Chiral binaphtholate ligands

An alternative reaction, the addition of an olefinic C—H bond to C=0, is catalyzed by Lewis acidic titanium aryloxide complexes chiral binaphthol ligands give high optical yields.96... 

Chiral (helical) Lewis acids for asymmetric Diels-Alder catalysis are prepared from titanium tetraisopropoxide 5 and a chiral binaphthol ligand 4 [13]. The titanium reagent 6 plays an important role as chiral template for the fixation of a,fi-unsaturated aldehydes and thereby for the enantioface recognition of substrates. The asymmetric Diels-Alder reaction, e. g., of cyclopentadiene 7 and acrolein 8, is effected in the presence of catalytic (P)-6 (10 mol%), producing the endo adduct 9 (R =R2=H) in 88% ee (Scheme 1). 

Fluorous chiral binaphthol ligands have been used for enantioselective addition of diethyl zinc to aldehydes in a biphasic system [20] (Scheme 3.5). 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

The synthesis of a series of chiral organophosphine oxide/sulfide-substituted binaphtholate ligands has recently been reported by Marks and Yu and their corresponding lanthanide complexes characterized. These complexes, generated in situ from Ln[N(TMS)2]3, cleanly catalysed enantioselective intramolecular hydroamination/cyclisation of 1-amino-2,2-dimethyl-4-pentene albeit with a low enantioselectivity of 7% ee (Scheme 10.82). 

Uemura and coworkers utilized (R)-binaphthol 85 as chiral ligand in place of DET in association with Ti(IV)/TBHP, which not only mediated the oxidation of sulfides to (R)-configurated sulfoxides, but also promoted the kinetic resolution of sulfoxides (equation 50). In this latter process the two enantiomers of the sulfoxide are oxidized to sulfone by the chiral reagent at different rates, with decrease of the chemical yield, but increase of the ee values. Interestingly, the presence of ortho-nilro groups on the binaphthol ligand lead to the reversal of enantioselectivity with formation of the (5 )-configurated sulfoxide. Non-racemic amino triols and simple 1,2-diols have been successfully used as chiral mediators. 

Chiral Ligand of LiAlH4 for the Enantioselective Reduction of a,p-Unsaturated Ketones. Enantioselective reductions of a,p-unsaturated ketones afford optically active ally lie alcohols which are useful intermediates in natural product synthesis. Enantioselective reduction of a,p-unsaturated ketones with LiAlH4 modified with chiral amino alcohol (1) affords optically active (S)-allylic alcohols with high ee s. When 2-cyclohexen-l-one is employed, (5)-2-cyclohexen-l-ol with 100% ee is obtained in 95% yield (eq 2). This is comparable with the results obtained using LiAlH4-chiral binaphthol and chiral 1,3,2-oxazaborolidine. ... 

These reactions can also be made enantioselective (in which case only one of the four isomers predominates) ° ° by using chiral enol derivatives, chiral aldehydes or ketones, or both. ° " Chiral bases can be used, such as prohne, ° prohne derivatives, or chiral additives, used in conjunction with the base. ° A chiral binaphthol dianion has been used to catalyze the reaction. Chiral auxiharies ° ° have been developed that can be used in conjunction with the aldol condensation, as well as chiral catalysts and chiral ligands " ... 

Methyl- and phenyl-Ti complexes with different chiral ligands have been tested in reactions with aromatic aldehydes. The enantioselectivity of the addition of chiral Ti reagents having a bidentate l,T-binaphthol ligand system to aromatic aldehydes (equation 55) has been also extensively examined. 

Similar results for the asymmetric hydrocyanation of norbornene (up to 70% yield with up to 38% ee) were obtained using an air-stable chirally modified Ni(0)L2 complex as the catalyst precursor39. The chiral (7 )-2,2 -binaphthol-derived aryl diphosphite (L1), contains three 1,1 -binaphthyl units. Its Ni(0)L2 complex is prepared from Ni(cod)2 and can be activated with trip hen ylboron39. Use of a similar chiral monophosphite ligand [L2, also derived from (/ )- 2,2 -... 

Quinine [1, (8a,9/Q-6 -methoxy-9-cinchonanol] is the most familiar of the cinchona alkaloids. Quinine has been used as a catalyst in the enantioselective addition of zinc alkyls to aldehydes (together with its acetic ester) (Section D. 1.3.1.4.), for the addition of thiols and selenols to activated double-bond systems (Sections D.2.1., D.5. and D.6.), and as a chiral ligand for cobalt catalysts in the hydrogenation of 1,2-diketones to a-hydroxycarbonyl compounds (Section D.2.3.1.) and C-C double bonds (Section D.2.5.1.2.2.). Quinine and quinidine can also be incorporated into more complex systems (forming ethers and esters with its hydroxy function) where they may act as a chiral leaving group. This technique has been applied to the synthesis of chiral binaphthols (Section D.1.1.2.2.). 

Breuzard et al. [25] prepared chiral polyether ligands derived from (S)-binaphthol and combined with the [Rh(cod)2]BF4 complex. This system has been used in the catalytic enantioselective hydroformylation of styrene in thermoregulated phase-transfer conditions, but the ee value is less than 25%. 

Chiral Oxygen Ligands. Among the oxygen donor ligands, the BINOL-type ones are predominant (68). The parent compound and its 3,3 -dibromo-substituted derivatives form efficient catalysts with titanium and zirconium (Fig. 8). Binaphthol-modified titanium(lV) compounds are efficient catalysts for ene reactions of glyoxylic esters and olefins. The replacement of isopropoxide of the titanium precursor, TiCl2(OiPr)2, is a crucial step in the formation of the catalytic species. 

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