Onium chiral

For enantioselectivity to be possible multipoint interaction between the catalyst and the reactant in the transition state is necessary. The most effective chiral onium salts are derivatives of cinchona alkaloids (see Fig. 3.59). 

Abstract Phase transfer catalysts including onium salts or crown ethers transfer between heterogeneous different phases and catalytically mediate desired reactions. Chiral non-racemic phase transfer catalysts are useful for reactions producing new stereogenic centers, giving chiral non-racemic products. Recent developments in this rapid expanding area will be presented. 

Asymmetric phase-transfer catalysis using chiral nonra-cemic onium salts or crown ethers has now grown into a practical method whereby a large number of reactions can be performed and some optically pure compounds can be produced effectively on a large scale. 

The remaining three chapters (Chapters 20-22) concentrate fully on onium ion chemistry. In Chapter 20, H.-J. Frohn and V. V. Bardin describe synthesis and multi-nuclear NMR studies of organoxenonium salts. Chapter 21 by R. S. Brown et al. focuses on the synthesis of chiral... 

The reaction conditions are rather mild, so that the possibility of side reactions, such as catalyst decomposition, is considerably reduced. The major challenge associated with these reactions is an absence of prochirality in the anionic species. Namely, the chiral onium anion must discriminate the enantiotopic face of the distant electronically neutral reaction partner in the organic phase. 

Onium ions are cationic species with expanded valencies. The most important onium ions relevant to cationic polymerizations are oxonium, sulfo-nium, ammonium, and phosphonium ions. Ammonium, phosphonium, and sulfonium ions are tetrahedral with angles of approximately 109.5° between substituents. These species may be chiral if all... 

ENAMINE-BASED ORGANOCATALYTIC MICHAEL REACTIONS USING A CHIRAL ONIUM ION-TAGGED PYRROLIDINE... 

Preparation.—Conventional quaternization reactions of phosphines with alkyl halides have been used for the preparation of chiral P-hydroxyalkylphosphonium salts for use in prostaglandin synthesis and of the salts (111), (112), and (113). This approach has also been used for the preparation of polymer-bound phosphonium salts for use in subsequent Wittig reactions and of a range of co-dialkylaminoalkylphosphonium salts. The salt (114), of limited thermal stability, is formed on treatment of the parent phenylphosphaferrocenophane (67, R = Ph) with iodomethane. The oxonium salt (115) is converted into the mixed onium salt (116) on treatment with triphenylphosphine. A range of... 

Quaternary onium salts. Quaternary ammonium salts include trioctymethylammo-nium chloride (Starks catalyst), Aliquat 336, tricaprylmethylammonium chloride, tetrabutylammo-nium hydrogen sulfate (Brandstrom s catalyst), and benzyltrimethylam-monium chloride (Mqkosza s catalyst) quaternary ammonium salts can also be generated in situ from trialkylamines, etc. Other quaternary onium salts include tetrabutylphospho-nium bromide, tetraphenylphosphonium bromide, triphenylbenzylphosphonium chloride, tetraphenylarsonium chloride, and triphenylsulfonium chloride, etc. Special quaternary salts are 4-aminopyridinium salts, bis-(quaternary ammonium) [R3N -(CH2) -NR3" R3N -(CH2) -0-(CH2) -NRj ] salts, 4,4 -dialkylbipyridinium salts, cluster quaternary ammonium [e.g., P(C6H4S03"NR4)3] salts, crown-quaternary salts [e.g. (18-crown-6)-(CH2)9PBuj Br ], and chiral V-(4-trifluoromethyl)benzylcinchonium bromide, etc. 

Non-crosslinked polystyrene with salen ligand Non-crosslinked polystyrene with onium salt residues Non-crosslinked polystyrene copolymerised with a chiral phosphine ligand 14,765 0.5-3.8" 0.7-2.3 CH2Cl2,THF, EtOAc, DMF DMF,DMSO,DMA, toluene, anisole, MeOH,MeCN, diglyme Asymmetric epoxidation Regioselective addition reaction of phenyl glycidyl ether with S-phenyl thioacetate Pt-catalysed asymmetric hydroformylation of olefins Precipitation (methanol) (r) Precipitation (diethyl ether) (r) [68] [142] [143]... 

Recent advances in asymmetric phase-transfer catalysis using chiral nonracemic onium salts and crown ethers 07AG(E)4222. 

The remarkable inclusion capacity of organic onium salts has been shown for numerous representatives of this family of host compounds Thus it is obvious that optically pure onium salts, preferrably to be taken from the chiral pool, are (very) promising host resolving agents. 

Fig. 5 Typical cyclophane receptors for metal ion complexation (siderophore 31. torand 32, crown ether 36), onium ion-rt interactions (33, crpptophane 38). hydrogen bonding interactions (34-37), and chiral recognition (36-38). Self-assembled cyclophanes 39 and 40.

Among other polymer-bound catalysts, onium compounds can be used successfully in halogen exchange reactions between activated and nonactivated halides. This is the case, for instance, in additions to the double bonds of dichlorocarbene to form substituted cyclopropanes and in C-alkylating nitriles. When optically active polymeric ammonium compounds are used as catalysts in carbene addition reaction, chiral products form. ... 

Over the past decades, quaternary ammonium- and phosphonium salts have been widely employed as effective phase-transfer catalysts in reactions between snb-stances located in different immiscible phases. Recently, efforts have been made to unlock the full potential of chiral non-racemic onium salts as versatile catalysts for asymmetric carbon-carbon bond formation. These reactions can be condncted nnder mild biphasic conditions and the phase-transfer catalysts can often be derived from readily available naturally occurring alkaloids. The reaction proceeds since the catalyst forms a well-defined chiral ion pair with the electrophile. As a result one enantiotopic face is shielded and enantioselective carbon-carbon bond formation can be realized. 

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