Chiral oxazoline ligands

Aromatic and vinylic sulfides take part in cross-coupling reactions with Grignard reagents in the presence of Ni catalysts.336,337 This reaction has been applied to the enantioselective synthesis of binaphthyls using a standard chiral oxazoline ligand (Equation (25)) 338... 

An alternative protocol for treating alkylzinc bromides with primary and secondary alkyl bromides and iodides was described by Zhou and Fu.409 In the study, a combination of Ni(cod)2 and a chiral oxazoline ligand 306 was used as a catalytic system providing 62-88% yield of product 307 A,A-dimethylacetamide (DMA) was the reaction solvent (Scheme 156). 

When trimethylaluminum reacted with 3-methyl-cyclohexa-2,5-dienone in the presence of chiral oxazoline ligands 86, the conjugate addition proceeded efficiently at the less-substituted double bond with up to 68% ee (Scheme 43).129,129a tw0 0 0-substituents on the phenyl ring of ligands 86 were considered to be important for selectivity. 

Recently, asymmetric NHK reactions have been investigated. Among them, catalytic versions of this reaction have been successful a Cr-chiral salen complex 127,226-228 a Cr-chiral sulfonylamide complex 128,229 a Cr-tridentate ligand 129 complex,230,231 and a Cr-chiral oxazoline ligand 130 complex232 were found to be effective to achieve good to high enantioselectivity (Scheme 57). 

A very useful extension of the de Mayo reaction has been recently introduced by Blechert et al. (Scheme 6.26) [78]. The retro-aldol fragmentation was combined with an intramolecular enantioselective allylation (asymmetric ring-expanding allylation) catalyzed by a chiral Pd complex. Bicycloheptane 68, for example, was accessible by intermolecular [2 + 2]-photocycloaddition of cyclopentenone 67 with allene. Further transformation in the presence of Pd2(dba)3 (dba = dibenzylideneacetone) and the chiral oxazoline ligand 69 (tBu-phox) resulted in the enantioselective formation of cycloheptadione 70. 

Some typical examples of PS-PEG resin-supported chiral oxazoline ligands [39], which have aquacatalyhc potential as well as stereoselechve abilihes, are illustrated in Eigure 6.9. 

Azidations with azidobenziodoxoles can be effectively catalyzed by iron salts, such as iron(II) propionate, Fe(02CEt)2. In the presence of chiral oxazoline ligands (boxmi ligands), the Fe-catalyzed azidations of... 

Chiral oxazoline ligands 24a,bearing a chiral sulfinyl group provided enantiose-lectivity of (5)-2 with 88% or 55% ee, respectively, in the Pd-catalyzed alkylation of 1 with dimethyl malonate using [Pd(7r-allyl)Cl]2, BSA, and KOAc in dichloromethane... 

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. ... 

Since Evans s initial report, several chiral Lewis acids with copper as the central metal have been reported. Davies et al. and Ghosh et al. independently developed a bis(oxazoline) ligand prepared from aminoindanol, and applied the copper complex of this ligand to the asymmetric Diels-Alder reaction. Davies varied the link between the two oxazolines and found that cyclopropyl is the best connector (see catalyst 26), giving the cycloadduct of acryloyloxazolidinone and cyclopentadiene in high optical purity (98.4% ee) [35] (Scheme 1.45). Ghosh et al., on the other hand, obtained the same cycloadduct in 99% ee by the use of unsubstituted ligand (see catalyst 27) [36] (Scheme 1.46, Table 1.19). 

Non-covalently Immobilized Catalysts Based on Chiral Bis(oxazoline) Ligands... 

Metal complexes of chiral bis(oxazoline) ligands, in most cases Cu(II) complexes, have been supported by cationic exchange on inorganic, organic, and composite anionic solids. 

Fig. 18 Chiral iminobis(oxazoline) ligands used for immobilization...

In 1997, Uozumi and Hayashi found high enantioselective Wacker-type cycUza-tion of o-allylphenols or o-homoaUylphenols by using Pd(II) catalysts coordinated with chiral bis(oxazoline) ligands based on the 1,1 -binaphthyl backbone (Eq. 6.36)... 

In 1999, Ikeda et al. reported a new type of sulfur-oxazoline ligands with an axis-fixed or -unfixed biphenyl backbone prepared in good yields by coupling reactions of methoxybenzene derivatives substituted with a chiral oxazoline and a sulfur-containing Grignard reagent. These ligands were subsequently evaluated for the test palladium-catalysed asymmetric allylic alkylation... 

In addition, Bryce and Chesney have developed chiral oxazolines linked to tetrathiafulvalene in order to use these ligands as redox-active ligands. When applied to the test reaction, these ligands gave only low enantioselec-tivities (<21% ee), as shown in Scheme 1.32. 

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