Photochemical & Photobiological Sciences www.rsc.org/pps PAPER Regiospecific [2 + 2] photocyclodimerization of trans-4-styrylpyridines templated by cucurbit[8]uril†‡ Asao Nakamura,* a Hiromi Irie, a Shuhei Hara, a Mai Sugawara b and Shinji Yamada b Received 23rd February 2011, Accepted 26th April 2011 DOI: 10.1039/c1pp05079f Addition of HCl accelerated the photocyclodimerization of trans-4-styrylpyridine 1a in methanol and increased the yield of syn-head-to-tail (syn-HT) dimer 2a through the effect of cation-p interactions between the pyridinium ion of one molecule and the phenyl group of the other. We examined the photoirradiation products of derivatives of 1a having alkyl substituents on the phenyl group (1b–1f). The effect of the alkyl substituent on product distribution was rather limited for the photoreaction in MeOH solutions. However, the substituents had a distinct effect on the product distribution for the photoreaction of the inclusion complexes of hydrochloride salts of trans-4-styrylpyridines with cucurbit[8]uril in aqueous solutions. Introducing an alkyl group at the 2- or 3-position of the phenyl group completely shifted the major product from the syn-HT dimer to the syn-head-to-head (syn-HH) dimer. By adjusting the balance of host–guest interactions and cation-p interactions between guest molecules through systematic changes in the substituents on the phenyl ring of trans-4-styrylpyridine, we could change the orientation of the reactant molecules in the host cavity, resulting in a change of the major regioisomer of the photocyclodimerization products. Introduction In 1984, Tamaki et al. reported that the photocyclodimerization of 2-anthracenesulfonate and 2-anthracenecarboxylate in aqueous solutions is accelerated by the addition of g -cyclodextrin, and that the reaction proceeds regio- and stereospecifically in the presence of g -cyclodextrin. 1 In the following two decades, large numbers of papers have been published that focus on the acceleration and regio- and stereospecific control of [2 + 2] and [4 + 4] photocyclodimerization reactions utilizing different host–guest complexes. 2–22 In these studies, g -cyclodextrin, 2–11 calixarenes, 12 cavitands, 13,14 nanocages, 15,16 and cucurbiturils 17–22 have been used as hosts that accelerate photocyclodimerization in solution by accommodating two guest molecules in the same cavity. In recent years, controlling the reactivity of olefins in the solid state by constructing cocrystals with template molecules has at- tracted much attention. 23–25 In these cocrystals, template molecules direct stacking of the olefin molecules via hydrogen bonds or coordination bonds, which enables photocyclodimerization. a Department of Chemistry, College of Engineering, Shibaura Institute of Technology, Saitama-shi, Saitama, 337-8570, Japan. E-mail: asao@ sic.shibaura-it.ac.jp; Fax: +81 48-687-5013; Tel: +81 48-687-5032 b Department of Chemistry, Faculty of Science, Ochanomizu University, Bunkyo-ku, Tokyo, 112-8610, Japan. E-mail: yamada.shinji@ocha.ac.jp † This article is published as part of a themed issue in honour of Yoshihisa Inoue’s research accomplishments on the occasion of his 60th birthday. ‡Electronic supplementary information (ESI) available: Details of the synthesis of trans-4-styrylpyridines, outline of the analytical methods for the assignment of the isomers, and properties and NMR spectra of the isomers of cyclodimers. See DOI: 10.1039/c1pp05079f Photoirradiation of dilute neutral solutions of trans-4- styrylpyridine (Scheme 1) 1a gives only the cis isomer 5a through trans-cis photoisomerization. However, the hydrochloride salt of the trans-4-styrylpyridine is easily photocyclodimerized in solu- tions, preferentially producing syn-head-to-tail (syn-HT) dimer 2a. 26 The BF 4 - salt of protonated trans-4-styrylpyridine gives the syn-HT dimer 3a upon photoirradiation in the solid state. 26 Yamada et al. reported that cation-p interactions between reactant molecules are responsible for the large regiospecificity of the photocyclodimerization of the protonated trans-4-styrylpyridine both in solution and crystal states. 27 Scheme 1 Photocyclodimerization and photoisomerization of trans-4-styrylpyridines. If the photocyclodimerization of trans-4-styrylpyridines is performed in the cavity of host molecules, the regiospecificity may be governed not only by the cation-p interactions between 4-styrylpyridine molecules but also by the balance of host– guest interactions and the cation-p interactions between guest 1496 | Photochem. Photobiol. Sci., 2011, 10, 1496–1500 This journal is © The Royal Society of Chemistry and Owner Societies 2011