Crown ethers phenolic

Organic acids, phosphates and phosphonates amino acids, aminocarboxylates, (aza)crown ethers, phenols, heterocyclic and aromatic compounds... 

Chiral crown ethers possessing two chiral cw-l-phenylcyclohexane-l,2-diol moieties as well as a p-(2,4-dinitrophenylazo)phenol chromophore were prepared and with chiral alkylamines were observed to show enantiomeric selectivity <96JCS(P1)383>. 

The 1,3-xylyl trick was also used for the incorporation of phenols into crown ethers. Three classes of phenols [46a]-[46c] have been investigated. They differ by their substituents in the 4-position. In Table 22 the pX a values of different macrocyclic phenols [46a]-[46c] are compared. The data obtained for the phenol-containing crowns [46a] and [46b] show very little evidence for a macrocyclic effect. No extra stabilization of the protonated (acidic) form by a macrocycle of appropriate ring size was found. The acidities of the macrocyclic phenols [46a] and [46b] were independent of the ring size and comparable to non-macrocyclic analogues. However, the azo-substituted crowns [46c] showed a difference of 0.8 pACg units which was not expected from the pAfa values of [46a] and [46b]. TTiis different behaviour of [46c] is not yet understood. 

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

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). 

General. Toluene, chlorobenzene, and o-dichlorobenzene were distilled from calcium hydride prior to use. 4-Dimethylaminopyridine (Aldrich Chemical Co) was recrystalled (EtOAc), and the other 4-dialkylaminopyridines were distilled prior to use. PEG S, PEGM s, PVP s, and crown ethers were obtained from Aldrich Chemical Co., and were used without purification. BuJ r and BU. PBr were recrystallized (toluene). A Varian 3700 VrC interfaced with a Spectraphysics SP-4000 data system was used for VPC analyses. A Dupont Instruments Model 850 HPLC (also interfaced with the SP-4000) was used for LC analyses. All products of nucleophilic aromatic substitution were identified by comparison to authentic material prepared from reaction in DMF or DMAc. Alkali phenolates or thiol ates were pre-formed via reaction of aqueous NaOH or KOH and the requisite phenol or thiophenol in water under nitrogen, followed by azeotropic removal of water with toluene. The salts were transferred to jars under nitrogen, and were dried at 120 under vacuum for 20 hr, and were stored and handled in a nitrogen dry box. 

The distribution ratios obtained were compared to the corresponding distribution ratios between water and Toctanol. The authors also calculated solvent parameters of [C4Cilm][PFg] (Section 9.3). It was shown that phenolate-ion associates with [C4CiIm][PFg] more strongly than other ions. The authors also mention the possibility of extraction of amino acids into [C4CiIm][PPg] in the presence of crown ether dibenzo-18-crown-6, though at rather moderate efficiency. 

Changes in the electron density of crown ether oxygen atoms greatly affect the stability constants for cation complexation. The electron density on phenol ether oxygens is less than that on alkyl ether oxygens consequently dibenzo[18]crown-6 (2) shows weaker complexing ability than [18]crown-6 (1) and hexabenzo[18]crown-6 does not complex cations at all (80APO(17)279). 

The yield of crown ether can be increased to more than 80% by using a longer stepwise route in which the initial catechol is partially protected. Reaction of the phenol (34) with bis(2-chloroethyl) ether (33) forms intermediate (35) which after deprotection can react with a further molecule of bis(2-chloroethyl) ether to give dibenzo[18]crown-6. 

Apart from their obvious utility in separating mixtures of cations,68 crown ethers have found much use in organic synthesis (see the discussion on p. 363). Chiral crown ethers have been used for the resolution of racemic mixtures (p. 121). Although crown ethers are most frequently used to complex cations, amines, phenols, and other neutral molecules have also been complexed69 (see p. 133 for the complexing of anions).70... 

Quite unexpected turned out to be the recently discovered molecular nature of the solvates of the most electropositive metals, such asLiOMe 2MeOH [1521, 1643] (Fig. 4.6 b) and Mg(OMe)2 3.5MeOH [1524] (the single molecules being connected by the hydrogen bonding). The polymer structures have been observed for the [K( j.-OBu )(p- BuOH)] [352] and also the adducts of phenoxides with phenols, water, alcohols, and crown-ethers [482, 548, 858, 974, 1483, 1720] (see Table 12.1). 

The parent / -f-butylcalix[n] arenes (n = 4, 6, 8) are almost completely insoluble in water. However, their resemblance to crown ethers and spherands makes them interesting from the point of view of applications as phase transfer catalysts (Section 3.8.2). Table 3.20 shows the selectivity of calixarene 3.118 and its hexameric and octameric homologues for the extraction of various metal hydroxides into an organic receiving phase such as chloroform. Fortunately, in aqueous base the calixarenes are sufficiently soluble to act as phase transfer catalysts as a consequence of deprotonation of one of their phenolic hydroxyl groups. This solubility contrasts to [18] crown-6, which is much more effective in neutral solution. 

KI and KSCN are dissolved by 5.12 but without complexation of the boron atom. The stabilisation of the K+ ion by the crown ether moiety is apparently sufficient in these cases.12 Similarly, the 18-membered phenolic crown ether analogue can be metalated (= exchange of a proton for a metal ion in this case the OH proton) with trimethylaluminium to give 5.13 which forms a ditopic complex with LiCl in solution and in the solid state. 

Common guests Benzene, phenol Napthalene, l-anilino-8-napthalenesulfonate Anthracene, crown ethers. l-anilino-8-napthalene-sulfonate... 

Davis, B., Crown ether catalyzed derivatization of carboxylic acids and phenols with pentafluorobenzyl bromide for electron capture gas chromatography, Anal. Chem., 49, 832, 1977. 

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