Solvation diastereomers

Fig. 31.4 Solvate dihydrides characterized at low temperature [S = CH3OH or CD3OD]. Dihydrides formed in situ react rapidly with the common substrates of enantioselective hydrogenation, and the reduced product is formed with high enantios-electivity. At ambient pressure, the proportion of dihydrides is 45% at -100°C for (a) with a 10 1 diastereomer ratio, and 40% for (b) at —40°C, with a 2 1 diastereomer ratio.

Thus in the presence small cations where coordination with the penultimate 2-pyridyl group is most likely, the stereoselectivity of these systems is readily understood by a cation side (syn) attack of electrophyle on the preferred diastereomer [10]. With large or more extensively solvated cations such as Rb+ or (Na, CE)+, coordination with the... 

Intramolecular solvated tetramers are observed for 3-lithio-l-methoxybutane (49) and frrom 1-di-methylamino-3-lithiopropane (50) in the solid state. Hiese tetramers are shown in generalized form as (51) and (52), respectively. Note the significant difference between the aggregates (51) and (52). Variable temperature Li NMR as well as NMR suggest that although the major form of 1-dimethylami-no-3-lithiopropane (50) is the diastereomer (51), this structure is presumed to be in equilibrium with (52)... 

Long-lived diastereomers are generated by chemical derivatization of the enantiomers with a chiral reagent. They may be separated subsequently by achiral means. Their formation energies have no relevance to their chromatographic separation it is, rather, due to the difference in their solvation energies. Differences in their shape, size, or polarity will affect the energy needed to displace solvent molecules from the stationary phase [1]. 

Nevertheless, it is possible to convert a racemic sample with chiral reagents into diastereomers or simply to dissolve it in an enantiomericaUy pure solvent R or S following this process, solvation diasteromers arise from the racemate (RP + SP) of the sample P, e.g. R RP and R SP, in which the enantiomers are recognisable because of their different shifts. Compounds with groups which influence the chemical shift because of their anisotropy effect (see Sections 2.5.1 and 2.5.2) are suitable for use as chiral solvents, e.g. 1-phenylethylamine and 2,2,2-trifluoro-l-phenylelhanol. ... 

A further step was taken when first Halpern [28] and then Brown [29] were able to identify a further intermediate, the rhodium alkyl hydride formed by addition of dihydrogen to the enamide complex with transfer of a single hydride to the benzylic carbon. For the simple dppe complex studied by Halpern, the interpretation of the experiment was straightforward, but the intermediate derived from DIPAMP by Brown and Chaloner provided a major surprise only the disfavored minor diastereomer of the enamide complex was reactive towards H2. The major/minor equilibrium is so strongly biased towards the former below -50 °C that reaction with H2 is undetected. Only when the solvate complex is allowed to react with the dehydroamino acid derivative under H2, well below -50 °C (under which conditions up to 35% of the minor diastereomer is initially observed) is the alkyl hydride observed, concomitant with disappearance of that minor diastereomer. This reactive intermediate was characterized by its H-NMR (hydride), the distinctive P-NMR and by both heteronuclear coupling and chemical shifts in the C-NMR spectra of alkyl hydrides derived from singly and doubly labeled dehydroamino esters. 

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