RCNNCRCNNCRCNCNDDDHAs-cis-VHFreactives-trans-VHFunreactivehv
Scheme 1. Dihydroazulene / vinylheptafulvene photo/thermoswitch.
A wide variety of derivatives has been synthesized, incorporating both electron-withdrawing and –donating groups in the five-membered ring7 and very recently also in the seven-membered ring.8 To exploit the DHA/VHF system in molecular electronics a handle in both the seven- membered and the five-membered ring is needed. We reported in 2009 a procedure for incorporating a bromine atom selectively at C-7 (Figure 1) of the seven-membered ring of the DHA.8a This functionality was next employed for incorporation of an arylethynyl group at C-7 by a Sonogashira cross-coupling reaction.8b Studies on electron-donating and –withdrawing aryl substituents have shed light on the mechanism for the ring-closure reaction.8b Thus, the first step of ring-closure involves a change of conformation from an unreactive s-trans to a reactive s-cis conformation (Scheme 1) and the ring-closure goes through a zwitterionic transition state or intermediate.8 The kinetics of the ring-closure was shown to follow a Hammett correlation when plotting ln(k) against .p+ (Hammett constant including through-conjugation).8b The most advanced systems actually provide the possibility to control the switching abilities by the redox- state of the substituent,9 or by the pH of the solution.8b
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Figure 1. Numbering of the DHA.
The DHA photoswitch has been much less studied than the two aforementioned systems. This is most likely due to the lack, until recently, of a protocol for functionalizing the system in both the five- and seven-membered rings. Moreover, access to DHA in large scale is a prerequisite for its wider applicability in advanced systems. 2-Phenyl-1,8a-dihydroazulene-1,1- dicarbonitrile 1 was one of the first photochromic DHAs to be reported by Daub and coworkers.10 Different synthetic protocols for obtaining DHAs were developed. One protocol involves a [8+2] cycloaddition between 8-methoxyheptafulvene and a derivative of 1,1- dicyanoethylene, followed by elimination of methanol (Scheme 2).10 In this route the DHA is formed directly in the final elimination step. In another route, the VHF precursor was first prepared.11 The first step was here an acid-catalyzed nucleophilic addition of an acetophenone
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