Dimer chirality effect

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. A similar effect is observed when the proton in the proton-bound dimers is replaced by lithium or ammonium ion. These observations are attributed to a dramatic change in the basket-type... 

The dimer chirality effect, Aihomo/ hetero = 0.33 corresponds... 

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. 

Table 6 Chirality effects in the dimerization of homologous tartrate esters... 

For a mixture of enantiomers it is thus possible to determine the ee-value without recourse to complicated calibration. The fact that the method is theoretically valid only if the g factor is independent of concentration and if it is linear with respect to ee has been emphasized repeatedly.84-89 However, it needs to be pointed out that these conditions may not hold if the chiral compounds form dimers or aggregates, because such enantiomeric or diastereomeric species would give rise to their own particular CD effects.88 Although such cases have yet to be reported, it is mandatory that this possibility be checked in each new system under study. 

By constrast, the methylzinc complexes bearing the sterically more encumbered anisyl fencholates with tert-butyl and trimethylsilyl substituents dimerized preferentially in the heterochiral form, resulting in the more common chiral amplification effects observed for classical dimethylaminoisoborneol systems. 

As shown in the previous two sections, rhodium(n) dimers are superior catalysts for metal carbene C-H insertion reactions. For nitrene C-H insertion reactions, many catalysts found to be effective for carbene transfer are also effective for these reactions. Particularly, Rh2(OAc)4 has demonstrated great effectiveness in the inter- and intramolecular nitrene C-H insertions. The exploration of enantioselective C-H amination using chiral rhodium catalysts has been reported by several groups.225,244,253-255 Hashimoto s dirhodium tetrakis[A-tetrachlorophthaloyl-(A)-/ r/-leuci-nate], Rh2(derived rhodium complex, Rh2(i -BNP)4 48,244 afforded moderate enantiomeric excess for amidation of benzylic C-H bonds with NsN=IPh. 

Although relatively weak, it is this last interaction that is essential for determining chiral discrimination. The superior chiral recognition achieved when ref has an aromatic side chain (Table 10) suggests that 7r-cation interactions play an important role in the stereoselectivity. Evidence for such a rr-cation interaction is observed in the CID spectra of the dimeric [A Me° ref-H]and [A -Me -ref-H] diaster-eomers, in which one ligand is an aromatic amino acid, and is supported by ab initio calculations. When an L-aromatic amino acid, such as L-phenylalanine, is used as ref, these interactions are disrupted by the side group on the a-asymmetric carbon of the L-analyte, whereas the side-chain group in the D-analyte has little steric effect on... 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

Osmium tetroxide hydroxylations can be highly enantioselective in the presence of chiral ligands. The most highly developed ligands are derived from the cinchona alkaloids dihydroquinine and dihydroquinidine.40 The most effective ligands are dimeric derivatives... 

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