Resolution, chiral cations

Many chemical reactions and processes yield cationic racemic products, and either a resolution or a stereoselective synthesis must be envisaged to obtain the chiral cations in an enantioenriched or enantiopure form. Resolution has been strongly studied [130] and selected representative examples of such processes mediated by chiral P( VI) anions are presented. 

The lipophilicity of the TRISPHAT anion 8 also confers to its salts an affinity for organic solvents and, once dissolved, the ion pairs do not partition in aqueous layers. This rather uncommon property was used by Lacour s group to develop a simple and practical resolution procedure of chiral cationic coordination complexes by asymmetric extraction [134,135]. Selectivity ratios as high as 35 1 were measured for the enantiomers of ruthenium(II) trisdiimine complexes, demonstrating without ambiguity the efficiency of the resolution procedure [134]. 

The chiral ( + )589-a-bromocamphor-7t-sulfonate ion ([BCS]-) has been widely used as a resolving agent for resolutions of cationic species.1 Here an improved and simplified method for the preparation of its ammonium salt is presented as well as a method for the synthesis and the resolution of the cis-[Cr(en)2Cl2] + ion. 

Fiedler, D., Leung, D.H., Bergman, R.G. and Raymond, K.N. (2004) Enantioselective guest binding and dynamic resolution of cationic ruthenium complexes by a chiral metal-ligand assembly. J. Am. Chem. Soc., 126 (12), 3674-3675. 

Lurie IS, Klein RFX, Dal Cason TA, LeBelle MJ, Brenneisen R, Weinberger RE (1994) Chiral resolution of cationic drugs of forensic interest by capillary electrophoresis with mixtures of neutral and anionic cyclodextrins. Anal Chem 66 4019-4026. 

The standard method for resolution of coordination complexes involve crystallization of salts with chiral anions such as tartrate, ((+)-tartrato)antimonate(IlI) [19], or tn s-(cysteinesulphinato)cobaltate [20], or rf-[Co(en)2(N02)2] as a chiral cation... 

The reported preparations of enantiomerically pure chiral iron-acyl complexes have relied upon resolutions of diastereomers. One route1415 (see also Houben-Weyl, Vol. 13/9 a, p 421) employs a resolution of the diastereomeric acylmenlhyloxy complexes (Fe/ )-3 and (FeS )-3 prepared via nucleophilic attack of the chiral menlhyloxide ion of 2 at a carbon monoxide of the iron cation of 1. Subsequent nucleophilic displacement of menthyloxide occurs with inversion at iron to generate the enantiomerically pure iron-acyl complexes (i>)-4 and (f )-4. 

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

Chiral crown ethers have been employed extensively (48-53, 56-60, 86, 90, 93-95, 107, no, 116, 117, 128, 143, 144, 152-155, 158-161, 163, 164, 212-227) for enantiomeric recognition of racemic primary alkyl ammonium cations including those associated with amino acid ester salts. Resolutions have been effected employing both bulk and chromatographic procedures. 

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... 

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