Chiral recognition in ionic clusters

In principle, mass spectrometry is not suitable to differentiate enantiomers. However, mass spectrometry is able to distinguish between diastereomers and has been applied to stereochemical problems in different areas of chemistry. In the field of chiral cluster chemistry, mass spectrometry, sometimes in combination with chiral chromatography, has been extensively applied to studies of proton- and metal-bound clusters, self-recognition processes, cyclodextrin and crown ethers inclusion complexes, carbohydrate complexes, and others. Several excellent reviews on this topic are nowadays available. A survey of the most relevant examples will be given in this section. Most of the studies was based on ion abundance analysis, often coupled with MIKE and CID ion fragmentation on MS and FT-ICR mass spectrometric instruments, using Cl, MALDI, FAB, and ESI, and atmospheric pressure ionization (API) methods. 

The detection of chiral recognition with a mass spectrometer was reported first in 1977 by Fales and Wright. Their study showed that the chirality of dialkytartrates (T) strongly influences the stability of their diastereomeric proton-bound dimers, 

In order to differentiate the protonated species of the homochiral self-dimers 

The l homo/ hetero was also estimated by B/E linked scanning of FAB-MS.  

It can be expressed as fhomo/. hetero = [Td-H] /[Td-To-H]+ / [To-H] + - -[T -H] /[To-T -H], where [To-H] and [T -H] correspond to the peak intensities of monomer ions produced by unimolecular decomposition of the relevant protonated dimer species. The experimental results (l homo/ hetero = 0-67 (diPT-D-tartrate - - diPT-D-tartrate-di4) 0.77 (diEt-D-tartrate - - diEt-D-tartrate-dio)) confirm the higher stability of the homochiral dimer relative to the heterochiral one by indicating that the latter has a higher tendency toward unimolecular decomposition. 

More comprehensive CIMS investigations on tartrate systems indicate that the dimer chirality effects disappear when the ester functions of tartrates is replaced by H or an alkyl function, e.g., methyl or cyclohexyl.359 A similar effect is observed when the proton in the proton-bound dimers is replaced by lithium or ammonium 

The same CIMS approach has been used for investigating the self-recognition processes in proton-bound tartrate trimers.359-363 The trimer chirality effect is consistent with the heterochiral trimers as more stable than the homochiral ones. The reverse is true when the proton in the proton-bound trimers is replaced by hydronium, ammonium ion, or primary aminium ions.364,365 This changeover is 

The relative stability of the homochiral and the heterochiral dimers arising from self-CI of an equimolar mixture of the L and the D enantiomers of dimethyl- and di-isopropyltartrate has been evaluated by Nikolaev et al. using the FT-ICR technique.366-369 The dimer chirality effect, Khomo/= 0.33 corresponds to a AAG°98 = — RT ln(Arhomo/Arhelero) = 0.65 kcal mol-1 value at 20 °C, a value which is slightly larger than those measured in the CIMS experiments (0.25-0.50 kcal mol-1).358,359 The lack of chirality effects, observed when the used tartrates are replaced by the L and the D enantiomers of methyl lactate, alaninamide, and Af-acetyl-a-methyl-benzylamine, is attributed to their extensive racemization after protonation. 

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