New calix [4]pyrroles bearing dipyrrolylquinoxaline as strapping elements have been synthesized and characterized by spectroscopic means. The binding behavior of these receptors at 25 °C was investigated first by proton NMR spectroscopy in CD 3 CN/DMSO-d 6 (9:1 v./v.), as well as by UVvis spectroscopy and isothermal titration calorimetry (ITC) in CH 3 CN/DMSO (97:3 v./v.). The receptors displayed a selective colorimetric response when exposed to the fluoride, dihydrogen phosphate, and acetate anions (studied in the form of the corresponding tetrabutylammonium salts) and an enhanced affinity as compared to a comparable calix [4]pyrrole system lacking the dipyrrolylquinoxaline-containing strap. KeywordsStrapped-calix [4]pyrrole; anion sensing; dipyrrolylquinoxaline; colorimetric sensor Motivated in part by an increasing appreciation of the importance of anions in the environment and biology, considerable continues to be devoted to the design and synthesis of anion receptors possessing high affinity and selectivity. 1,2 A variety of systems that are capable of recognizing anions are now known, and most general classes have been discussed at some length in a variety of recent reviews. 3,4 Much of our own work of late has focused on a set of modified calix [4] pyrrole-based receptors known as strapped-or capped-calix [4]pyrroles. These modified calix [4]pyrrole derivatives generally display affinities and selectivities towards anions that are significantly enhanced relative to simple calix[4]pyrroles. They thus offer the attractive possibility of tuning the anion recognition properties of this well-studied class of receptors. 5-9 However, to be useful in certain applications, the augmented affinity of the strapped calix[4]pyrroles would need to be coupled with a "read out" element that would allow the anion binding event to be followed easily. Within the context of this general goal, we are particularly interested in developing strapped calix [4]pyrrole-based receptors that are capable of producing a colorimetric response upon anion recognition. In this paper, we wish to report the design, synthesis, and anion binding properties of calix [4]pyrroles that contain dipyrrolylquinoxalines as a part of the strap as shown in Scheme 1. These systems, which produce an easy-to-see visual response in the presence of certain anions, offer a further potential advantage in that they possess additional pyrrole subunits on the strap that are expected to interact with the chhlee@kangwon.ac.kr. quinoxaline chromophore via a variety of second order interactions, including through conjugation and anion-pi effects. NIH Public AccessThe synthesis of receptor 4 starts with ketone 1, a species that was prepared by the reaction of oxalyl chloride with two equivalents of 2-(3-oxobutyl)pyrrole. 11 Once in hand, 1 was reacted with 1,2-diamino-4-nitro-benzene in the presence of acid to afford bisketone 2 in 24% yield. 10 Treatment of this latter intermediate with neat pyrrole in the presence of trifluoroacetic acid afforded the bis-dipy...
The synthesis and characterization of a new sapphyrin analogue, dioxabenzosapphyrin, are reported. The benzodifuran moiety upon which this system is based leads to the incorporation of two oxygen atoms within the central macrocyclic core, thus replacing two protonated nitrogen centers found in normal pentaaza sapphyrin derivatives, including those derived from benzodipyrroles. As expected, the loss of these two NH hydrogen bond donor sites greatly reduces the anion affinity for the diprotonated form, even though the overall charge is the same as in the corresponding sapphyrins. Interestingly, dioxabenzosapphyrin, but not the corresponding all-aza systems, is found to bind neutral guests, such as phenol and 4-nitrophenol, albeit weakly. This latter finding highlights a potentially new application for core-modified expanded porphyrin derivatives, namely, as receptors for hydrogen bond donating neutral substrates.
New core-modified, meso-alkylidenyl porphyrinoids bearing multiple exocyclic double bonds were synthesized and characterized. The synthesis was accomplished using a typical "3 + 1"-type condensation approach. Stable exocyclic tautomers bearing double bonds at the meso positions, as well the corresponding endocyclic tautomers, were isolated in the case of both thiabenziporphyrin and thiapyriporphyrin products prepared in the course of this study. On the other hand, only the exocyclic tautomer was isolated in the case of the congeneric oxapyriporphyrin and oxabenziporphyrin. Expanded analogues of the exocyclic forms of oxabenziporphyrin and thiabenziporphyrin were also isolated as minor products. A single-crystal X-ray diffraction analysis of the expanded thiabenziporphyrin (20) revealed that all four pyrrole rings displayed an inverted geometry, presumably reflecting the strong hydrogen-bonding extant between the pyrrole N-H proton and the carbonyl group of the malonate moiety in the solid state. On the other hand, the expanded oxabenziporphyrin (14) was found to possess a severely distorted geometry with only one pyrrole ring being inverted. Careful analysis of the structure revealed that the solid-state geometry of the expanded macrocycles correlates well with the internal angle defined by the 2- and 5 substituents and the centers of the furan (14) or thiophene (20) subunits.
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