cadiot–Chodkiewicz, and Suzuki couplings and for the latter
reaction; optimized conditions were developed by varying the palladium
catalyst. In general, our focus was on reducing the formation of fully
unsaturated azulene byproducts.