Cation optically active, resolution

Because the optically active forms of the (ethylenediamine)bis-(oxalato)cobaltate(III) ion have been, and will continue to be, excellent resolving agents for many cationic cobalt(III) complexes, it is only proper that a detailed resolution procedure be developed to produce both enantiomorphic forms in useful quantities. 

The idea of lone pairs was originated by W. J. Pope of Cambridge in 1900 who extended the concept of the three-dimensionality of carbon and nitrogen compounds to those of sulfur. His resolution of sulfonium cations RR R"S+ with three different substituents into optically active enantiomers suggested that these species were tetrahedral with an invisible substituent. The influence of these lone pairs can hardly be detected in transition metal compounds, but the situation is different for post-transition group central atoms such as Ge(II) As(III), Se(IV), and Br(V) with 30 electrons, In(I), Sn(II), Sb(III), Te(IV), I(V), and Xe(VI) with 48 electrons, and Au( —I), T1(I), Pb(II), and Bi(III) with 80 electrons (90). 

Diastereoisomer formation with the optically active complex Z-tris(l,10-phenanthroline)nickel(II) cation has been shown by Dwyer and Sargeson to provide a rapid general resolution method for trioxalato complexes of Co(III), Cr(III), and Rh(III). The method, with modifications, is given below. 

Tetrachloroauric(III) acid in aqueous ethanol reacted with an excess of the phosphine-arsine undergoing reduction and chelation to give the cation, LXVII, precipitated as the colorless, highly stable iodide. The 4-coordinate aurous atom in this compound has the tetrahedral configuration, and attempts were made to resolve the cation into optically active forms. Fractional recrystallization of eight salts having different optically-active anions failed to give any indication of resolution. 

The interaction of optically active CoNe trications with two chiral anions was investigated under different conditions of chromatographic resolution of enantiomers. Comparison of [Sb i(d-tart)2] and [As2(d-tart)2] dianions as potential chiral eluents indicated that the latter is the more efficient one [312]. The efficiency of [Sb2(d-tart)2] dianion increases when it is employed in the form of a salts with hydrophobic cations in reversed-phase ion-pair chromatography (RPIPC). 

Quinolizidine behaves as a tertiary cycloaliphatic amine, and its protonation leads to the quinol-izidinium cation (126). This basicity allows resolution of optically active quinolizidines with chiral acids. As an example, optical resolution of butaclamol analogues (127) was accomplished with (-1-)-and (— )-binaphthyl phosphoric acids <5lJA368l>. 

In 1966, [Co(en)3] Bfj was separated into its optically active enantiomers on a colunm of anion-exchange resin, which was in advance loaded with tartrate or anti-monyltartrate ions. The resolution was partial. In the same year, Brubaker et al. achieved the total resolution of a trinuclear cobalt(III) complex, hexakis(2-amino-ethanethiolato)tricobalt(III) bromide on a column of a cation-exchange cellulose (Bio-Rad Cellex CM) by eluting with 0.1 mol/dm NaCl. 

Thus, chromatography on Sephadex ion-exchangers is very effectively applied to multivalent complex cations. Jensen and Woldbye have reviewed the optical activity of coordination compounds resolutions of racemic octahedral transition metal complexes through both diastereoisomer and chromatographic techniques are summarized. 

This paper traces the development of optical activity in inorganic compounds to the point where Alfred Werner was able to use optical resolution to such striking effect in proving the octahedral stereochemistry of several metal ions by resolving chelated compounds, in particular the fully inorganic cation now called "Werner s hexol". This achievement came to fruition (1) in the years just prior to World War I. 

Following this line, a great variety of optically resolved tuptieally active) crown compounds were prepared for the resolution of racemic cationic substrates. 

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