Itaconic enantioselective hydrogenation

The efficiency of the new ligands was examined in enantioselective hydrogenation of some prochiral substrates. Itaconic ester hydrogenation using in situ prepared Rh-complexes was the first test reaction chosen. The best results from... 

Table 24.1. Enantioselective hydrogenation of dimethyl itaconate using Rh(l)-complexes ...

Togni and co-workers have used the convergent methodology to link phosphine-containing chiral ferrocene ligands on the cyclophosphazene core to obtain dendrimeric structures of the type 37 (Fig. 21) (201). The reaction with the cyclophosphazene end occurs by the replacement of the P-Cl bond and by the formation of the P-0 bond. The dendrimers contain twelve and sixteen ferrocene moieties respectively. The phosphine units present can coordinate to Rh(I) to afford metallic dendrimers, which have been shown to be excellent catalysts for the enantioselective hydrogenation of dimethyl itaconate. The product... 

In the early 1990s, Burk introduced a new series of efficient chiral bisphospholane ligands BPE and DuPhos.55,55a-55c The invention of these ligands has expanded the scope of substrates in Rh-catalyzed enantioselective hydrogenation. For example, with Rh-DuPhos or Rh-BPE as catalysts, extremely high efficiencies have been observed in the asymmetric hydrogenation of a-(acylamino)acrylic acids, enamides, enol acetates, /3-keto esters, unsaturated carboxylic acids, and itaconic acids. 

During the 1980s, Achiwa and colleagues examined a number of derivatives of DIOP, and found that MOD-DIOP (12c) allowed for the enantioselective hydrogenation of itaconic acid derivatives with >96% ee [68-75]. 

The R,S-family 33, and of course its enantiomer, provide high enantioselectiv-ities and activities for the reductions of itaconic and dehydroamino acid derivatives as well as imines [141], The JosiPhos ligands have found industrial applications for reductions of the carbon-carbon unsaturation within a,/ -unsaturated carbonyl substrates [125, 127, 131, 143-149]. In contrast, the R,R-diastereoisomerof30 does not provide high stereoselection in enantioselective hydrogenations [125, 141]. 

The enantioselective hydrogenation of a,fj- or / ,y-unsaturated acid derivatives and ester substrates including itaconic acids, acrylic acid derivatives, buteno-lides, and dehydrojasmonates, is a practical and efficient methodology for accessing, amongst others, chiral acids, chiral a-hydroxy acids, chiral lactones and chiral amides. These are of particular importance across the pharmaceutical and the flavors and fragrances industries. 

Table 26.7 Enantioselective hydrogenation of itaconic acid derivatives.

A PYRPHOS ligand was found to be effective for the hydrogenation of a / -aryl- or alkyl-substituted monoamido itaconate [107]. A MeO-BIPHEP-Ru catalyst was successfully applied for the enantioselective hydrogenation of an intermediate for the drug candoxatril in a mixed solvent (THF/H20) (Scheme 26.12) [108]. 

Salzer et al. prepared a set of planar-chiral diphosphine ligands based on the arene chromium tricarbonyl backbone (Fig. 36.3) [21]. The straightforward four-step synthetic route allowed the preparation of 20 ligands of this family. These ligands were tested in Ru- and Rh-catalyzed enantioselective hydrogenation of various substrates, including the standard C=C substrates (dimethyl itaconate, methyl-2-acetamidocinnamate, methyl-2-acetamidoacrylate) as well as MEA-imine (l-(methoxymethyl)ethylidene-methylethylaniline) and ethyl pyruvate. Moderate conversions and ee-values were obtained. 

In the studies conducted by Reetz, rhodium catalysts based on mixtures of monodentate phosphites, monodentate phosphonites and combinations of the two were screened in the enantioselective hydrogenation of a- and /9-N-acetyl-de-hydroamino acid esters, enamides and dimethyl itaconate [40], and a number of the more striking positive results are listed in Table 36.3. An enhanced ee-value was found mostly with combinations of two phosphonites, or one phosphonite and one phosphite, in particular when one of the ligands carries a bulky substituent and the other a small one. 

