Tartrate sensing

Figure 13.21 Indicator displacement assay for tartrate sensing (upper panel). Displacement isotherms at 597 nm for the addition of (R,R)- and (S,S)-tartaric acid 74 to the complex of 72 and bromophenol blue (73) (c72 and 73=1 x 10 4mol/dm3)...

Chirality, in its many and varied manifestations, is ubiquitous a concept rooted in mathematics, it permeates all branches of the natural sciences.1 In 1848, Louis Pasteur announced his epochal discovery of a causal relationship between the handedness of hemihedral sodium ammonium tartrate crystals and the sense of optical rotation of the tartrates in solution.2 This discovery, which marks the beginning of modem stereochemistry, connected enantiomorphism on the macroscopic scale to enantiomorphism on the molecular scale and thus led to Pasteur s recognition that the optical activity of the tartrates is a manifestation of dissymetrie moleculaire, 3 that is, of molecular chirality. 

When the allyUc alcohol has a chiral substituent, diastereofacial control by the chiral center (substrate control) competes with facial control by the titanium-tartrate complex. When the senses of two facial controls match, high enantiose-lectivity can be expected but, when mismatched, enantioselectivity suffers. The strength of substrate control depends strongly upon the location of chiral center in the substrate. 

Noteworthy among these examples is the ability to achieve high diastereoselec-tivity for both the 3,4-syn and 3,4-anti isomers, almost independent of the chirality sense of the aldehyde. Comparison of several examples show the expected trends for matched and mismatched pairs (c/. entry pairs 1/2, 4/6, 5/7, 9/12, 16/17). Note that either 3,4-anti diastereomer can be obtained with 96% ds (entries 8 and 12) the two 3,4-syn isomers are also available selectively (entries 13-16 and 17), although only one ligand (5.1i) is selective for the 3,4-syn-4,5-syn product (entry 17) that is a mismatched pair cf. entry 16). Note that with Roush s tartrate ligand (Figure 5.1c), the -crotyl mismatched pair is more selective than the matched pair (entries 8/11 for a rationale, see ref. [33]), and the matched and mismatched pair give the same major product isomer with the Z-crotyl compound (entries 14/15). 

Despite the complexity of the active catalyst, the sense of asymmetric induction in Sharpless asymmetric epoxidation reactions can be rehably predicted using the model shown in Figure 4.2. In order for the model to predict the stereochemical outcome correctly, only two points need to be remembered. The allyhc hydroxy group resides in the bottom right corner and D-(-)-diethyl tartrate (which has the (S,S)-configuration) attacks from above the plane. 

A racemate can be resolved with ease if it happens that the enantiomers form separate crystals - a so-called conglomerate. It becomes possible to separate, physically, the enantiomorphic crystals - a process sometimes referred to as triage. This is what Pasteur famously achieved in 1848 with a sample of ammonium sodium tartrate [6]. Such good fortune is quite rare and is in any case not a synthetic method in the strictest sense (nor is it practical on a large scale). Other physical processes (alone, without any attendant synthetic process) such as evaporation or sublimation can be used to increase the enantiomeric excess of organic compounds, including amino acids [7]. 

Schmuck, C. Schwegmann, M. A naked-eye sensing ensemble for the selective detection of citrate-hut not tartrate or malate-in water based on a tris-cationic receptor. Org. Biomol. Chem. 2006, 4, 836-838. 

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