Crown complexes

This work was later expanded to include a larger crown ether system functionalized with either the above-mentioned cyclene macrocycle (2), two cyclene macrocycles (4), or a triazacyclononane macrocycle (3) [32]. Unfortunately, the Cu-32+ complex did not display any appreciable affinity for the phosphate species (NaH2P04, KH2P04, sodium glycerophosphate, or disodium 4-nitrophenylphosphate). 

A tetraamidinium functionalized, bowl-type cavitand (receptor 8) was developed by Diederich and Sebo [47]. This receptor was found to complex 1,3-dicarboxylate anions with good selectivity and a 1 2 binding stoichiometry both in CD3OD and D20, as revealed by standard Job plot analysis. In contrast, various nucleotide phosphates were found to be bound with a 1 1 stoichiometry in D20. In the case of the adenosine phosphates, the association constants increased as a function of nucleotide charge [i.e., the affinity order (K., M-1) was cAMP (1400) AMP (10000) ADP (48 700) ATP (660000) in pure D20], This charged receptor (8) also showed moderate selectivity towards AMP (Ka = 10 000 M-1) over other nucleotide monophosphate anions, such as GMP (FCa = 5200 M-1), CMP (fCa = 3500 M-1), TMP (K l — 5900 M ), and UMP (Ka-3800M ) in D20 containing TRIS buffer (2.5 mM, pH 8.3). 

The doubly functionalized calix[4]arene 9, bearing two chiral arms , was prepared recently [48]. This neutral system proved not only to be a good receptor for a-phenylglycine anions, it was also found to display good enantioselectivity towards the L-isomer fCa(L)// l(D) = 4.76. It was thus proposed by the authors, that this presumably preorganized receptor could function as a chromogenic sensor for nonracemic a-phenylglycine anions. 

Another strategy for functionalizing calixarenes is to strap the macrocydes. Several systems bearing a diamido strap have been prepared and have shown a selectivity for acetate anions (e.g., receptor 10 Ka=103M 1 in CDC13) [49]. Similar selectivity preferences were observed for thiacalix[4]arene receptors functionalized with four ureido or thioureido units on the lower rim [50]. 

Recently, the effect of two proximate cations on anion-binding was investigated [53], Toward this end, the basic calix[4]arene framework was functionalized on the lower rim with two methyl ester units and in two separate positions with two urea-linked bipyridine-functionalized polyether units to give 13. This receptor was designed to allow for the coordination of hard metal cations via the ester groups, as 

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Nucleophilic substitution reactions of halide anions in aprotic solvents are often accompanied by elimination reactions. For instance, reactions of secondary alkyl halides with potassium fluoride solubilized in acetonitrile with the aid of 18-crown-6 [3] give olefins as the main reaction product (Liotta and Harris, 1974). Similarly, the dicyclohexyl-18-crown-6 complex of potassium iodide acted exclusively as a base in its reaction with 2-bromo-octane in DMF (Sam and Simmons, 1974). The strongly basic character of weakly solvated fluoride has been exploited in peptide synthesis (Klausner and Chorev, 1977 Chorev and Klausner, 1976). It was shown that potassium fluoride solubilized... 

Aromatic halides react with crown ether-complexed K02 by an electron-transfer mechanism and not by nucleophilic attack, as was shown by Frimer and Rosenthal (1976) using esr spectroscopy. The corresponding phenol is the main reaction product (Yamaguchi and Van der Plas, 1977). Esters are saponified by the K02/18-crown-6 complex in benzene, presumably by an addition-elimination pathway (San Fillippo et al., 1976). The same complex has been used to cleave cr-keto-, or-hydroxy-, and or-halo-ketones, -esters, and -carboxylic acids into the corresponding carboxylic acids in synthetically useful quantities (San Fillippo et al., 1976). 

Reactions.—Aldehydes. The stereochemistry of the alkene produced from ylides generated by using 18-crown-6 complexes of potassium carbonate or butoxide, depends upon the solvent used. In THF typical salt free distributions are obtained whereas in dichloromethane reversal of product distributions is observed.17 A simplified method (Scheme 4) for preparing para-substituted styrenes in high... 

In 1981 we reported (2, 3) the first examples of free radical polymerizations under phase transfer conditions. Utilizing potassium persulfate and a phase transfer catalyst (e.g. a crown ether or quaternary ammonium salt), we found the solution polymerization of acrylic monomers to be much more facile than when common organic-soluble initiators were used. Somewhat earlier, Voronkov and coworkers had reported (4) that the 1 2 potassium persulfate/18-crown-6 complex could be used to polymerize styrene and methyl methacrylate in methanol. These relatively inefficient polymerizations were apparently conducted under homogeneous conditions, although exact details were somewhat unclear. We subsequently described (5) the... 

Fig. 4-6. X-ray analyses of [18]crown-6 complexes with dimethyl acetylene di-carboxylateU), malonodinitrilen), and benzenesulfonamide 13). In the latter case the crown is in a conformation intermediate between A and B (Fig. 3e in 8)...

The [18]crown-6 (/) complexes with dimethyl acetylene dicarboxylate (1 1) U), malo-nodinitrile (1 2) 12), and benzenesulfonamide (1 2)13) were the first to be revealed by X-ray structural analyses. Figures 4-6 show the stereochemistry of these neutral complexes within the crystal1. In the first two complexes the CH3 and CH2 groups... 

Fig. 12. Structure of 4-nitro-l,2-benzenediamine [18]crown-6 complex 22) a a perspective view, b as viewed almost perpendicular to the plane of the crown, hydrogen bonds marked with dotted lines...

Dibenzo[ 18]crown-6 and [18]crown-6 Complexes with [trans-PtCh (PMe2) NHi) ]... 

Fig. 23. a the 1 2 complex between [18]crown-6 and /trans-PtCh PMe3)NHi] 35 1. b Schematic representation of the three bifurcated hydrogen bonds in the 1 1 [trans-PtCl2(PMe3)NH3] dibenzo[18]crown-6 complex 35 >. 

Figure 3 Crystal structures of (a) [18]crown-6 complex of benzylammonium thiocyanate (b) pyrido[18]crown-6 complex of r-butylammonium perchlorate (reproduced with permission from 80JCS(P2)1529 and 79AX(B)2233)...

Table 3 [18]Crown-6 Complexed Alkali Metal Fluoride Reactions (74JA2250)...

Acetonitrile, phenyl-, 55,91, 94 Acetonitrile-18-crown-6 complex, 57, 31 Acetophenone, 58, 57, 61 ACETOPHENONE, 2-(2-ACETAMIDO-ETHYLE4,5-DIMETHOXY-, 56, 3 Acetophenone, 2-(2-acetamidoethyl)-4,5, 6-trimethoxy-, 56, 7 Acetophenone, 4-chloro, 55, 40 Acetophenone, 4-chloro-, oxime, 55, 39, 40 7-Acetoxy-4,4,6,7-tetramethylbicyclo-[4 2 0] octan-2-one, 57, 113 Acctylacetone, 58, 52, 56 Acetyl chloride, a rert-butyl-o-cyano-, 55, 38... 

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