Bailar inversion reactions

Substitution reactions of optically active metal complexes to yield reaction mixtures with some remaining optical activity usually owe this to the occurrence of optical retention reactions. Only five examples of optical inversion reactions have been reported, and all but one of these were discovered by Bailar 3) and his students. This is a somewhat surprising result because the types of rearrangements described above must also be involved in optical inversion reactions. In some of these cases the authors prefer to explain the mechanism of inversion on the basis of a trans-attack displacement 19, 33), One reason for this may be due to the erroneous assumption that a dissociation process could not provide a path... 

Even though the activation energy of this process may be less than that for other paths, a low frequency factor would allow other paths to compete at higher temperatures. This postulate would also explain the temperature dependence of the Bailar inversions. An alternate, oriented ion-pair mechanism has been suggested previously (P, 26 6S) but the observations with [CoCl2(en)2] in methanol ( 21 make this less probable because both the ion pair and the solvated ion appear to give trans and racemic cis isomers under the conditions studied. Some orientation effects would be expected for the ion-pair reaction if it were operative in the inversion reactions. 

Although the Bailar inversion (an octahedral complex substitution D L conversion, where d represents a right-handed screw relative to the C2 symmetry axis of an octahedral complex ion of the type cis-[Co(en)2Cl2] in which en = ethylenediamine, as shown in Fig. 1) was first reported by John C Bailar, Jr. and R. W. Auten in 1934, the exact mechanism of this type of reaction is still unknown. Originally, the reaction was considered to be due to the reaction of Ag2C03 with the [Co(en)2Cl2] ion... 

Most of the inversion reactions related to the above Bailar inversion have also been studied either by Bailar, his students, and his students students, or by the capable Australian chemists following in the footsteps of the late Frank Dwyer. Bailar has reviewed the contributions made at the University of Illinois at a Dwyer memorial lecture. ... 

The reaction between D -[Co(en)2Cl2] and ammonia also shows a Bailar inversion. Correlation of the appropriate optical rotatory dispersion curves and the temperature dependence of the optical rotatory dispersion... 

The involvement of conjugate base species in such base hydrolysis reactions must be considered in arriving at a mechanistic path for the Bailar inversion. And even though a d and l inversion can be accommodated by an unsymmetrical trigonal bipyramidal conjugate base intermediate, " the concentration and silver ion dependence of the Bailar inversion require other explanations. Four such possibilities can be imagined for the concentration dependent reaction. 

Circular dichroism has been used to study the base hydrolysis of the ri.-[Co(en)2-CI2I+ ion, a reaction which proceeds in a concerted way with Bailar inversion ... 

Concerted base hydrolysis and Bailar inversion of the /l-cis -[Co(en)2Cl2]+ ion has been established by a circular dichroism study [equation (18)]. Base hydrolysis of the /l-c/j-[Co(en)aCl(OH)]+ ion is reported to proceed with complete retention of configuration in contrast to an earlier investigation of this reaction. The results of these studies were considered previously. ... 

Optical inversions in the reactions of cobalt complexes. J. C. Bailar, Rev. Pure Appl. Chem.. 1966, 16, 91-101 (16). 

Werner realized the importance of absolute configuration in establishing reaction mechanisms he found (16) examples of reactions which occurred with apparent inversion. Bailar and his school (17,18) observed two cases which can only have involved inversion1). 

John C. Bailar, Jr and Robert W. Auten Optical inversion in reactions of cobalt complexes (inorganic Walden inversion)... 

Reaction Mechanisms. Bailar (2 3) studied the formation of [Co (en) 2003]" from optically active m-[Co(en)2Cl2] and found that products of opposite configurations were obtained under different conditions. He proposed a Walden type inversion for the substitution process. The configurations of the original complex and the products were related using optical rotatory dispersion curves. Dwyer (19) studied these reactions in detail and concluded that the inversion occurs through a trans displacement process involving both Ag+ and OH ... 

Closely related inversions were observed by Bailar and Peppard for the bromochloro and dibromo ions of the [Co(en)2XY] series. The rotations observed for the bromochloro reaction were similar to those reported earlier for the dichloro complex, and the one rotation reported for the dibromo complex ion reaction was appreciably more negative than any observed for the other starting materials. Insufficient details are available to draw definite conclusions for these reactions or for the corresponding inversion observed for optically active [Co([-]pn)2Cl2], where [-]pn = 1,2-diaminopropane, specifically the isomer which rotates 589 nm light to the left. As discussed later in this paper, definitive results for this reaction should be most informative. 

The base hydrolysis stereochemistry for D -a-cis- and L -j8-ds-[Co(trien)Cl2] and the corresponding chlorohydroxo ions, where trien = triethylenetetramine, have been investigated by Kyuno and Boucher in collaboration with Bailar. Whereas, the p isomers undergo base hydrolysis with complete retention of configuration, the a isomers show inversion during base hydrolysis. Furthermore, the products of the a isomer reactions were shown to be exclusively D -a-cis and L -)S-cis isomers. The rotations of the pure D -a-[Co(trien)(OH)2] ion ([a]o = +1950°), the pure l -) -[Co(trien)(OH)2] ion ([ajo = —700°), the reaction... 

Eishin Kyuno and J. C. Bailar, Jr., The Stereochemistry of Complex Inorganic Compounds. XXXI. Optical Inversions in the Reactions of Some Optically Active a-Dihalo-triethylenetetraminecobalt(III) Cations with Ammonia and Ethylenediamine, J. Am. Chem. Soc. 9S 1125 (1966). 

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