Competition between Inter- and Intramolecular Photocycloaddition Reactions of 9-Substituted Anthracenes

Abstract

I n t r a -a n d I n t e r m o l e c u l a r P h o t o c y c l o a d d i t i o n o f 9 -S u b s t i t u t e d A n t h r a c e n e sAbstract: A number of 9-[(arylmethoxy)methyl]anthracenes were prepared and their photoreactivity was studied. The intramolecular [4p+4p]cycloaddition competes with the intermolecular head-totail [4p+4p] cyclodimerization. The aryl substituents control the selectivity; concentration only plays a role in selectivity in certain cases. The complete reversibility of the cycloaddition makes this process suitable as a molecular switch, provided that traces of acids are avoided; when acid is present the cyclomers pursue an irreversible enol ether cleavage route.Reversible photochemical reactions between compounds that have different absorption (and/or emission) are of great interest for optical switching and optical data storage processes. 1 Recently, we developed such a system, which is based on the intramolecular [4p+4p] cycloaddition of benzene and anthracene moieties. 2 A prerequisite condition for this reaction is that the two chromophores are linked by a three-atom chain, like CH 2 -O-CH 2 . Useful switching systems should have clean, uniform reactions in both directions. The [4p+4p] cyclodimerization is an alternative photoreaction of anthracenes and is an old and well-known process. 1e,3 Therefore, we have now studied the competition between the desired intramolecular [4p+4p] cycloaddition and possible intermolecular [4p+4p] cyclodimerizations. For this purpose, 9-[(arylmethoxy)methyl]anthracenes 3 were generated. Their preparation was based on the nucleophilic substitution of 1 and 2 (Williamson synthesis), whereby the role of nucleophile and electrophile can be exchanged (Table 1, Methods A and B). The best results were obtained by using phase-transfer conditions (KOH, TBAB).The possible photoproducts of 3a-n are shown in Scheme 1. 4-8 Apart from the cyclomers 4, head-to-tail dimers 5 and head-to-head dimers 5¢ can be expected. 9,10 Compared to 5, the dimers 5¢ are highly thermolabile. In the past such compounds were usually detected in solution by NMR measurements of the crude product mixture. Nevertheless, many head-to-head dimers 5¢ have been reported. 4,11,12 In the first approximation, the compounds 3a-n contain two separate chromophores. Therefore, selective irradiation (l ≥300 nm) into the anthracene chromophore is possible. This precondition is necessary in order to prevent the light-induced reverse reactions 4,5,5¢ → 3. Table 2 shows the product ratios 4/5 that were obtained in the irradiation experiments of 3a-h. The halogen-containing compounds 3i-k are highly light sensitive. However, their irradiation yields product mixtures in which dimers, but no cyclomers, can be detected by 1 H NMR and FD-MS measurements; 3k reacts even in daylight. Compound 3l was studied in another context. 7 For the compounds 3a and 3m, no cyclomers 4a and 4m were reported, but mixtures of dimers 5a/5¢a and 5m/5¢m, respectively, were found; 4 on irradiation compound 3n shows a quantitative intram...