Acids binaphthyl phosphoric

This compound, in common with other suitable substituted biphenyls, possesses a chiral axis (p. 6) and is isolated from the reaction as a racemate. Although several resolution procedures have been reported, the superior method to date4 is that in which the binaphthol is first converted by treatment with phosphorus oxychloride into the binaphthyl phosphoric acid (14). Resolution is then effected by formation of diastereoisomeric salts with (+ )-cinchonine, appropriate fractional crystallisation and recovery of the (S)-( + )-binaphthyl phosphoric acid. Suitable hydrolysis gives (S)-( — )-l,l -bi-2-naphthol (15). 

In addition, chiral Schiff base catalysts, which were developed previously for the Strecker reaction, were also found to be suitable catalysts for the Mannich reaction starting from imines and enolates [36, 37]. Very recently, further efficient organocatalysts for the Mannich reaction, such as chiral pyrrolidinyltetrazole and chiral binaphthyl phosphoric acids, have been reported [38]. 

Another type of organocatalyst, which is suitable for the Mannich reaction with ketene silyl acetals, is a chiral binaphthyl phosphoric acid [38c]. Very recently, it has been reported that high enantioselectivity of up to 96% ee can be obtained with this type of catalyst [38c]. 

A heterocyclic pentahelicene structure hides behind the cyclic binaphthyl phosphoric acid derivatives 170a,b, which have been applied as reagents for the separation of enantiomers [ot] ) = +530 (c = 1.35, in methanol). The enantiomer separation of 170 was achieved using the cinchonine salt... 

Fig. 66. Cyclic binaphthyl phosphoric acid derivatives (BPA, 170a, h )...

More effective regio- and enantiosclcctivc asymmetric synthesis of branched optically active free carboxylic acids via palladium-catalyzed hydrocarboxylation of alkenes is achieved in the presence of (-)-(/ )- or ( + )-(5)-2,2/-(l,r-binaphthyl)phosphoric acid23. 

Quinolizidine behaves as a tertiary cycloaliphatic amine, and its protonation leads to the quinol-izidinium cation (126). This basicity allows resolution of optically active quinolizidines with chiral acids. As an example, optical resolution of butaclamol analogues (127) was accomplished with (-1-)-and (— )-binaphthyl phosphoric acids <5lJA368l>. 

At almost the same time, You and coworkers developed the same type of asymmetric reaction by employing the binaphthyl phosphoric acid Ig. In their case, the isopropyl ester group was the best substituent as for the enantioselectivity [25]. 

A well-defined chiral pocket produced by the binaphthyl skeleton and the appended bulky 3,3 substituents, (iii) A ring structure attached to the phosphoric acid moiety to prevent free rotation at the a-position of the phosphorus center. This feature is not found in other Bronsted acids such as carboxylic and sulfonic acids (Figure 5.2). 

In Ught of the recent developments in thiourea, diol, and phosphoric-acid-mediated catalysis, far fewer studies have focused on the use of chiral carboxyhc acids as suitable hydrogen bond donors. To this end, Mamoka synthesized binaphthyl-derived dicarboxylic acid 49 which catalyzes the asymmetric Mannich reaction of N-Boc aryl imines and tert-diazoacetate (Scheme 5.65) [120]. The authors postulate that catalytic achvity is enhanced by the presence of an addihonal car-boxyhc acid moiety given that use of 2-napthoic acid as catalyst provided only trace amounts of product... 

For the 3,3 -dimethyl derivative 8, several other methods have been developed. The racemic 3,3 -dimethy] compound can be prepared by aminomcthylation of binaphthol with subsequent conversion of the aminomethy] function to a methyl group, and resolved analogously to binaphthol via the phosphoric acid derivative15. Another possibility is the preparation [oxidation of 3-hydroxy-2-naphthoic acid with iron(III) chloride] and resolution [with (S)-leucine methyl ester] of 2,2 -dihydroxy-1,l -binaphthyl-3,3 -dicarboxylic acid (7). 

The chiral catalysts that have been used in nitroaUcene conjugate additions include bw-oxazolines with Cu(OTF)2 [281] or Zn(OTf)2 [282], tridentate bw-oxazolines with Zn(OTf)2 [283], mixed thiazoline-oxazolines with Zn(OTf)2 [284], imidazoUne-aminophenols with CuOTf [285], bis-trifluoromethylsulfonamides [286], binaphthyl sulfonamides [287], binaphthyl imines [288], thioureas [289], and quinoUnyl thioureas [290]. A BINOL-phosphoric acid with 3A molecular sieves gave ee values consistently at 90% and above with both p-alkyl and p-aryl nitroalkenes [291]. 

Excellent enantioselectivity has also been obtained using 3,3 -6 s-(l-naphthyl) BINOL-phosphoric acids [311]. A -Tosyl imines of aryl aldehydes were also examined using a binaphthyl Pd(II) carbene complex as the catalyst. Enantioselectivity in the 50-75% range was obtained [312]. Imines formed from ot-phenylethylamine and ethyl 3,3,3-trifluoropyruvate give adducts with 85-97% de in the presence of TEA [313]. The chiral auxiliary can be removed by hydrogenolysis. 

There is also one example in which a chiral phosphoric acid has been employed as catalyst in the reaction. In particular, the addition of several cyclic p-ketoesters to methyl vinyl ketone was found to occur smoothly in the presence of several chiral phosphoric acids (Scheme 4.35). As mentioned earlier, a key feature of the chiral phosphoric acid catalyst is the backbone binaphthyl axial chirality together with the incorporation of bulky substituents at the 2 positions. In this case, 60b was identified as an appropriate promoter of the reaction leading to the corresponding Michael adducts in excellent yields, although with moderate enantioselectivity. In addition, the authors succeeded in applying this reaction to a procedure to carry out a subsequent Robinson-type annulation. 

Several chiral organocatalysts such as chiral squaramide 31, chiral phosphoric acid (S)-32a, and binaphthyl-based chiral sulfonimide (i )-33 have also been successfully applied in the AFC reaction of indoles with N-sulfonyl imines (Scheme 6.14). 

Other chiral Bronsted acid organocatalysts developed more recently include chiral binaphthyl-derived disulfonic acids [162] and sulfonimides [163] and SPINOL-derived phosphoric acids [164]. 

Examples of efficient catalysts for the asymmetric hydrogenation of aromatic and heteroaromatic compounds are quite rare, even with hydrogenation procedures catalyzed by chiral Rh, Ru, and Ir complexes. Therefore an important breakthrough was by Rueping s group in 2006 the development of an enantioselective phosphoric acid-catalyzed partial reduction of quinoline derivatives [62]. This represents the first example of a metal-free reduction of heteroaromatic compounds. (/ )-(—)-9-phe-nanthryl-l,l -binaphthyl-2,2 -diyl hydrogenphosphate was selected as chiral element of choice to perform stereocontrol (97% ee. Scheme 15.29). 

As protons are the simplest and most easily available Lewis acids available to catalyze organic transformations, the use of a chiral Br0nsted acid combines the potential of proton catalysis with asymmetric induction achieved through the choice of the proper coimteranion. It is without doubt that chiral phosphoric acids have been the most successfully used chiral Br0nsted acids so far (369, 371). With respect to the chiral backbone employed therein, binaphthyls have been found to be the most promising substances for this purpose. 

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