TRISPHAT

An overall efficiency of TRISPHAT 8 and BINPHAT 15 anions as NMR chiral shift agents for chiral cations has been demonstrated over the last few years. Additions of ammonium salts of the A or A enantiomers of 8 and 15 to solutions of racemic or enantioenriched chiral cationic substrates have generally led to efficient NMR enantiodifferentiations [112-121]. Well-separated signals are usually observed on the spectra of the diastereomeric salts generated in situ. 

TRISPHAT anion 8 seems to be more particularly efficient with cationic metallo-organic and organometallic substrates. BINPHAT 15 has often-supe-rior chiral shift properties than 8 when associated with organic cations such as ammonium cation 68 (Fig. 24). In all these examples, solvent polarity influences the quality of the separation since ion association is crucial. Solvent or solvent mixtures of low polarity are preferred for these experiments. 

Recently, Lacour, Sauvage and coworkers were able to show that the association of chiral [CuL2] complexes (L=2-R-phen,6-R-bpy and2-iminopyridine) with TRISPHAT 8 leads to an NMR enantiodifferentiation, which allows the determination of the kinetics of racemization of the complexes (bpy=2,2 -bipyri-dine phen=l,10-phenanthroline) [119]. This type of application has recently been reported in conjunction with chiral sandwich-shaped trinuclear silver(l) complexes [122]. Several reports, independent from Lacour s group,have confirmed the efficiency of these chiral shift agents [123-127]. Finally, TRISPHAT can be used to determine the enantiomeric purity of (r] -arene)chromium complexes. These results broaden the field of application of 8 to chiral neutral, and not just cationic, species [114,128,129]. 

Preparative chromatographic resolution procedures have overall freed chemists from the constraint of dependency on crystallization. They are most often performed with covalent diastereomer mixtures but ionic salts can also be separated. Recently, it was found that the lipophilicity of TRISPHAT anion 8 profoundly modifies the chromatographic properties of the cations associated with it and the resulting ion pairs are usually poorly retained on polar chromatographic phases (Si02, AI2O3) [131]. Using enantiopure TRISPHAT anion. 

The lipophilicity of the TRISPHAT anion 8 also confers to its salts an affinity for organic solvents and, once dissolved, the ion pairs do not partition in aqueous layers. This rather uncommon property was used by Lacour s group to develop a simple and practical resolution procedure of chiral cationic coordination complexes by asymmetric extraction [134,135]. Selectivity ratios as high as 35 1 were measured for the enantiomers of ruthenium(II) trisdiimine complexes, demonstrating without ambiguity the efficiency of the resolution procedure [134]. 

Unfortunately, in most of the previous examples, the extent of the asymmetry-induction was determined by chiroptical measurements (ORD, CD) that gave qualitative and not quantitative information. The NMR chiral shift efficiency of TRISPHAT 8 and other hexacoordinated phosphate anions was therefore considered as an excellent analytical tool to provide accurate measurement of the induced selectivity by NMR spectroscopy. 

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

The Z)3-symmetric TRISPHAT anion 88, tris(tetrachlorobenzenediolato)phosphate, is chiral and configurationally stable. It can be resolved by association with chiral ammonium salts such as cinchonidine <1997AGE608, 2004JOC8521>. TRISPHAT 88 displays high selectivity for cinchonidine and does not associate with the related diasteromer cinchonine <1998TL4825>. The selective ion pairing behavior of TRISPHAT has been exploited in... 

ENANTIOSELECTIVE EPOXIDATION OF OLEFINS USING PHASE TRANSFER CONDITIONS AND A CHIRAL [AZEPINIUM][TRISPHAT] SALT AS CATALYST... 

The enantioselective epoxidation using [diphenylazepinium] [TRISPHAT] salts as catalysts is an easily reproducible procedure that requires no particular precautions except in the handling of Oxone . Although moderate levels of enantiomeric excess are observed, this reaction can be applied to a wide range of olefins, and both enantiomers of the catalyst are readily available through the use of the S) or (R) enantiomers of 3,3-dimethylbutan-2-amine. The results of a small screen using [6-A-((5)-3,3-dimethylbutan-2-yl)-5/7-dibenz[c,e]azepinium][rac-TRISPHAT] salt as catalyst are reported in Table 6.1... 

Table 6.11 Enantioselective epoxidation of olefins 1 to 5 in the presence of [6-N-((S)-3,3-dimethylbutan-2-yl)-5/f-dibenz[c,e]azepinium][rac-TRISPHAT] as catalyst."...

However, very efficient deracemization was reported in tris(4,4 -dimethyl-2,2 -bipyridine)iron(II), [Fe(4,4 -Me2bpy)3]2+, with tris(trichlorobenzenedi-olato)phosphate (trisphat), where trisphat is shown in Scheme 16 [42]. Though this is not a photoreaction but a thermal reaction, we briefly discuss this result because this deracemization is very interesting and provides an interesting idea of photoinduced deracemization. 

Lacour et al. have shown that the ion pairing of Bulman Page s iminium cation 66 with a tris(tetrachlorobenzenediolato)phosphate(v) (TRISPHAT) anion allows the use of strict biphasic conditions leading to higher enantioselectivities when compared to a tetraphenylborate counterion (Scheme 26) <2002TL8257>. 

View of the Cinchonidinium cation (left) and of the Trisphat anion (right). 

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