Itaconic acid ester hydrogenation

Scheme 6 Process for itaconic acid ester hydrogenation (BASF)...

The amide group is superior to the ester as a directing group (equation 4).i5i. 52 The cationic rhodium complex (15) catalyzes the hydrogenation of 3-substituted itaconic acid esters with high diastereoselectivity. When a chiral rhodium complex is employed, effective kinetic resolution occurs. 

Stereoselective hydrogenation of itaconic acid esters Rhodium- organic catalyst Improved selectivity, high rate 45... 

With 56g as the heterogeneous catalyst, asymmetric hydrogenation of a-dehydroamino acid esters 48b and 48c, enamide 50a, and itaconic acid ester 57 were performed in toluene at room temperature under 40 atm of H2 to afford the corresponding products with full conversion in 90-97% ee, which were comparable or even superior to those obtained with their homogeneous counterpart (MonoPhos)2/Rh(I). Upon completion of the reaction, the catalyst... 

Prochiral organic acids were hydrogenated on clay-supported Rh-chiral phosphine complexes.205,206 Hectorite-supported chiral Rh(I)-phosphine complexes were used for the asymmetric hydrogenation of a,P-unsaturated carboxylic acids.207 It was found that the interaction between the a-ester group of itaconates and phenyl groups of phosphine can play an important role in the determination of the configuration of products. 

Hydride-promoted reactions are also well known, such as the acrylic and vinylacrylic syntheses (examples 7-10, Table VII). Some less-known compounds, which form in the presence of halide ions added to tetracar-bonylnickel, have been described by Foa and Cassar (example 11, Table VII). Reaction of allene to form methacrylates, and of propargyl chloride to give itaconic acid (via butadienoic acid), have been reported (examples 13 and 14, Table VII). 1,5-Hexadiene has been shown to be a very good substrate to obtain cyclic ketones in the presence of hydrogen chloride and tetracarbonylnickel (example 15, Table VII). The latter has also been used to form esters from olefins (example 16, Table VII). In the presence of an organic acid branched esters form regioselectivity (193). 

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. 

In contrast to the many successful examples for hydrogenation of the parent itaconic acid or its dimethyl ester, only a few ligands have been reported to be efficient for the hydrogenation of / -substituted itaconic acid derivatives. Rh complexes with chiral ligands such as MOD-DIOP,69,69a 69h BPPM,246 Et-DuPhos,247 and TangPhos116 are... 

Table 7 Asymmetric hydrogenation of itaconic acid or dimethyl ester...

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. 

Many chiral phosphorus ligands have shown excellent reactivities and enantio-selectivities in the Rh-catalyzed hydrogenation of itaconic acids or esters. Some successful (>95% ee) hydrogenations of itaconic acid or its dimethyl ester with different chiral phosphorus ligands are listed in Table 26.7. High reactivity is observed with electron-rich phosphane ligands such as BICHEP [7c[. 

In contrast to the many successful examples of hydrogenation of the parent itaconic acid or its dimethyl ester, only a few ligands have been reported to be... 

Table 28.3 Enantioselective hydrogenation of itaconic acid and its dimethyl ester.

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. 

Excellent results have been obtained in the asymmetric hydrogenation of itaconic acid (97% ee) and dimethyl ester (94% ee) using Rh/MonoPhos.31 The latter substrate could be hydrogenated in 99% ee using the piperidine ligand 24d.40 These hydrogenations are relatively fast and have been carried out on a 100-g scale with an S/C of 10,000. Preliminary results with some alkylidene and benzylidene succinates were also very promising. 

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). 

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