Rhodium unsaturated carboxylic acid

Scheme 5.11 Reaction routes for various saturated and unsaturated carboxylic acids and alcohols using a rhodium catalyst and a lipase. s-g indicates sol-gel encapsulation of the catalyst superscript u and s indicate unsaturated and saturated compounds,...

Th effect of pH on the rate of hydrogenation of water-soluble unsaturated carboxylic acids and alcohols catalyzed by rhodium complexes with PNS [24], PTA [29], or MePTA r [32] phosphine ligands can be similarly explained by the formation of monohydride complexes, [RhHPJ, facilitated with increasing basicity ofthe solvent. 

Hydrogenation of unsaturated carboxylic acids, such as acrylic, methacryUc, maleic, fumaric, cinnamic etc. acids was studied in aqueous solutions with a RhCU/TPPTS catalyst in the presence of p-CD and permethylated P-cyclodextrin [7]. In general, cyclodextrins caused an acceleration of these reactions. It is hard to make firm conclusions with regard the nature of this effect, since the catalyst itself is rather undefined (probably a phosphine-stabilized colloidal rhodium suspension, see 3.1.2) moreover the interaction of the substrates with the cyclodextrins was not studied separately. 

Asymmetric catalysis undertook a quantum leap with the discovery of ruthenium and rhodium catalysts based on the atropisomeric bisphosphine, BINAP (3a). These catalysts have displayed remarkable versatility and enantioselectivity in the asymmetric reduction and isomerization of a,P- and y-keto esters functionalized ketones allylic alcohols and amines oc,P-unsaturated carboxylic acids and enamides. Asymmetric transformation with these catalysts has been extensively studied and reviewed.81315 3536 The key feature of BINAP is the rigidity of the ligand during coordination on a transition metal center, which is critical during enantiofacial selection of the substrate by the catalyst. Several industrial processes currently use these technologies, whereas a number of other opportunities show potential for scale up. 

Ruthenium and rhodium complexes that contain TMBTP have shown utility in the asymmetric hydrogenation of allylic alcohols,155,156 P-keto esters,155,157 and a,P-unsaturated carboxylic acids.155... 

Reduction of a, fi-Unsaturated Carboxylic Acids with the Rhodium(I)-(-)-MDPP Catalyst0... 

Enantiomeric excesses up to 43% were obtained in the catalytic transfer hydrogenation of some a, j9-unsaturated carboxylic acids in water using sodium formate in the presence of rhodium complexes associated with chiral sulfonated ligands such as Cyclobutanediop 3 [48]. 

Coordination of Unsaturated Carboxylic Acids in Rhodium Biphosphine Complexes... 

Figure 4. Rhodium biphosphine complexes of unsaturated carboxylic acids and carboxylates phosphorus-31 NMR spectra, MeOH, conditions as shown.

The corresponding rhodium complexes L Rh(I)nbd+Tf (L = diphosphine) were prepared in situ and appeared to form homogeneous solutions in water, although in some cases formation of micelles was observed, especially at higher temperatures. Not only were these cationic rhodium complexes shown to be cata-lytically active in homogeneous hydrogenation reactions in water using several substrates, such as unsaturated carboxylic acids, but also they may catalyze the water-gas shift reaction [13]. 

At lower rhodium concentrations, the activity of the catalyst decreases, the decrease is probably caused by the low pH of the aqueous solution in the presence of carbon dioxide and the limited stability of the active species of the hydroformy-lation cycle under acidic conditions. More recently, it has been shown that similar catalytic systems show excellent activity and very promising recycling characteristics in the hydrogenation of a,(3-unsaturated carboxylic acids such as itaconic acid [Eq. (14)] [42],... 

Various rhodium catalysts, such as [(l,5-hexadiene)RhCl]2, RhCl3, or Rh black, in methanol solution under H2 reduce and esterify unsaturated carboxylic acids. Even the phenyl ring of cinnamic acid derivatives is reduced at ambient conditions as shown in equation (3). The substrate must contain a C=C double... 

In the last 20 years, the variety of ligands available for rhodium and ruthenium-catalysed hydrogenations has increased to the point where the right combination of metal and ligand will reduce almost any unsaturated carboxylic acid derivative in high enantiomeric excess. Details are beyond the scope of this book, but we leave you with four examples, all from industrial drug syntheses, to illustrate how versatile the method can be. 

Similar metallacycles have also been prepared from unrelated substrates (a) Reaction of y,8-unsaturated sulfinilic acids with platinum(O) complexes Hallock, J. S. Galiano-Roth, A. S. Collum, D. B. Organometallics 1988, 7, 2486 (b) Reaction of y,8-unsaturated carboxylic acids with rhodium(I) complexes Marder, T. B. Chan, D. M.-T Fultz, W. C. Milstein, D. J. Chem. Soc., Chem. Commun. 1988,996 (c) Activation of a, P, y, 8, or e C-H bonds with platinum(II) complexes Kao, L.-C. Sen, A. J. Chem. Soc., Chem. Commun. 1991,1242. 

One of the standard methods for preparing enantiomerically pure compounds is the enantioselective hydrogenation of olefins, a,/3-unsaturated amino acids (esters, amides), a,/3-unsaturated carboxylic acid esters, enol esters, enamides, /3- and y-keto esters etc. catalyzed by chiral cationic rhodium, ruthenium and iridium complexes ". In isotope chemistry, it has only been exploited for the synthesis of e.p. natural and nonnatural H-, C-, C-, and F-labeled a-amino acids and small peptides from TV-protected a-(acylamino)acrylates or cinnamates and unsaturated peptides, respectively (Figure 11.9). This methodology has seen only hmited use, perhaps because of perceived radiation safety issues with the use of hydrogenation procedures on radioactive substrates. Also, versatile alternatives are available, including enantioselective metal hydride/tritide reductions, chiral auxiliary-controlled and biochemical procedures (see this chapter. Sections 11.2.2 and 11.3 and Chapter 12). 

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