Chiral lanthanide shift reagents CLSRs

The mechanism of chemical shift non-equivalence induction is more complicated in the case of CLSRs than with CSAs. The shift reagents themselves can exist as a number of rapidly interconverting isomers. Frequently, several equivalents of CLSR must be added to a solution of the chiral substrate in a bulk solvent before anisochronicity is observed. The precise nature of the association complexes formed varies from one example to the next. This makes assignment of absolute configuration on the basis of the sense of non-equivalence unreliable even in structurally related series of compounds. 

The magnitude of the chemical shift non-equivalence depends on the strength of the complexation, the proximity of the stereogenic centre to the complexation site and the identity of the substrate functionalities involved in electron donation. Non-polar solvents 

Reported CLSR enantiomeric purity determinations are in good agreement with the results from other methods ( 2% in 30%). The best claimed results are 2% in the range 40-60% e.e. but it has been claimed that for e.e. 90% the error can be as large as 10%. 

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There are three types of chiral auxiliary that are used chiral derivatizing agents (CDAs), chiral lanthanide shift reagents (CLSRs) and chiral solvating agents (CSAs)75. Chiral derivatizing agents (CDAs), such as the enantiomers of o -methoxy-o -(trifluoromethyl)phenylacetic acid (MTPA, 83)76, require the separate formation of discrete... 

The use of chiral lanthanide shift reagents (CLSRs) for NMR enantiomeric purity determination has become very popular (6) since the first of these compounds (54a) was reported by Whitesides and Lewis (96). Reagents 54b [Eu(hfbc)3 or Eu(hfc)3] and 54c [Eu(facam)3 or Eu(tfc)3] subsequently independently introduced by Fraser (97) and Goering (98), are most widely used, and are commercially available. 

The second technique is to use a chiral lanthanide shift reagent (CLSR). Addition of certain lanthanide chelates, most commonly those of europium(III), to a solution of an organic compound in CDC13 or... 

Three types of chiral auxiliary are widely used. Chiral derivatising agents (CDAs)[ l form diastereomers while chiral solvating agents (CSAs)t l and chiral lanthanide shift reagents (CLSRs)t l form diastereomeric complexes in situ with the substrate enantiomers. 

CLSR chiral lanthanide shift reagent amination... 

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