Amide-based receptors

8-tt Kang, S. O., Begum, R. A. and Bowman-James, K., Amide-based ligands for anion coordination , Angew. Chem. Int. Ed. 2006, 45, 7882-7894. 

The secondary amide group, -C(0)NHR, is ubiquitous in protein structures where hydrogen bonding from the amide NH group to the carbonyl oxygen atom is responsible for much of the secondary structure of proteins such as a-helices and i-sheets. The electron withdrawing effect of the carbonyl oxygen 

The range and versatility of amide anion binding hosts is very extensive. Macrocyclic amides have also been reported, such as 4.42 which forms a remarkable sandwich compound with sulfate in the solid state. Sulfate binding is enhanced by protonation of the tertiary amine groups and the host is thus a hybrid between amide and ammonium binding sites. The triethylbenzene scaffold has been used to prepare 

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Hughes, M. P., Smith, B. D., Enhanced carboxylate binding using urea and amide-based receptors with internal Lewis acid coordination A cooperative polarization effect. J. Org. Chem. 1997, 62, 4492—4499. 

Although the works concerning amide-based receptors for anions are rather rare, several very interesting publications dealing with this topic can be found. One review on peptide and glycocalixarenes has recently appeared [63]. 

The binding affinity of amide-based receptors can be increased by appending Cr(CO)3 groups to the phenyl rings of the original cleft, as in 2.62, and their organometallic derivatives bind Cl with > 10 M in acetonitrile solution. 

The ability of the heteroditopic tris-urea and tris-amide-based receptors XCU and XCA to recognize an ammonium ion simultaneously to an anion was investigated by NMR spectroscopy in chloroform [24, 37]. In aU cases, the complexation of the ammonium ion only proceeds when an anion is simultaneously bound in the H-bonding donor cap (Fig. 6.6). This positive cooperativity is due to the close... 

In early 1980s, chemists started to develop amide-based receptors for selective recognition of anions. In 1986, Pascal and co-workers reported a pioneering work about the synthesis and X-ray crystal structure of an amide-based cryptand 1 [12]. Although it was not certain that fluoride was contained within its cavity, and NMR studies suggested that 1 could bind to fluoride through H-bonding. 

As a seminal contribution to anion recognition, in 1997, Crabtree and co-workers reported the synthesis and anion-binding properties of isophthalamide-linked bidentate amide-based receptors 4a,b [21]. Crystal stmcture of 4a Br showed 1 1 complexation with Br (Fig. 5.2). In the stmcmre, the bromide was held with two H-bonds by the amide NH groups at N Br distances of 3.436 and 3.634 A, and N-H—Br angles of 116 and 172 . The isophthalamide spacer could be replaced with a 2,6-diamidopyridine unit to give 4b, which was selective for fluoride anion in CD2CI2 ( Ta = 2.4 x 10 M ) [22]. 

Jurczak and co-workers reported cyclic and acyclic amide-based receptors 9a-e, 10 and 11 [33, 34]. The crystal stmctures of 9a F and 9a-Cl indicated that the anion was coordinated by four H-bonds with N F distance ranging from 2.742 to 2.881 A and N C1 from 3.211 to 3.520 A, respectively (Figs. 5.6a and 5.7a). However, the fluoride ion was almost in the plane of the macrocycle (Fig. 5.6b), while the larger chloride hovered above the plane of the macrocycle (Fig. 5.7b). Solution-phase binding studies also proved that the smaller fluoride ion fit better to the macrocycle K = 830 M ) than chloride K = 65 M ). Base on these results, Kanbara et al. transformed amide 9a to thioamide 9f which was found to exhibit strong affinity toward anions [35]. Although acyclic receptors 10 and 11 have... 

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