Pasteur-type resolution

An extension of the above principles of morphological modification to racemic mixtures, in which each enantiomer (R or S) crystallizes separately in enantio-morphic crystals (d or l), naturally suggests the possibility of performing by these means a new manual Pasteur-type resolution (Scheme 7), as well as a kinetic resolution (54). 

A surprisingly different approach was followed for obtaining both enantiomers of l,2-bis(di-phenylphosphino)cyclopentane 26. a ligand used in catalytic hydrogenations (Section D.2.5.1.2.1.1.) and hydrosilylation (Section D.9.). The racemic product was prepared by the reaction of cyclopentene with white phosphorus and phosphorus trichloride at elevated temperatures, followed by treatment with phenylmagnesium bromide. The racemate formed a complex with nickel(II) bromide which undergoes a Pasteur-type spontaneous resolution on crystallization from dichloromethane. The crystals were separated by hand27. 

Resolution can be thought of as the converse of racemization (Section 2.4). One starts with a 50 50 mixture of both enantiomers and separates this mixture into the individual enantiomers. Of course, for some purposes one may only want one enantiomer, and recovery of the second enantiomer can be painstaking. Since enantiomers have identical properties, including solubility, separation of enantiomers by recrystallization is quite rare. It was, however, such a crystallization by Pasteur in 1848 that opened up the field of resolution. Pasteur s key observation was that two distinct but related types of crystal were obtained from an aqueous solution of the sodium ammonium salt of racemic tartaric acid. The two types of crystal were related as object and non-superimposable mirror image, and one type was identical to the dextrorotatory crystals of sodium ammonium tartrate obtained from (+)-tartaric acid, itself obtained as a by-product of wine-making. 

The separation of a racemic mixture of enantiomers, or racemate, is a process known as resolution. The first resolution of a pair of enantiomers was achieved by Pasteur in 1848 who observed the racemic conglomerate of a sodium ammonium salt of tartaric acid (4, Fig. 4.1). This is an equimolar mechanical mixture of crystals where each crystal only contains a single enantiomer. The enantiomorphous crystals could be observed under magnification and separated manually by hand. Examples of this type are not very common and the separation is labour intensive. However, the principle has been used in the development of classical resolution. 

One can imagine the homodimers continuing to grow selective and finally yield a crystal containing only As and a separate crystal containing only Bs. Perhaps an extreme example of such a self-assembly process is the formation of enantiopure crystals from a racemic solution. This spontaneous resolution of enantiomers was first observed in 1848 with Louis Pasteur s physical separation of hemihe-dral crystals of two types of enantiopure tartaric acid. ... 

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