Chiral cation

Diels-Alder reactions Neutral ionic liquids have been found to be excellent solvents for the Diels-Alder reaction. The first example of a Diels-Alder reaction in an ionic liquid was the reaction of methyl acrylate with cyclopentadiene in [EtNH3][N03] [40], in which significant rate enhancement was observed. Howarth et al. investigated the role of chiral imidazolium chloride and trifluoroacetate salts (dissolved in dichloromethane) in the Diels-Alder reactions between cyclopentadiene and either crotonaldehyde or methacroline [41]. It should be noted that this paper describes one of the first examples of a chiral cationic ionic liquid being used in synthesis (Scheme 5.1-17). The enantioselectivity was found to be < 5 % in this reaction for both the endo (10 %) and the exo (90 %) isomers. 

Scheme 17 Temperature-induced epimerization of chiral cationic phosphate 62...

As already mentioned, chiral cations are involved in many areas of chemistry and, unfortunately, only few simple methods are available to determine their optical purity with precision. In the last decades, NMR has evolved as one of the methods of choice for the measurement of the enantiomeric purity of chiral species [ 110,111 ]. Anionic substances have an advantage over neutral reagents to behave as NMR chiral shift agents for chiral cations. They can form dia-stereomeric contact pairs directly and the short-range interactions that result can lead to clear differences in the NMR spectra of the diastereomeric salts. 

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. 

Sometimes, a direct ion-pairing of the chiral cations and anions 8 or 15 is necessary to maximize the NMR separation of the signals [115,116]. Cationic species as different as quaternary ammonium, phosphonium, [4]heterohelice-nium, thiiranium ions, (rj -arene)manganese, ruthenium tris(diimine) have been analyzed with success (Fig. 23). 

Fig. 23 Selected examples of chiral cations analyzed successfully with anions 8 or 15...

Many chemical reactions and processes yield cationic racemic products, and either a resolution or a stereoselective synthesis must be envisaged to obtain the chiral cations in an enantioenriched or enantiopure form. Resolution has been strongly studied [130] and selected representative examples of such processes mediated by chiral P( VI) anions are presented. 

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]. 

For cations 74-75 (Fig. 27), low temperature NMR experiments were necessary to reveal stereodynamical behaviors and allow the observation of split signals for the enantiomers [38,144]. Stereoselective recognition between the chiral cations and anions was observed in essentially all cases as integration of the split signals revealed the preferential occurrence of one diastereomeric salt over the other. 

Diastereomeric ratios as high as 20 1 can be observed for some of the substrates, e.g., salt [72] [A-8] [41,141]. The selectivity strongly depends upon the polarity of the solvent medium. An increase in the diastereoselectivity is usually observed upon the decrease of solvent polarity. This is interpreted as the result of closer interactions between the ions. In most cases, induced CD spectra could also be measured allowing the determination of the preferred configuration of the chiral cations. 

Few of these studies (460, 462) dealt with the Michael reaction one study (461) with the Henry reaction. The efficiency, stereoselectivity, and enantiose-lectivity of this process are rather high. The mechanism of the transformations is poorly known. Presumably, the chiral cation should shield the Si surface of nitronate, thus providing the Re approach of the substrate. In addition, the approach of the reagents, resulting in generation of syn isomers, is considered less favorable than the approach yielding anti isomers. 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

In this case, the material s specific chiral cavities are created by utilizing phenyltrimethoxysilane and TMOS as the monomers and the chiral cationic surfactant A-dodecyl-TV-methylephedirnium (DMB) as template,... 

Ghosez and co-workers (223) illustrated that azadienes could be efficiently used as heterodienes with acryloylimide dienophiles in the presence of chiral cationic Cu(II) catalysts, Eq. 183. The reaction was found to be highly exo- and enantiose-lective, provided the azadiene contains an aryl or vinyl group in the 1-position. Acryloylimide as dienophile affords equal enantioselectivities albeit reduced dias-tereoselectivities (typically 6 1). 

This volume seeks in a small way to bridge the wide gap between organic chemistry in the gas and condensed phases. The same types of chiral ion-dipole complexes that form as intermediates of solvolysis may be generated in the gas phase by allowing neutral molecules to cluster with chiral cations. The reactions of these chiral clusters have been characterized in exquisite detail by mass spectrometry. The results of this work are summarized by Maurizio Speranza in a chapter that is notable for its breadth and thoroughness of coverage. This presentation leaves the distinct impression that further breakthroughs on the problems discussed await us in the near future. 

Within the area of molecular magnets, oxalate-bridged 3D networks of the type X" "[M M C204)3] are important, and of significance is the introduction of chiral cations X" ". Examples have included enantiomers of [Ru(bpy)2L]+ where HL = 2-phenylpyridine or 8-hydroxyquinoline, which are able to act as chiral templates in the construction of 3D polymeric networks. ... 

Recently the group of D. W. Armstrong exploited the enantiopure ionic liquid 76 in the photoisomerization of dibenzobicyclo[2.2.2]octatrienes, and up to 12% ee was reported (Scheme 83). The obtained ee was possible due to the addition of base in order to deprotonate the carboxylic acid function of 74 resulting in a strong anion-chiral cation interaction. In the absence of a base, lower values of ee were obtained, and in the case that ester functions instead of carboxylic acid groups were present in the molecule, only racemic product was found. Ionic liquid 77 gave up to 6.8% ee. 

In an extension of the work to chiral chemistry 244), the imidazolium cation of the ionic liquid was modified to carry a chiral substituent. The high cost of the chiral cation could be justified as the chiral ionic liquid, [MBMIM ] , can be reused. In the ATRP of soluble methyl acrylate, a small effect of the chiral ionic liquid on the polymer tacticity was observed. The use of a chiral ionic liquid as a solvent could lead to applications in other areas of catalysis. The synthesis of imidazolium-containing ionic liquids functionalized with chiral natural amino acids has already been reported 245), as have less expensive chiral ionic liquids(2- (5). 

Generation of Chiral Cation from the Bislactim Ether and Its Use for... 

However, in the presence of Lewis acids such as AlBr3, without the presence of the CpFe(CO)2+ Lewis acid the [3 + 2[-cycloaddition process was more effective, giving the product in 45% yield. The application of a chiral cationic CpFe(diphosphine) complex as the catalyst (Scheme 9.37) for the asymmetric [3 + 2]-cycloaddition of nitrones to acrolein derivatives was described by Kiindig and coworkers [95],... 

A chiral cationic rhodium complex has been shown to catalyse the enantioselective conjugate addition of silyl anion equivalents to cyclic a,fl-unsaturated ketones and esters, thus providing a facile access to chiral organosilicon compounds.247... 

S. Oi, E. Terada, K. Ohuchi, T. Kato, Y. Tachibana, and Y. Inoue, Asymmetric hetero Diels-Alder reaction catalyzed by chiral cationic palladium(II) and platinum(II) complexes, J. Org. Chem., 64 (1999) 8660-8667. 

In the phase-transfer processes discussed in Section 11.2 it is assumed that the anionic hydride source, i.e. borohydride or a hypervalent hydrosilicate, forms an ion-pair with the chiral cationic phase-transfer catalyst. As a consequence, hydride transfer becomes enantioselective. An alternative is that the nucleophilic activator needed to effect hydride transfer from a hydrosilane can act as the chiral inducer itself (Scheme 11.6). 

Interesting recently published examples include ionic liquids with fluorinated [30], functionalized [31] and chiral cations [32] and anions [33],... 

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