Chiral binaphthol

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%). 

Allylstannane additions to aldehydes can be made enantioselective by use of chiral catalysts. A catalyst prepared from the chiral binaphthols R- or S-BINOL and Ti(0-i-Pr)4 achieves 85-95% enantioselectivity.187... 

Another useful reagent of this type is chiral binaphthol (see Structure 7.5). 

Yamamoto and colleagues prepared chiral titanium catalyst 420 from titanium tetraisopropoxide and chiral binaphthol 419 (equation 126). This catalyst gave high asymmetric inductions in various Diels-Alder reactions of a,/J-unsaturated aldehydes with cyclopen-tadiene and 1,3-cyclohexadiene260. 

Several chiral lanthanide(III) Lewis acid catalysts, derived from chiral binaphthols, have been used in the cycloaddition reactions of cyclopentadiene with substituted iV-acryloyl-1,3-oxazolidin-2-ones. A catalyst derived from ytterbium triflate, (R)-binaphthol... 

Enantioselective trimethylsilylcyanation of benzaldehydes has been achieved using a lanthanum alkoxide of a chiral binaphthol as catalyst. ... 

In an effort to impart a stronger twist sense bias on the mPE helix, a chiral binaphthol derivative was placed directly into the mPE backbone [64]. The chiral binaphthol unit was placed both at the center (47) and terminus (48) of the oligomer in order to determine how the position of the chiral unit would affect the twist sense bias and folded state stability. Solvent denaturation studies (UV... 

Terada and co-workers reported a novel guanidine catalyst with a chiral binaphthol backbone for the asymmetric addition of dicarbonyl compounds to nitro-olefins [126]. Substitution on the binaphthol backbone dramatically increased enantioselectivity. 

Asymmetric ene reactions can be achieved by using chiral Lewis acids as shown in Scheme 113 (275). Chiral binaphthol-based organoalu-... 

H MeLi, LiBr/chiral HNR2 chiral binaphthol-SnCU chiral imide Tetr Asym 4 35 (1993) (chiral) JACS 116 11179 (1994) (chiral) Angew Int 33 107 (1994) (chiral)... 

With Chiral DiolTTi Complexes Harada investigated the influence of the torsional angles of biaryl rings on the enantioselecdvity in the asymmetric Diels-Alder reaction of cyclopentadiene with methyl acrylate catalyzed by a series of Ti complexes possessing chiral 2,2 -biaryldiols. The best results were obtained by the use of chiral binaphthol or 6,6 -hexylenedioxy-2,2 -biphenyldiol [52] (Eq. 8A.29). 

The chiral boron complex prepared in situ from chiral binaphthol and B(OPh)3 is utilized for the asymmetric aza-Diels-Alder reaction of Danishefsky s diene and imines [67] (Eq. 8A.43). Although the asymmetric reaction of prochiral imine affords products with up to 90% ee, the double asymmetric induction with chiral imine by using oc-benzylamine as a chiral auxiliary has achieved almost complete diastereoselectivity for both aliphatic and aromatic aldimines. This method has been successfully applied to the efficient asymmetric synthesis of anabasine and coniine of piperidine alkaloides. 

A mechanism has been proposed for the enantioselective Mikami ene reaction of a terminal alkene with a glyoxylic aldehyde using a chiral binaphthol as Lewis acid.80... 

Chiral binaphthol(BINAP)-titanates (59 X = OR) have been used as asymmetric catalysts of additions to aldehydes, and show evidence of oligomeric Ti—O—Ti... 

The asymmetric oxidation of sulphides to chiral sulphoxides with t-butyl hydroperoxide is catalysed very effectively by a titanium complex, produced in situ from a titanium alkoxide and a chiral binaphthol, with enantioselectivities up to 96%342. The Sharpless oxidation of aryl cinnamyl selenides 217 gave a chiral 1-phenyl-2-propen-l-ol (218) via an asymmetric [2,3] sigmatropic shift (Scheme 4)343. For other titanium-catalysed epoxidations, see Section V.D.l on vanadium catalysis. 

The prototype of a first fluorescence sensor for enantioselective recognition of chiral amino alcohols was synthesised by Pu et al. by linking phenylacetylene dendrons to an axially chiral binaphthol core unit (see Fig. 4.71) [14c, d, 25],... 

Kagan and Schiffers carefully studied the effect of the lithium salts of BINOL (17) and related axially chiral binaphthols on the reduction of a variety of ketones with trialkoxysilanes [24]. They found that diethyl ether, with TMEDA as an additive, was the best solvent for asymmetric reduction of ketones. In the presence of 5 mol% of the monolithium salt of BINOL (17), acetophenone (1) could be reduced with trimethoxysilane in 80% yield and with 61% ee. Enantiomeric excesses > 90% were achieved under the same conditions with 2, 4, 6 -trimethyl-acetophenone (18) or a-tetralone (19) as substrates. Aliphatic ketones such as... 

Scheme 5.19 The achiral phosphine and chiral binaphthol-derived Bronsted acid-catalyzed MBH reaction of cyclohexenone and 3-phenyl propionaldehyde.

Typical Procedure for the Triethyl Phosphine and Chiral Binaphthol-Derived Bronsted Acid Co-Catalyzed Asymmetric MBH Reaction [43] (pp. 173 and 232)... 

S/Uj to Br rearrangement. A new route to hindered chiral binaphthols involves a 1,3-rearrangement of SiR3 with f-BuLi. 

The use of a chiral reagent for the oxidative coupling of naphthols has received much attention as the product chiral binaphthols are widely used in asymmetric synthesis [138]. The well-known oxidative coupling of 2-naphthol by dioxygen in the presence of a copper com-... 

The chiral BINAPHTHOL derivative (H)-ll and two equivalents of Zn(C2H5)2 produced an active catalyst for the asymmetric reduction of acetophenone with... 

MeLi, LiBr/chiral HNR2 chiral binaphthol-SnCl4 chiral imide... 

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 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. ... 

Vederas and co-workers have reported the ene reaction of an allylglycinate 262 with methyl glyoxylate using copper Lewis acids (Sch. 57) [104], Chiral binaphthol-titanium complexes did not catalyze the ene reaction and only the starting material was recovered. 

A mechanism has been proposed for the enantioselective Mikami ene reaction of a terminal alkene with a glyoxylic aldehyde using a chiral binaphthol as Lewis acid. Stereoselective synthesis of 5-amino esters via asymmetric aldol-type and aza-Diels-Alder reactions has been reviewed. Siliranes react cleanly with benzaldehyde to produce oxasilacyclopentanes—with inversion— under conditions of Bu OK catalysis enolizable aldehydes yield sdyl enol ethers. ... 

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