Isoinversion temperature

Optimization of Enantiomeric Ratio E by Choice of Temperature 5.7.3.1 Derivation of the Isoinversion Temperature... 

The results of the temperature dependence of the reaction rates of the enantiomers of secondary alcohols with a secondary alcohol dehydrogenase (SADE1) from the thermophilic bacterium Thermoanaerobacter ethanolicus demonstrated a temperature-dependent reversal of stereospecificity (Pham, 1990) (Figure 5.16). At T < 26°C, (S)-2-butanol was a better substrate than (i )-2-butanol on the basis of kCSLt/KM values however, at T> 26°C, (R)-2-butanol was a better substrate than (S)-2-butanol. (S)-2-Pentanol was the preferred substrate at T < 60°C however, the data predict that (i )-2-pentanol would be preferred at T > 70°C. (S)-2-Elexanol was predicted to be the preferred enantiomer only at T > 240°C. Therefore, the concept of isoinversion temperature is as valid for enzyme reactions as for others only the range of catalytically accessible temperatures is smaller. 

From a liner plot of the difference between 5AAH and 5AAH, where 5AAH and 5AAH are the differences in the activation parameters for the high- and low-temperature regions, an isoinversion temperature 7] = 218K can be defined [47]. 

Samarium iodobinaphtholate (222) has been reported to act as an efficient enantioselective catalyst for the Michael addition of aromatic amines to fumaryl oxazolidinone (220), affording the aspartic acid derivatives (221) in good yields. Elucidation of the influence of temperature on the addition of p-anisidine revealed an isoinversion effect with a maximum ee of 88% at —40 °C.257 A non-enantioselective version of this reaction has also been reported.258... 

An interesting result has been obtained with respect to the stereoselectivity of the addition of benzoyl nitrene to compounds 104a-104d The ratio of the two diastereomers depends on the temperature, such that an inversion of the diastereoselectivity can be observed at a distinct temperature (isoinversion principle [47]). Such an effect may indicate a reversible first bond formation between the benzoyl nitrene and the alkene [45]. 

Frequently, the differences in entropy between two diastereomeric transition states are small, and the enthalpy term predominates. However, the entropy term cannot always be neglected, and sometimes it may even dominate according to the reaction conditions [2], In some cases, the observed selectivity can be inverted by changing the temperature of the reaction, and the temperature at which AAH = TAAS is called the isokinetic temperature (isoinversion). The occurrence of an isokinetic temperature in the normal operating range is not veiy frequent, but it is observed sometimes in multistep processes (see below). The ratio of the diastereo-isomeric products formed in kinetically controlled reactions is given by the relationship... 

Based on the pioneering study of the influence of the reaction temperature on diastereoselectivity137, 138, it is now known that enthalpy and entropy effects oppose each other in the transformation of 1 to 2136. Comparison of different systems at a single temperature may therefore be meaningless, since the results obtained below an isoselective temperature140 will be reversed above that temperature136. This principle of isoinversion may have to be taken into account in a comprehensive discussion of the above reactions141. 

Once again it is emphasized that the temperature dependence of these reactions can be treated according to the principle of isoinversion (see also Section 2.1.5.)136 U1. 

Glyoxylates of chiral alcohols have been intensively studied as carbonyl components. According to the mechanistic studies of Scharf and coworkers, the outcome of the cycloaddition is in particular depending upon the reaction temperature (isoinversions... 

Imidazolide ionic liquids [Bmimjlm (2 mol%) have been reported to catalyse aza-Michael addition of, for example, imidazole to acrylates and acrylonitriles. Samarium diiodide has been identified as an efficient catalyst for the aza-Michael addition of 0-benzylhydroxylamine to a,/8-unsaturated Af-acyloxazolidinones. The 1,1-bi-2-naphthol (BINOL)-Sml complex afforded the Michael adducts with <88% ee. An isoinversion effect with temperature was observed for the highest enantiomeric excess at -40°C.205... 

---