Synthesis of cesium selective pyridyl azocalix[n]arenes Har Mohindra Chawla, * Suneel Pratap Singh and Shailesh Upreti Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India Received 9 September 2005; revised 2 December 2005; accepted 5 January 2006 Available online 2 February 2006 Abstract—A series of pyridylazo calix[n]arenes (nZ4, 6, 8) including the first examples of mixed hetroaryl azocalix(n)arenes have been synthesized by coupling calix[n]arenes with diazonium salts derived from amino pyridines. It has been observed that the coupling reaction of diazonium salt obtained from 3-aminopyridine with calix[n]arene gives tetrakis-, hexakis- and octakis (pyridylazo)calix[n]arenes (nZ4,6,8) while those derived from 4-aminopyridine give partially substituted (4-pyridylazo)calix[n]arene analogs. There is no reaction of calix(n)arenes with diazonium salts derived from 2-aminopyridine under identical conditions of experiments. The conformational analysis of synthesized compounds have been ascertained by detailed spectral measurements and single crystal X-ray analysis of 5-(3 0 -pyridylazo)- 25,26,27,28-tetrahydroxycalix[4]arene. A rational explanation for the observed partial and exhaustive coupling reaction in the synthesis of heteroaryl azocalix(n)arenes has been suggested. Preliminary evaluation of synthesized derivatives as molecular receptors for metal ions indicates that they have good potential to function as selective ionic filters for cesium ions. q 2006 Published by Elsevier Ltd. 1. Introduction Calix[n]arenes (nZ4–20, where n is the number of phenyl units, Fig. 1a) are phenolic [1n]-metacyclophanes that can be easily obtained by base or acid-catalyzed condensation of p-substituted phenols with formaldehyde or paraformalde- hyde. 1 They are known to provide useful building blocks for hollow molecular scaffolds with easily functionalizable hydrophilic and hydrophobic lower and upper rims, respectively. 2–4 Introduction of azo groups into the calix[n]arene framework confer chromogenicity that can be employed for the development of molecular diagnostics and sensor materials for metal ions and organic molecules. 5–7 For example, typically substituted azocalix[n]arenes (Fig. 1b) have been studied by various researchers in the recent past for their use in ionic and molecular recog- nition. 8,9 However, despite such studies, predictive infor- mation on the usefulness of coupling reaction between calix[n]arenes and diazonium salts to provide regioselective azo substituted calix[n]arenes is very limited. For example, it has been reported that the reaction of calix[4]arene with substituted benzene diazonium flouroborate in the presence of pyridine provides tetrakis(phenylazo)calix[4]arene 10–11 in good yield but the same reaction with benzene diazonium salt derived from aniline gives a very poor yield of the related product. Since the base-catalyzed diazotization reaction involving diazo flouroborates usually led to the formation of tetrakis(arylazo)calix[4]arenes, Shinkai et al. ascribed this outcome to autocatalysis of the coupling reaction and suggested that deprotonation of a phenolic group by the basic solvent facilitates the reaction through hydrogen bond assistance. 12 Whether such an influence of hydrogen bonds is operative in all diazocoupling reactions is as yet a matter of speculation. During recent years, several reports have appeared in the literature 13–15 wherein the synthesis and characterization of partially substituted azo calixarenes have been utilized to elicit photoresponse to ionic or molecular recognition events. Differential observations and outcome of the coupling reaction of calix[n]arenes with diazonium salts derived from different aryl amine structures do not seem to have been explained. In the present work, we have attempted to understand the diazocoupling reaction of calix[n]arenes to rationally obtain chromogenic molecular filters that may be selective for metal ions and organic substrates. The choice of diazonium cations derived from aminopyridines 16,17 was 0040–4020/$ - see front matter q 2006 Published by Elsevier Ltd. doi:10.1016/j.tet.2006.01.022 Tetrahedron 62 (2006) 2901–2911 Figure 1. (a) p-tert Butyl calix[n]arene nZ4–20, (b) p-substituted azo calix[n]arene nZ4,6,8. Keywords: Calix[n]arenes; Diazotization; Cesium; Hydrogen bond. * Corresponding author. Tel.: C91 11 26591517; fax: C91 11 26591502; e-mail: hmchawla@chemistry.iitd.ernet.in