Interconversion of Isomers

The conversion of any optically active isomer to its enantiomorph can be carried out by a successive number of pseudorotations. Thus in Equation 13.115, five successive BPR, taking different equatorial arms as pivots, will interconvert the two enantiomorphs based on five different ligands. 

The same interconversion can be carried out with a larger number of successive TR. The spontaneous racemisation of a given enantiomer, without bond breaking, is thus possible via pseudorotation processes. In some cases, however, such pseudorotation may prevent the isolation of specific isomers. 

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Tautomerism, strictly defined, could be used to describe the reversible interconversion of isomers, in all cases and under all conditions. In practice, the term has increasingly been restricted to isomers that are fairly readily interconvertible, and that differ from each other only (a) in electron distribution, and (b) in the position of a relatively mobile atom or group. The mobile atom is, in the great majority of examples, hydrogen, and the phenomenon is then referred to as prototropy. Familiar examples are / -ketoesters, e.g. ethyl 2-ketobutano-ate (ethyl acetoacetate, 23), and aliphatic nitro compounds, e.g. nitro-methane (24) ... 

The usually tedious work of synthetizing the stereochemical probes is rewarded in most cases by information that is not obtainable by other means. However, the mechanistic interpretation of the results may not be simple as the data are sometimes obscured by side reactions, interconversion of isomers, and the multistep character of catalytic reactions. 

This is an unusual reaction, in that the energy of the products will be identical to the energy of the reactants, though the interconversion of isomers involves an activation energy that must be overcome by the application of heat. 

Dimethomorph. Dimethomorph (Figure 4.36) is effective against Oomycetes except Pythium spp. The compound, which is a cinnamic acid derivative, operates as a protectant but also has some curative activity that can be modified by means of formulation. Only the Z isomer is active but because of the rapid interconversion of isomers in the light, there is no practical advantage in its specific synthesis. 

The importance of equilibria in the chemistry of pentacoordinated phosphorus arises from the possibility of passing from a given structure to another, even if the latter has a totally different molecular geometry and distribution of electrons, by a mere shift of hybridization. This has important consequences on the chemical properties of phosphoranes and on the possibilities of interconversion of isomers by bond rupture and recombination processes akin to regular true stereomutations, i.e. without bond cleavage. 

The interconversion of isomers in the case of optical or geometric pairs, if structurally feasible, may only take place by the breaking of a- or 7c-bonds. However, there is a further area of stereoisomerism wherein the isomers are interconvertible by rotation about a single (conformational isomers or conformers. 

Alternative syntheses have been proposed, including the interconversion of isomers,162 by a number of academic and process groups.163-178... 

Valence tautomerism - This refers to the interconversion of isomers simply by reorganization of bonding electrons and without any accompanying rearrangement including proton migration. For examples see Sections 2.2.5.3, 2.4.54, and... 

Valence tautomerism refers to the interconversion of isomers without any accompanying rearrangement including proton transfer. Heterocyclic examples are essentially electrocyclic reactions (Scheme 15). If one of the isomers is colored and the position of the equilibrium can be changed by heat or light, the heterocycles can have useful thermochromic or photochromic applications. 

The relationship between the rate of interconversion of isomers and the existence of polymorphic modifications has been discussed by Dunitz [25b], Crystals of isomers that interconvert rapidly in solution would be classed as polymorphs whereas those of slowly interconverting isomers would be classed as different... 

Pentacoordinate centers are mainly exemplified by phosphorus, whereas some pentacoordinate sulfur derivatives exist, and a few other elements can be envisaged [36], In the case of phosphorus derivatives (phosphoranes), the trigonal-bipyramidal arrangement is the low-energy geometry, while interconversion of isomers occurs by pseudorotation through the tetragonal-pyramidal transition state [23,36]. 

Macrocycles 158 and 159 containing alternating thiophene and pyrrole rings are obtained by a Rothemund-type synthesis <2001CEJ5099>. The so-called tetrathiaoctaphyrin can be reduced to the dihydrotetrathiaoctaphyrin. Two dithiaporphyrin-like pockets behave as independent proton acceptors. Stepwise protonation leads to several cationic species. The interconversion of isomers in solution has been discussed and a new mechanism proposed. 

Cai, T, Slebodnick, C., Xu, L. etal. (2006) A pirouette on a metallofullerene sphere interconversion of isomers of A-tritylpyrrolidino I, Sc3N C8o- Journal of the American Chemical Society, 128, 6486-6492. 

Six-coordinate tin complexes of benzoylacetoacetonate are fluxional on the NMR timescale at ambient temperatures. The mechanism of interconversion of isomers of Ph(Cl)Sn(Bzacac)2 has... 

In this example, the interconversion of isomers requires more extensive nuclear movement. What the PMH does predict is that each isomer is a local minimum on the energy-nuclear position hypersurface. The hardness will be a maximum with respect to all asymmetric distortions from the geometry of the local energy minimum. These predictions have been verified by accurate calculations on all five Pg isomers.As expected, for symmetric distortions, no maxima are found. 

Preliminary investigations demonstrate that [19] is an excellent reactant for forming heterocycles. Hvdrolysis of [19] occurs quantitatively, to give the known (15) furan derivative [24]. Indeed, it is difficult to avoid forming some of the furan unless solvents are kept scrupulously dry. Obviously, for the cyclisation to proceed, then there must be interconversion of isomers and we can think of alternate processes for forming [24], as shown. 

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