Solvents separation, diastereomeric

Complexation of an enantiopure diene derived from (+)-L-arabinose was a modestly diastereoselective process (equation 54). The major rf-iron diene complex formed (224) was likely the result of heteroatomic dehvery and installation of the Fe(CO)3 unit syn to the benzyloxy group. The diastereoselectivity of this transformation was solvent dependent this may be result of conformational differences in different solvents. The diastereomeric iron diene complexes were separable by chromatography and each could be transformed into various derivatives under carefully controlled conditions without loss of optical integrity. ... 

To a 0.5 M solution of A -[(/ )-2-methylamino-2-oxo-l-phenylethyl -4,5-hexadienamine in dry CH/T is added silver tetrafluoroborate (50 mol%), the reaction mixture is flushed with N2 and stirred in the absence of light at 20 "C until completion of reaction (3 h). H20 and CH2C12 are then added and the organic layer separated and the aqueous phase is extracted a further two times. The combined extracts are dried and after removal of the solvent a diastereomeric mixture is obtained in 90% yield d.r. [(25)/(22 )] 90 10 fH NMR). The isomers are not easily separated and are characterized as the mixture. 

Irradiation (313 nm) of 0.001 M 3-benzoylpropanoic acid amide in Et O under an Nj atmosphere at 20°C for 15-18 h followed by evaporation of the solvent afforded diastereomeric mixtures of cyclopropanes, which were separated and purified by recrystallization (EtOH/pentane). 

The hallmark of a concerted process is stereoselectivity. Crich and Gastaldi investigated the cine substitution reaction with the diastereomeric probes 27 and 28 and found partial (i.e. incomplete) scrambling in the products 29 and 30 (Scheme 26) [50], This result is best interpreted in terms of the evolving general mechanistic picture and stereoselective capture of the two diastereomeric contact or solvent-separated ion pairs by the nucleophile. The possibility remains, however, of a concerted mechanism in which the nucleophile does not distinguish between the two lobes of the singly occupied p-orbital in the initial radical. 

Diastereomeric salt formation results from complete proton transfer between CSA and solute. There is rapid exchange between the free acid and base and that constituting a close ion-pair. Solvation of such systems can be problematic since by its nature a salt requires a reasonably polar solvent. However, such solvents tend to dissociate the close ion-pairs to give solvent-separated ion-pairs in which the stereochemically dependent interaction responsible for induction of anisochronicity is lost. This limitation can be overcome in some cases by using mixed achiral solvents such as dg-benzene and ds-pyridine. 

The role of HMPA as solvent in the addition of organolithium reagents to enones has been explored by applying a multinuclear NMR technique to quantify the amount of solvent-separated ion pairs (SIP) in solution and to corrolated this with changes in regioisomeric and diastereomeric product ratios.Contact ion pairs (CIP) have been found to react exclusively by 1,2-addition, presumably via a four-centre transition state as hypothesized. However, the situation for SIPs is more complicated and clean 1,4-addition occurs only in the absence of lithium catalysis. Well stabilized anions react by 1,2- and 1,4-addition in the absence of HMPA, when lithium catalysis is possible and SIPs are energetically accessible intermediates. 

A closely related method does not require conversion of enantiomers to diastereomers but relies on the fact that (in principle, at least) enantiomers have different NMR spectra in a chiral solvent, or when mixed with a chiral molecule (in which case transient diastereomeric species may form). In such cases, the peaks may be separated enough to permit the proportions of enantiomers to be determined from their intensities. Another variation, which gives better results in many cases, is to use an achiral solvent but with the addition of a chiral lanthanide shift reagent such as tris[3-trifiuoroacetyl-Lanthanide shift reagents have the property of spreading NMR peaks of compounds with which they can form coordination compounds, for examples, alcohols, carbonyl compounds, amines, and so on. Chiral lanthanide shift reagents shift the peaks of the two enantiomers of many such compounds to different extents. 

One of the most powerful methods for determining enantiomer composition is gas or liquid chromatography, as it allows direct separation of the enantiomers of a chiral substance. Early chromatographic methods required the conversion of an enantiomeric mixture to a diastereomeric mixture, followed by analysis of the mixture by either GC or HPLC. A more convenient chromatographic approach for determining enantiomer compositions involves the application of a chiral environment without derivatization of the enantiomer mixture. Such a separation may be achieved using a chiral solvent as the mobile phase, but applications are limited because the method consumes large quantities of costly chiral solvents. The direct separation of enantiomers on a chiral stationary phase has been used extensively for the determination of enantiomer composition. Materials for the chiral stationary phase are commercially available for both GC and HPLC. 

In principle, separation of resonances of diastereomeric compounds (such as dl and meso isomers) may be increased simply through use of an appropriate achiral solvent. Chiral solvents may in some cases be especially effective in producing a separation, particularly if the diastereomers differ in configuration about a center that is amenable to analysis by the CSA method. Kaehler and Rehse (89) give a detailed account of conditions necessary for measurement of the ratio of meso- and dZ-tartaric acid employing A,N-dimethyl PEA. Bomyl acetate used as solvent for l,2-difluoro-l,2-dichloroethane (90) allows measurement of the diastereomeric composition. Paquette and co-workers (91,92), using TFAE, were able to determine the diastereomeric purity of the recrystallized adducts 47 of... 

The practical difficulty with carrying out a crystalhzation DTR process is the need to operate under conditions that allow selective crystalhzation of the least soluble diastereomer while permitting the racemization to take place. Amine racemization catalysts, such as SCRAM , Shvo, Pd/C, and Adam s, are more active at higher temperatures, which runs counter to the conditions required for crystaUization. A solution to this problem is to separate the diastereomeric resolution and racemization steps but couple them with a flow engineering design. In this way each reaction can be operated under optimal conditions for example, temperature, concentration and solvent, via an intermediary solvent exchange unit Since the racemization catalyst itself may affect the crystalhzation (or indeed the crystalhzation may affect the catalyst), it is preferred to keep them separate. This can be achieved by having the catalyst or product either permanently or temporarily in a different phase by immobilization, extraction, precipitation, distil-... 

One possibility to separate the enantiomers of rac- 1-phenylethanamine is to form diastereomeric salts with an enantiomerically pure chiral acid, e.g. (R,R) tartaric acid or (S)-2-hydroxysuccinic acid. These can be separated from each other by recrystallisation as a consequence of their different solubilities. Note, however, that the separation process is not complete at this stage since the amines are now present as salts. The separated salts must be treated with a strong base, e.g. aqueous sodium hydroxide, to convert them back to the free amines which can then be extracted into an organic solvent. After drying the extract distillation of the solvent leaves the pure amine. 

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