PAPER www.rsc.org/pps | Photochemical & Photobiological Sciences Templating photodimerization of stilbazoles with water-soluble calixarenes†‡ Raja Kaliappan, Lakshmi S. Kaanumalle, Arunkumar Natarajan and V. Ramamurthy* Received 10th May 2006, Accepted 2nd August 2006 First published as an Advance Article on the web 15th August 2006 DOI: 10.1039/b606658e Water soluble six and eight membered calixarenes template the dimerization of trans-stilbazoles. In the absence of calixarenes at the concentrations employed stilbazoles mainly isomerize to the coresponding cis isomers. Calixarenes help to localize the olefins and orient them in a specific geometry to yield anti-head–tail dimers. Electrostatic interaction between the sulfonate anion and the pyridinium ion of the olefin and hydrophobic interaction between the olefin and the host cavity arebelieved to be responsible for the observed selectivity. 1 H NMR spectra provide evidence for complexation but do not suggest the exact structure of the host–guest complex. Introduction Olefins upon excitation undergo a variety of reactions of which addition of an excited olefin to another in the ground state has been extensively investigated both in the solid state 1 and solution. 2 Despite such a long history controlling the regioselectivity during photocycloaddition continues to be a challenging task. Early work on this topic pursued by Gerhard Schmidt and co-workers resulted in the formulation of topochemical rules for solid state photodimerization reactions. 3–5 However, in the absence of a thorough understanding of the crystal engineering principles, con- trolling the regioselectivity during solid state photodimerizations remains a difficult problem. Following their extensive studies on the photocycloaddition of enones to olefins de Mayo and co- workers approached this problem from the solution chemistry perspective. 6–8 They were able to achieve modest control on the process using a micellar medium. The molecules were aligned at the interface between the hydrophobic interior and hydrophilic exterior of a micelle. Similar attempts by Whitten and co- workers to obtain selective dimers from stilbazoles in conventional micellar media were not successful. 9 Based on their observations in micellar media with stilbazoles they concluded that “merely orienting chromophores at a hydrophilic–hydrophobic interface are insufficient to force bimolecular interactions”. 9 In contrast to normal micelles, when Whitten and co-workers employed a reverse micelle with much smaller size as the reaction medium syn HH dimer was obtained from stilbazole. 10 Whitten et al. speculate that the well ordered and more constrained water pool in reverse micelles is able to provide better orientation than conventional micelles. The above results of de Mayo and Whitten’s groups suggest that the hydrophobic–hydrophilic interface could be used to orient molecules but the reaction space must be more rigid and less dynamic than a conventional micelle. During the last decade approaches based on the principles of supramolecular chemistry, i.e., controlling chemistry through weak interactions Department of Chemistry, University of Miami, Coral Gables, FL, 33124, USA. E-mail: murthy1@miami.edu † This paper was published as part of the special issue in honour of the late Professor George S. Hammond. ‡Dedicated to the memory of Professor G. S. Hammond, a pioneer in photochemistry, a great teacher and an outstanding individual. (e.g., hydrogen bonding, electrostatic interactions, etc.), have been pursued both in solution and solid state with moderate success. 11–23 The above studies led to the understanding of the basic features needed to control the photodimerization process: the two olefins must be pre-aligned within a reactive distance and they should possess very little freedom during their excited state lifetime. Continuing our interest in ‘controlling chemistry through confinement in aqueous medium’ we have explored the use of organic hosts to control regioselectivity during the photodimerization of olefins. We present below our results with water-soluble calixarenes as templating nano reaction vessels (Scheme 1). In this study stilbazoles, whose photochemistry has been investigated in solution, 24–29 are employed as models to explore the use of calixarenes as reaction vessels (Scheme 2). As presented in the next section water-soluble calixarenes were able to template stilbazoles to yield selectively anti-head–tail (anti-HT) dimers and permit photoreactions to be conducted in water. 30 Scheme 1 Scheme 2 This journal is © The Royal Society of Chemistry and Owner Societies 2006 Photochem. Photobiol. Sci., 2006, 5, 925–930 | 925