Table 42.4 Enantioselective hydrogenation of itaconic acid using sol-gel-entrapped Rh complexes [51].

Breakthroughs that took place around the year 2000 have shown, in contrast to the common view, that indeed chiral monodentate phosphorus ligands can also lead to high enantioselectivities in a number of asymmetric hydrogenations. In the years following, monophosphines, monophos-phonites, monophosphoramidites, and monophosphites have been successfully used in the enantioselective hydrogenation of a-dehydroamino acids and itaconic acid derivatives [25],... 

Marinetti [53] and Burk [54] reported the preparation of chiral l,l -bis(phos-phetano)ferrocenes (FerroTANE) independently, in which Et-FerroTANE demonstrated excellent enantioselectivity in the rhodium-catalyzed hydrogenation of itaconates. Zhang has reported a l,T-bis(phospholanyl)ferrocene hgand (f-KetalPhos) with ketal substituents at 3,4-positions [55], which proved an excellent ligand for the enantioselective hydrogenation of a-dehydroamino acid derivatives [56]. 

As enantioselective hydrogenations of prochiral substrates are undoubtedly the most common applications of chiral diphosphine ligands, a broad screening of our ligands was undertaken with some commonly used standard substrates. As substrates for the hydrogenation of C=C double bonds dimethyl itaconate (DlMl), methyl 2-acetamidoacrylate (MAA), methyl acetamidocinnamate (MAC) as an a-amino acid precursor, and ethyl (Z)-3-acetamidobutenoate ( 3-ENAM1DE) as a p-amino acid precursor were chosen (see Eig. 1.4.5). 

Several diphosphine ligands have been applied and the corresponding complexes have been tested for the immobilization (Fig. 2.1.6.3). The activity of different free and immobilized complexes in the enantioselective hydrogenation of dimethyl itaconate and methyl a-acetamidoacrylate was investigated. In blank reactions over pure mesoporous materials no reaction took place. When rhodium supported on carriers was used as catalyst, no enantiomeric excess was observed. 

Finally, several examples of the enantioselective hydrogenation of unsaturated substrates without any heteroatom attached to the olefinic double bond are noteworthy. Of particular relevance to the production of pharmaceutics, agrochemicals, flavors and aroma stuffs is the formation of the chiral 2-substituted succinates based on relevant itaconic acid derivatives. Burk et al. demonstrated that a rhodium(I) catalyst derived from (5,5)-ethyl-DuPHOS is able to hydrogenate aryl- or alkyl-substituted itaconic... 

Table 4 Enantioselective hydrogenation of itaconic acid derivatives with Rh[(S,S)-ethyl-DuPHOS](cod) +BF4 (MeOH, 25 °C, 0.56 MPa Hz)...

The continuous enantioselective hydrogenations of dimethyl itaconate (DMI) with Ru-BINAP (MW = 929 Da) and of methyl 2-acetamidoacrylate (MAA) with... 

Yonehara, K., Ohe, K. and Uemura, S.J. (1999) Highly enantioselective hydrogenation of enam-ides and itaconic acid in water in the presence of water-soluble rhodium(I) catalyst and sodium dodecyl sulfate. Org. Chem., 64, 9381. 

Scheme 12.17. Enantioselective hydrogenation of itaconic acid as a mechanistic probe for hydrogen transfer processes in SCCO2.

A rhodium complex with chiral phosphane ligand was also intercalated into sodium hectorite by cation exchange (Sento et al. ). The intercalated compound was characterized by FTIR, XRD, and TEM and the basal spacing of the compound was estimated to be 2.29 nm. This novel het-erogenized eatalyst exhibited a characteristic chiral as well as size recognition of the substrate molecule (like the "Tailor-made compounds" method used earlier by Balandin in the hydrogenation of tripticene derivatives over Ni) and was used in enantioselective hydrogenation of the itaconates (methylene-succinic acid esters). 

Sol-gel physically entrapped chiral rhodium and ruthenium complexes are used as recyclable and effective catalysts in enantioselective hydrogenation of itaconic acid up to 78% chiral methylsuccinic acid is obtained [65]. 

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