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.

二氢薁（dihydroazulene, DHA）/乙烯基庚富烯（vinylheptafulvene, VHF）光热开关体系在分子电子学与先进材料领域受到广泛关注。此前研究表明，两种异构体之间的开关行为强烈依赖于给体与受体基团的存在。光学性能与开关特性的精细调控，有赖于通过高效合成策略便捷获取新型衍生物。以苯乙酮与薁鎓（tropylium）底物为起始原料，经四步反应即可便捷制备核心DHA骨架。本研究详细考察了该反应的结果随苯乙酮苯环单元上取代基性质的变化规律。该合成路径可兼容多种官能团（硝基、氰基、卤代基团、烷基、酰胺基与硫醚基），能够合成大量2位取代的DHA衍生物。随后，本研究针对这些化合物在七元环上的后续官能化能力展开探究，采用区域选择性溴化-消除策略在7位引入溴取代基。对卤代DHA底物，分别开展了钯催化氰化、薗头偶联（Sonogashira）、卡迪奥-乔德凯维奇偶联（Cadiot–Chodkiewicz）以及铃木-宫浦偶联（Suzuki）反应，其中针对铃木-宫浦偶联反应，通过调控钯催化剂种类优化了反应条件。整体而言，本研究的核心目标之一是抑制全不饱和薁类副产物的生成。