Asymmetric reactions ligand applications

Chiral oxazolines developed by Albert I. Meyers and coworkers have been employed as activating groups and/or chiral auxiliaries in nucleophilic addition and substitution reactions that lead to the asymmetric construction of carbon-carbon bonds. For example, metalation of chiral oxazoline 1 followed by alkylation and hydrolysis affords enantioenriched carboxylic acid 2. Enantioenriched dihydronaphthalenes are produced via addition of alkyllithium reagents to 1-naphthyloxazoline 3 followed by alkylation of the resulting anion with an alkyl halide to give 4, which is subjected to reductive cleavage of the oxazoline moiety to yield aldehyde 5. Chiral oxazolines have also found numerous applications as ligands in asymmetric catalysis these applications have been recently reviewed, and are not discussed in this chapter. ... 

Abstract While the use of stoichiometric amounts of sparteine and related ligands in various asymmetric reactions often lead to highly enantioselective transformations, there have been far fewer applications of sparteine to asymmetric catalysis. The aim of this review is to highlight recent advances in the field of asymmetric transformations that use sparteine as chiral auxiliary, emphasizing the use of substoichiometric or catalytic amounts of this ligand. 

Shibasaki et al. have reported an asymmetric nitroaldol reaction catalyzed by chiral lanthanum alkoxide 18 to produce an optically active 2-hydroxy-1-nitroalkane with moderate-to-high enantiomeric excesses (Scheme 8B1.10) [27]. Apparently this novel catalyst acts as Lewis base. The proposed reaction mechanism is shown in Scheme 8B1.11, where the first step of the reaction is the ligand exchange between binaphthol and nitromethane. This reaction is probably the first successful example of the catalytic asymmetric reaction promoted by a Lew i s base metal catalyst. Future application of this methodology is quite promising. 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

The synthesis of optically active selenium-containing reagents as well as their application to stereoselective synthesis is of high current interest. Several products also contain a selenium functionality and their use as chiral ligands and catalysts in asymmetric reaction is promising. Various reactions of this type are known and some recent developments in this novel area are summarized here. 

Initial successes with these ligands came independently from the groups of Jacobsen and Katsuki in the asymmetric epoxidation of unfimctionalized olefins. Since these seminal works in 1990, metal salen complexes have become workhorse in asymmetric catalysis, finding applications in a wide variety of reactions. In Figure 1 is illustrated a variety of metal salen complexes. Scheme 1 lists some of the transformations in which they have been used, demonstrating the broad utility of these complexes. 

Miscellaneous. There are several other reports on the application of this ligand to catalytic asymmetric reactions, although enantioselectivities are modest. Those reports include the Mukaiyama-Michael reaction, allylation of aldehydes, asymmetric Diels-Alder reaction, Mukaiyama-Aldol reaction of ketomalonate, aziridination reaction of a-imino esters, and asymmetric hetero-Diels-Alder reaction. ... 

Asymmetric synthesis has recently been the focus of intense interest. Especially noteworthy is the development of homogeneous catalytic asymmetric reactions, in which a small amount of chiral ligand can induce asymmetry in a given reaction. Possible applications depend on the selectivity of the homogeneous catalysts, which are therefore of great interest because they provide simple methods for synthesizing complex molecules in which enantiocontrol is needed. 

Several other chiral boron reagents are available for asymmetric aldol reactions however, each of these compounds must be synthesized in the laboratory. In certain situations, some will give higher stereocontrol than the Ipc ligands, and hence for a given reaction their application could be pursued. Chiral reagents 53 and 54 have been used in the synthesis of bryostatin 7 [36] and the Taxol side-chain [37], respectively, while bis-sulfonamide 55 has been used in the synthesis of a C24-C35 segment of FK-506 (Scheme 9-18) [38]. 

Syntheses and applications for asymmetric reactions of chiral transition metal Lewis acids bearing te(oxazolinyl)phenyl as an anionic pincer ligand 03YGK343. 

Both enantiomers of BINAP are very useful ligands for various catalytic asymmetric reactions.5 However, the scarce supply and high cost of BINAP somewhat limit their wide application. A previously reported synthesis of BINAP was not easy to scale up because of potentially hazardous conditions (320°C with HBr evolution), and low overall yield.6 This procedure presents a short and efficient process to chiral BINAP from readily available chiral 1,1 -bi-2-naphthol. 

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