Resolution reactivity

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

This method is widely used for the resolution of chiral amines and carboxylic acids Analogous methods based on the formation and separation of diastereomers have been developed for other functional groups the precise approach depends on the kind of chem ical reactivity associated with the functional groups present m the molecule... 

A particularly useful property of the PX monomer is its enthalpy of formation. Conventional means of obtaining this value, such as through its heat of combustion, are, of course, excluded by its reactivity. An experimental attempt was made to obtain this measure of chemical reactivity with the help of ion cyclotron resonance a value of 209 17 kJ/mol (50 4 kcal/mol) was obtained (10). Unfortunately, the technique suffers from lack of resolution in addition to experimental imprecision. It is perhaps better to rely on molecular orbital calculations for the formation enthalpy. Using a semiempirical molecular orbital technique, which is tuned to give good values for heat of formation on experimentally accessible compounds, the heat of formation of /5-xylylene has been computed to be 234.8 kj/mol (56.1 kcal/mol) (11). 

EinaHy, kinetic resolution of racemic olefins and aHenes can be achieved by hydroboration. The reaction of an olefin or aHene racemate with a deficient amount of an asymmetric hydroborating agent results in the preferential conversion of the more reactive enantiomer into the organoborane. The remaining unreacted substrate is enriched in the less reactive enantiomer. Optical purities in the range of 1—65% have been reported (471). 

It is generally beheved that selectivity of hydrolytic enzymes strongly depends on the proximity of the chiral center to the reacting carbonyl group, and only a few examples of successful resolutions exist for compounds that have the chiral center removed by more than three bonds. A noticeable exception to this rule is the enantioselective hydrolysis by Pseudomonasfluorescens Hpase (PEL) of racemic dithioacetal (5) that has a prochiral center four bonds away from the reactive carboxylate (24). The monoester (6) is obtained in 89% yield and 98% ee. 

Kinetic Resolutions. From a practical standpoint the principal difference between formation of a chiral molecule by kinetic resolution of a racemate and formation by asymmetric synthesis is that in the former case the maximum theoretical yield of the chiral product is 50% based on a racemic starting material. In the latter case a maximum yield of 100% is possible. If the reactivity of two enantiomers is substantially different the reaction virtually stops at 50% conversion, and enantiomericaHy pure substrate and product may be obtained ia close to 50% yield. Convenientiy, the enantiomeric purity of the substrate and the product depends strongly on the degree of conversion so that even ia those instances where reactivity of enantiomers is not substantially different, a high purity material may be obtained by sacrificing the overall yield. 

The well-known fact that enantiomers exhibit different reactivity towards chiral reagents has been used to obtain optically active sulphoxides in a process which is called kinetic resolution. Kinetic resolution of sulphoxides usually involves either oxidation to the corresponding sulphones or reduction to sulphides by means of proper chiral oxidizing or reducing agents. 

Azetidine ring is an important structure because it is present in many compounds of pharmaceutical interest however, its manipulation must be done very carefully owing to the reactivity of these heterocycles of small size. An interesting application of the use ofbiocatalytic processes is the resolution of azetidine esters (Scheme 7.11). The procedure to choose for the resolution of these compounds is the enzymatic ammonolysis of the corresponding N-substituted azetidines [26]. 

The asymmetric ring opening (ARO) of racemic terminal epoxides with H2O via hydrolytic kinetic resolution provides an efficient synthetic route to prepare optically pure terminal epoxides. The dimeric type chiral Co(salen)AlX3 complex has great potential to catalyze HKR of terminal epoxides in a highly reactive and enantioselective manner in comparison to their monomeric analogy. 

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