Optical inversion, mechanism

The d5Tiamic stereochemistries of M(dtc)3 and [M(dtc)3] (M = Fe, Co, or Rh) complexes have been studied (315). The cobalt complex is non-rigid, but the mechanism of optical inversion could not be determined. The Rh complex is stereochemically rigid up to 200°. The optical inversion of (-l-)546 [Colpyr-dtcla] in chloroform has been studied, by loss of optical activity, by polarimetry (316). 

Mislow and co-workers (258) and Hammond (259) have shown that optically active diaryl sulfoxides, which are configurationally stable in the dark at 200°C, lose their optical activity after 1 hr at room temperature on irradiation with ultraviolet light. Similarly, an easy conversion of the trans isomer of thianthrene-5,10-oxide 206a into the thermodynamically more stable cis isomer takes place upon irradiation in dioxane for 2 hr. However, the behavior of a-naphthylethyl p-tolyl sulfoxide under comparable irradiation conditions is different, namely, it is completely decomposed after 4 min. These differences are not surprising because the photochemical racemization of diaryl sulfoxides occurs by way of the pyramidal inversion mechanism whereas decomposition of the latter sulfoxide occurs via a radical mechanism with the cleavage of the sulfur-carbon bond. It is interesting to note that photoracemization may be a zero-order process in which the rate depends only on the intensity of the radiation and on the quantum yield. 

A net C—O to C—C bond chirality transfer has been demonstrated in the preparation of a cyclopropane (equation 344).428 Another C—O to C—C chirality transfer was accomplished starting with an optically active alkylidenetetrahydrofuran 429 Not only was complete stereospecificity via a double inversion mechanism observed, but, in addition, regioselectivity had to be exercised to avoid the alternate cycliza-tion to give a seven-membered ring product (equation 345). 

Stefan Balint, Analysis and Numerical Computation of Solutions of Nonlinear Systems Modeling Physical Phenomena, Especially Nonlinear Optics, Inverse Problems, Mathematical Materials Science and Theoretical Fluid Mechanics, Proceedings of a conference held 19-21 May 1997, in Timisoara, Romania, University of the West Timisoara, Timisoara, Romania, 1997. 

Any detailed description of the mechanism of an octahedral substitution must also account for the stereochemical changes that accompany reaction. Werner recognized this and made use of it in his discussions of the stereochemistry of reactions of cobalt(III) complexes. The available experimental results can be explained on the basis of possible molecular rearrangements and some cautious predictions can even be made. The base hydrolysis of cobalt III)ammines appears to be unique in that it often occurs with rearrangement it also affords the few known examples of optical inversion. These results can be explained by formation of a 5-coordinated species with a trigonal bipyramidal structure. Optically active metal complexes racemize by either an intramolecular or an in-termolecular process. Substitution reactions of platinum metal complexes often occur with retention of configuration. 

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... 

Figure 6. Conjugate base mechanism for the optical inversion of D -cis-[Co en)2CU] into Jj cis-[Co en)2ClOHY via water attack of B) at Zl,3...

Recent studies (32) show that the base hydrolysis of D -a-[Co(trien)OHCl] also results in a net optical inversion reaction (Equation 10). On the basis of a conjugate base-dissociation mechanism this re-... 

Figure 7. Conjugate base mechanism for the optical inversion of [Co(trien(OCHl]- into ij p [Co(trien)(OH)2] by water attack of (B) at A 1,3] trien = NH2C2H HC2H HC2HJ H2...

On the other hand, when optically active 2-octyl triflate (197) was hydrolyzed in the solubilized state with surfactant concentrations greater than their CMC, the stereochemical course changed to net retention (ca. 48% for CTAB, and ca. 27% for NaLS)193 The rate constants, very much lower than those in the absence of micelles, indicate that the reaction proceeded in the micelles. The different behavior of (196) and (197), particularly in the presence of cationic micelles (CTAB), may be due to the greater efficiency of triflates in producing carbocations. The comparison of (196) and (197) then suggests that the solvation model of micellar stereochemical control applies to unassisted solvolyses whereas the double inversion mechanism operates with leaving groups which invite participation of a nucleophile. 

A modified Mitsunobu procedure in which 63 is first treated with the preformed complex 68 (prepared by reaction of triphenylphosphine and diisopropyl azodicarboxylate) and then cesium thioacetate leads to significant racemization [17]. However, if the free acid is reacted instead with an appropriate thioacid (rather than the ester and a cesium salt), optical yields improve significantly. Thus, thioacetylation of (S)-l can be accomplished by treating it with 68 followed by the addition of thioacetic acid in THF to provide in 48% yield (5)-2-(acet-ylthio)-2-phenylacetic acid (69) with 84% ee after recrystallization. The low yield is due in part to the unavoidable formation to the extent of at least 50% of a viscous, polymeric material. The reaction is complete in minutes, however, and proceeds with retention of configuration. Presumably this is a result of a double inversion mechanism that passes through an a-lactone. Interestingly, the corresponding reaction with lactic acid does occur with inversion [18]. 

Since the time of Werner, chemists have been fascinated by optical isomers of octahedral chelates with bidentate ligands and have puzzled over the possible mechanisms by which one isomer undergoes optical inversion to form its mirror image. Several possible mechanisms have been advanced, some requiring the rupture of one or more bonds plus bond reorganization, and others involving only internal reorganization of the bonds without bond rupture. In the Thomas mechanism, the bidentate... 

The case is selected in which the ligands are unsymmetrical so as to also show whether geometrical isomerization accompanies optical inversion. The twist mechanisms have been suggested by some authors as possible modes of racemization. For some jS-diketonates and other chelates inferential evidence has suggested that isomerizations occur principally by bond rupture plus bond reorganization rather than by bond reorganization without bond... 

The same nucleophilic substitution at the asymmetric Si atom takes place by an inversion mechanism called Sii 2-Si, similar to the organic S, 2 reaction and a retention mechanism called S -Si, rare in organic chemistry (Scheme 7, bottom) [1]. Thus, C and Si already behave differently. With the help of the new optically active transition metal compounds the stereochemical course of reactions of organometallic compounds can be studied. In Scheme 8 an overview shows that retention reactions, inversion reactions and racemization reactions of various types have been observed [12]. 

The stereochemical rigidity of NN-disubstituted dithiocarbamate complexes, [Cr(R R -dtc)3], has been established by n.m.r. in chloroform solution at temperatures up to 357 K. The relative rates of rearrangement by a trigonal-twist mechanism (resulting in optical inversion) have been established for a series of tervalent metal ions and the order is Ga, In, V > Mn > Cr. These results are discussed further in the next section. 

Typical conditions for these processes are simply to stir an aqueous solution of the metal salt of Y with the organic substrate alone, or in a solvent such as chloroform or benzene, in the presence of 5—10 mole % of a quaternary ammonium salt as catalyst. A recent example is to be found in a synthesis of alkyl azides from alkyl iodides (or other halides) using commercially available Aliquat 336 [mainly (1)] as catalyst, and a reaction temperature of 100 °C. The conversion of alkyl methanesulphonates to alkyl halides has been used to synthesize optically active secondary fluorides, chlorides and bromides via an 5 n2 inversion mechanism (the iodides racemize before isolation). Ammonium salt (1), or phosphonium salt (2), are used to catalyse these mesylate displacements. ... 

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