Ligand auxiliary

Progress has been made toward enantioselective and highly regioselective Michael type alkylations of 2-cyclohexen-l -one using alkylcuprates with chiral auxiliary ligands, e. g., anions of either enantiomer of N-[2-(dimethylamino)ethyl]ephedrine (E. J. Corey, 1986), of (S)-2-(methoxymethyl)pyrrolidine (from L-proline R. K. EHeter, 1987) or of chiramt (= (R,R)-N-(l-phenylethyl)-7-[(l-phenylethyl)iinino]-l,3,5-cycloheptatrien-l-amine, a chiral aminotro-ponimine G. M. Villacorta, 1988). Enantioselectivities of up to 95% have been reported. 

To overcome the problem of the insolubility of 1, different strategies have been developed. Synthesis of alternative soluble hydride reagents, substitution of cyclopen-tadienyl rings and/or changing the chloro-auxiliary ligand were widely investigated. 

When an auxiliary ligand is added to a chelate extraction system and it enhances the extractability and extraction rates, such a phenomenon is termed synergism. A typical... 

In cases of complexes bearing an exocyclic double bond directly coordinated to the metal center, the carbons of the double bond usually exhibit coupling with NMR-active metal centers and/or auxiliary ligands.6 14 18 19 The chemical shifts of the quaternary carbon atom vary from 66.976 to 82.2818 ppm, while the methylene group gives rise to signals at 29.16,14 41.91,6 or 51.3418 ppm in the 13C 1H NMR spectra. As one can see, the chemical shift variation is relatively broad and significantly affected by the nature of the metal center. 

Desimoni et al. have shown that the use of magnesium perchlorate or magnesium triflate, and three chiral to(oxazolines) and two equivalents of achiral auxiliary ligands such as water or tetramethylurea, induces a strong change of the enantiofacial selectivity with >94% ee in the... 

A monohydride mechanism is not operating in reactions catalyzed by these complexes. Noyori observed that the presence of an NH or NH2 in the auxiliary ligands was crucial for catalytic activity, the corresponding dialkylamino analogs being totally ineffective. These findings indicate a novel metal-ligand bifunctional cycle (Scheme 28) KOH reacts with the pre-catalyst (87)... 

Compounds presented in Scheme 9 have provided evidence for the influence of the otherwise rarely investigated />-anilino-ethynyl group and a variety of auxiliary ligands on the luminescence, NLO, and mesogenic properties of this type of complexes.98... 

Bis-ylides (Scheme 31) may function as four-electron donors and become geminally diaurated. The donor capacity of ylides is generally higher than that of most other ligands, and even powerful donors like tertiary phosphines can thus be readily replaced by non-stabilized ylides. Ylides may therefore be used as auxiliary ligands which are retained, while other components of the coordination sphere are subject to substitution. 

In general, gold cations Au+ and their adducts with auxiliary ligands [LAu]+, as well as gold(i) halides AuX, form only weak 7r-complexes with alkynes. Stable compounds have been isolated only with special combinations of components, but unstable species may nevertheless play an important role in gold chemistry as short-lived intermediates or transient species. 

Recently, a deeper understanding of the precise nature of metal-carbon bonding was achieved, enabling specific polymerization catalyst systems to be designed on a practical level. The metal-carbon bond of early transition metals is partially ionic, while that of late transition metal is generally covalent. The degree of ionicity is delicately dependent on the identity of metal, formal oxidation states and auxiliary ligands. 

Asymmetric carboxylation was also achieved by use of 7r-allyltitanium complexes bearing chiral ligands (Scheme 13).26 The enantiomeric selectivity was significantly influenced by the chiral auxiliary ligands used. In the case of 45, the chiral /3,7-unsaturated carboxylic acid 46 was obtained with excellent ee.27,28... 

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