Tartaric acid-based catalysts

Among the various strategies [34] used for designing enantioselective heterogeneous catalysts, the modification of metal surfaces by chiral auxiliaries (modifiers) is an attractive concept. However, only two efficient and technically relevant enantioselective processes based on this principle have been reported so far the hydrogenation of functionalized p-ketoesters and 2-alkanons with nickel catalysts modified by tartaric acid [35], and the hydrogenation of a-ketoesters on platinum using cinchona alk oids [36] as chiral modifiers (scheme 1). 

Pyruvic acid is the simplest homologue of the a-keto acid, whose established procedures for synthesis are the dehydrative decarboxylation of tartaric acid and the hydrolysis of acetyl cyanide. On the other hand, vapor-phase contact oxidation of alkyl lactates to corresponding alkyl pyruvates using V2C - and MoOa-baseds mixed oxide catalysts has also been known [1-4]. Recently we found that pyruvic acid is obtained directly from a vapor-phase oxidative-dehydrogenation of lactic acid over iron phosphate catalysts with a P/Fe atomic ratio of 1.2 at a temperature around 230°C [5]. 

In 2006, Wang et al. reported the synthesis of a new camphor-derived disulfonamide ligand based on L-tartaric acid that was employed in similar reactions to those described above, giving rise to enantioselectivities of up to 83% ee by using 5 mol% of catalyst loading (Scheme 3.43). ... 

In 1998, Ruiz et al. reported the synthesis of new chiral dithioether ligands based on a pyrrolidine backbone from (+ )-L-tartaric acid. Their corresponding cationic iridium complexes were further evaluated as catalysts for the asymmetric hydrogenation of prochiral dehydroamino acid derivatives and itaconic acid, providing enantioselectivities of up to 68% ee, as shown in Scheme 8.18. 

The directive effect of allylic hydroxy groups can be used in conjunction with chiral catalysts to achieve enantioselective cyclopropanation. The chiral ligand used is a boronate ester derived from the (VjA jA N -tetramethyl amide of tartaric acid.186 Similar results are obtained using the potassium alkoxide, again indicating the Lewis base character of the directive effect. 

The chapter Chiral Modification of Catalytic Surfaces [84] in Design of Heterogeneous Catalysts New Approaches based on Synthesis, Characterization and Modelling summarizes the fundamental research related to the chiral hydrogenation of a-ketoesters on cinchona-modified platinum catalysts and that of [3-ketoesters on tartaric acid-modified nickel catalysts. Emphasis is placed on the adsorption of chiral modifiers as well as on the interaction of the modifier and the organic reactant on catalytic surfaces. 

The development of a large scale manufacturing route to Esomeprazole is described by Federsel and Larsson ° using the titanium catalyst originally described by Kagan and Luukas. Employment of a tartaric acid derived chiral auxiliary, with the addition of a base such as diisopropylethylamine to the reaction mixture, resulted in a full-scale catalytic process capable of delivering multi-ton quantities of product with optical yields well above 90 %, a figure which could be raised to 99.5 % ee by recrystallization from methyl isobutyl ketone. 

In other diethylzinc studies, a neural network modelling approach has been used to predict the utility of new enantioselective catalysts,222 norephedrine-derived ligands with three stereogenic centres catalyse enantioselective addition to aldehydes and to chalcones,223 and a chiral sulfonamide ligand based on tartaric acid gives good ees in addition to both aldehydes and ketones.224... 

Highly porous silica gel served as a support for the TADDOL moiety derived from inexpensive and readily available i-tartaric acid, which provided access to htanium-based Lewis acid catalysts (Heckel, 2000). Such entihes are employed successfully for enantioselective reactions. TADDOLs were covalently attached to the trimethyl-silyl-hydrophobized silica gel, controlled-pore glass (CPG) at about 300 m2 g-1, at a loading of 0.3-0.4 mmol gl (Heckel, 2002). In a carefully monitored mulh-step immobilization procedure, the TADDOLs were titanated to yield dichloro-, diisopropyl-, or ditosyl-TADDOLates. These catalysts were employed in dialkylzinc addihon to benzaldehydes and diphenyl nitrone addihon to 3-crotonyloxazolidinone, a [3+2] cycloaddition. 

The asymmetric addition of glycine enolates to acrylates was also achieved by use of the tartaric acid-derived phase-transfer catalysts 27 and 28 (Scheme 4.9). Arai, Nishida and Tsuji [13] showed that the C2-symmetric ammonium cations 27a,b afford up to 77% ee when t-butyl acrylate is used as acceptor. The cations 28 are the most effective/selective PTC identified by broad variation of the substituents present on both the acetal moiety and nitrogen atoms [14], In this study by Shibasaki et al. enantiomeric excesses up to 82% were achieved by use of the catalyst 28a (Scheme 4.9) [14], Scheme 4.9 also shows the structure of the guanidine 29 prepared by Ma and Cheng in the absence of additional base this also catalyzes the Michael addition of the glycine derivative 22 to ethyl acrylate, albeit with modest ee of 30% [15],... 

The vast majority of successful chiral catalysts to date are based on tartaric acid, BINOL, or oxazolidinone derivatives (Table 26.1). Because derivatives of both of these compounds are commercially available, scale up should not present a problem. If the observed asymmetric induction is found to be low with catalysts based on tartaric acid or oxazolidinones, the sterically hindered titanium BINAP-type complexes should allow for increased selectivity. In addition, nontoxic metal counterions, such as iron and aluminum, do not appear to compromise the asymmetric induction. 

In the 1970s and early 1980s the development of new catalysts was mainly based on new optically active chelating phosphines used in Wilkinson-type catalysts. This era of design and synthesis of optically active bidentate phosphines started in 1971 with Kagan s tartaric acid derived ligand DIOP [59, 60]. Successful and well-known examples followed, namely DIPAMP [62], prophos [63], chiraphos [64], BPPM [65], BPPFA [66], norphos [67], and BINAP [68]. A selection is depicted in Figure 3. 

Yamamoto and coworkers have developed a practical Diels-Alder catalyst for aldehyde dienophiles. Treatment of a monoacylated tartaric acid with borane released ca. 2.2 equiv of H2 gas, affording a complex that has been assigned structure 7. Circrunstantial evidence for structure 7 was found in the comparable enan-tioselectivity of a catalyst in which the free carboxyl group was esterified (see below). The chiral (acyloxy)borane (CAB) complex is effective in catalyzing a number of aldehyde-based Diels-Alder reactions (Scheme 9) [56]. Reactions with... 

In contrast to Raney nickel catalysts ( 3.4.1), heterogeneous hydrogenation catalysts based on Pt, Rh or Pd do not induce asymmetry in the presence of tartaric acid [113, 578], Platinum catalysts modified by cinchona alkaloids 3.1 and 3.2 cause asymmetric hydrogenation of the carbonyl group of a-ketoesters with a high enantiomeric excess (> 90%). From other types of ketones, the enantioselectivities are lower. 

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