Crown ether formation

PEDERSEN Crown Ethers Crown ether formation and its use in substitutions, oxidations,etc... 

Whereas Gold and Sghibartz showed that cation complexation depressed the rate of crown-ether disrupture, there is convincing evidence that crown ether formation is facilitated by the presence of cations. The template effect, presumably due to complexation of the open-chain precursor and formation of a crown-type conformation, clearly emerged from studies in which the yield of crown ethers was related to the type of cations present (Reinhoudt et al., 1976). Kinetic evidence for the template effect was presented by Mandolini and Masci (1977), who showed that the rate of cyclization of the precursor of benzo-18-crown-6 [2061 decreased in the order Ba2+ > SrJ+ > K+ > Na+ > Li+. This sequence is the same as the one found for the stability constants of the 1 1 complexes of these cations with 18-crown-6 in water (Table 3). 

Rhenium Ores Benzene UV-Vis 4 mg kg 1 (dry basis) C-FLA membrane for phase separation prior crown ether formation for Mo(VI) separation [480]... 

Many of the crown ether syntheses with which we are concerned in this book are one form or another of the Williamson ether synthesis. Although the simplest example of such a reaction would involve an co-haloethylene glycol oligomer which undergoes intramolecular cyclization, it is more common for two new bonds to be formed in crown syntheses. An early example of the formation of a crown by a double-Williamson can be found in Dale s synthesis of 18-crown-6. The rather obvious chemical steps are shown in Eq. (2.1). 

Although cesium salts appear to be excellent reagents for the formation of crown ethers, it should be noted that such salts are used at least in equivalent amounts or in excess (4 eq.) . These salts are relatively expensive and will probably be reserved for use in the synthesis of less readily accessible crowns. 

Most of the compounds in this class have been prepared from preexisting crown ether units. By far, the most common approach is to use a benzo-substituted crown and an electrophilic condensation polymerization. A patent issued to Takekoshi, Scotia and Webb (General Electric) in 1974 which covered the formation of glyoxal and chloral type copolymers with dibenzo-18-crown-6. The latter were prepared by stirring the crown with an equivalent of chloral in chloroform solution. Boron trifluoride was catalyst in this reaction. The polymer which resulted was obtained in about 95% yield. The reaction is illustrated in Eq. (6.22). 

Furthermore, the molecular size of the Li+ -solvating solvents may affect the tendency for solvent co-intercalation. Crown ethers [19, 152-154, 196, 197] and other bulky electrolyte additives [196] are assumed to coordinate Li+ ions in solution in such a way that solvent co-intercalation is suppressed. The electrochemical formation of binary lithiated graphites Li tC6 was also reported for the reduction... 

Salts of diazonium ions with certain arenesulfonate ions also have a relatively high stability in the solid state. They are also used for inhibiting the decomposition of diazonium ions in solution. The most recent experimental data (Roller and Zollinger, 1970 Kampar et al., 1977) point to the formation of molecular complexes of the diazonium ions with the arenesulfonates rather than to diazosulfonates (ArN2 —0S02Ar ) as previously thought. For a diazonium ion in acetic acid/water (4 1) solutions of naphthalene derivatives, the complex equilibrium constants are found to increase in the order naphthalene < 1-methylnaphthalene < naphthalene-1-sulfonic acid < 1-naphthylmethanesulfonic acid. The sequence reflects the combined effects of the electron donor properties of these compounds and the Coulomb attraction between the diazonium cation and the sulfonate anions (where present). Arenediazonium salt solutions are also stabilized by crown ethers (see Sec. 11.2). 

Quinolizinium and other fused pyridinium salts are formed when a-methylheterocycles react with 2,4,6-triphenylpyrylium, which thus behaves as a C3-synthon <96MC99>. Pyrylium salts also feature in a stereocontroUed route to conjugated dienynes which has led to a synthesis of Carduusyne A, a marine metabolic fatty acid <96TL1913> and in the formation of pyridinium containing crown ethers <96LA9S9>. 

O- versus C-a kyIation product ratios in the methylation of desoxybenzoin by dimethyl sulphate can be varied between 0.75 and 63 by the choice of catalyst. The reaction can be steered towards enol-ether formation by large, sterically shielded ammonium ions, while C-alkylation is favoured by small ammonium ions (e.g. RMejN" ) and by crown ethers (Dehmlow and Schrader, 1990). 

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