Azulenium chemistry: towards new derivatives of photochromic dihydroazulenes

Anne Ugleholdt Petersen,Christian G Tortzen,Mogens Brøndsted Nielsen,Kurt V Mikkelsen,Martyn Jevric,Jonas Elm,Stine T Olsen,Anders Kadziola

doi:10.1039/c5ob02523k

Abstract

Here we present the preparation of a selection of azulenium ions by hydride abstraction from photochromic 1,8a-dihydroazulenes (1,8a-DHAs) incorporating two cyano groups at C-1. The reactivity of the electrophilic tropylium ring of these molecules towards lithium triisopropylsilylacetylide was investigated. The position of attack by the nucleophile depended on the substitution pattern of the azulenium cation but was in general found to occur preferentially at positions C-4, C-5, and C-6, and to a minor extent at positions C-7 and C-8. The outcome was a mixture of non-photochromic, regioisomeric DHAs. One of these compounds containing the ethynyl substituent at position C-4 was partly tautomerized to the photochromic 1,8a-dihydroazulene, which was isolated and its switching properties were investigated by UV-Vis absorption spectroscopy. Upon irradiation, it undergoes a ring-opening reaction to form a vinylheptafulvene (VHF), which in turn returns to the original DHA. The half-life of this reaction was significantly smaller than for a derivative with the alkynyl substituent placed at C-7. This fast switching behavior was according to the calculations explained by an enhancement in the stability of the reactive s-cis conformer of the VHF relative to the, still more stable, s-trans conformer, and by a smaller activation energy for this s-cis conformer to undergo ring-closure.

Highlights

Dihydroazulene 1 (DHA) is a photoswitch that upon exposure to light undergoes a ring-opening to vinylheptafulvalene 2 (VHF).1 VHF returns, in turn, to DHA (Scheme 1) with a rate of ring-closure that is solvent dependent and is believed to proceed via a zwitterionic transition state
DHA was subjected to the bromination–elimination protocol providing the intermediate 12, which was subjected to a Suzuki reaction with phenylboronic acid yielding DHA in reasonable yield over three steps
It is possible to synthesize a host of azulenium salts from their respective DHAs

Summary

Introduction

Dihydroazulene 1 (DHA) is a photoswitch that upon exposure to light undergoes a ring-opening to vinylheptafulvalene 2 (VHF).1 VHF returns, in turn, to DHA (Scheme 1) with a rate of ring-closure that is solvent dependent and is believed to proceed via a zwitterionic transition state. It is known from previous studies of the ring-closure of substituted phenyl VHFs that electron-withdrawing or donating groups can affect the rate of the thermal back reaction (Hammett correlations),6 yet in the present case the position of the substituent on the heptafulvene core seems of importance.

Results

Conclusion