Enantioselectivity catalytic hydrogenation

The hydrogenation of a cinnamate was also investigated as a first step to determine kinetics and finally to come to a quantitative determination of kinetic models and parameters in asymmetric catalysis [64]. The enantiomeric excess of enantioselective catalytic hydrogenations is known to be dependent on pressure, chiral additives and mixing. Such dependences are often due to kinetics, demanding appropriate studies. 

Fig. 29. Dendritically enlarged diphosphine enantioselective catalytic hydrogenations ai... 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Kagan et al. were the first to report the corresponding enantioselective catalytic hydrogenation using chiral metallocene derivatives [94, 95]. By using menthyl- and neomenthyl-substituted cyclopentadienyl titanium derivatives in the presence of activators (Scheme 6.5) [96], these authors observed low ee-values (7-14.9%) for the catalytic hydrogenation of 2-phenyl-l-butene into 2-phenylbutane. In contrast, no enantiomeric excess was obtained with the corresponding zirconocene derivatives. 

Enantioselective catalytic hydrogenation. The ruthenium(II) complexes of (R)- and (S)-l, bearing a chiral BINAP ligand, catalyze asymmetric hydrogenation of N-acyl-l-alkylidenetetrahydroisoquinolines to give (1R)- or (lS)-tetrahydroiso-quinolines in 95-100% ee.1 Thus the (Z)-enamide (2), prepared by acylation of 3,4-dihydropapaverine, is hydrogenated in the presence of (R)-l to (1R)-tetrahydroisoquinolines (3). The enantiomeric (lS)-3 is obtained on use of (S)-l as catalyst. 

Daley and Bergens [124] have used this approach to characterize the 3-D structure of the intermediate [Ru(ii)(alkoxide)(CH3CN)(Binap)]BF4, 84, in their study of an enantioselective catalytic hydrogenation of a ketone. The ortho-protons of the equatorial and axial P-phenyl rings provide the reporter protons. 

Enantioselective catalytic hydrogenation has also been apphed to the preparation of optically active duloxetine intermediates (Scheme 14.15). In one such report, (3-keto amine 31 was converted to amino alcohol 32a in excellent enantiomeric excess without... 

On the other hand, the preparation of TV-acetyl- and iV-benzoylferrocenylalanines 102 and 103 by enantioselective catalytic hydrogenation of the Z-alkenes 100 and 101 has been reported (Scheme 29).[6S1 In this case, a comparative study showed that among different chiral ligands for Rh, (+)- or (—)-Norphos were the ones giving the highest enantioselec-tivities (up to 94% ee) in the hydrogenation of 100. 

Enantioselective catalytic hydrogenations have also been accomplished with various substituted pyran-2-ones using ruthenium catalysts <1999JOC5768>. Hydrogenation was found to occur selectively at positions 5 and 6 with up to 97% ee. Substitution at position 3 influenced the selectivity and rendered further hydrogenation of the double bond at positions 3 and 4 much slower. 

Enantioselective catalytic hydrogenation. Hoffmann-LaRoche chemists have examined the efficiency of DIOP (1) and a number of related phosphines as ligands for asymmetric hydrogenation of acrylic acids for synthesis of (R)-6-methyl-tryptophan (6), of potential value as a non-nutritive sweetening agent. Of these, 2 and the less available 3 showed the most promise. Thus catalytic hydrogenation of 4... 

The investigation and development of the enantioselective catalytic hydrogenations of the allylic alcohols required the ready availability of a number of BIPHEP type diphosphanes for screening and optimization purposes as well the availability of the most interesting ligands, i.e., of (S)-MeOBIPHEP (S-29) and (S)-pTol-BI-... 

Asymmetric induction occurs by enantioselective catalytic hydrogenation of a cyclic anhydride using an Ru(II) chiral phosphine complex ... 

Enantioselective catalytic hydrogenation reactions have been reported and appear to offer synthetically useful results, although for a very limited set of alkenes [10,26]. Thus deuteration of styrene and hydrogenation of substituted styrenes, molecules with essentially no functional group available to serve as a... 

Among the many stereoselective routes currently employed to prepare enantiomerically pure NS AID a-arylalkanoic acids [63], a pre-eminent position is held by the enantioselective catalytic hydrogenation of a-arylsubstituted acrylic acids. 

The synthesis of the C20—C26 fragment started with a 4-alkylation of methyl aceto-acetate. The first stereocentre was introduced by enantioselective catalytic hydrogenation with Noyori s (S)-binap rhodium complex (cf. p. 102f.). Stereoselective Fr ter-Seebach alkylation with allyl bromide introduced the second stereooentre in 90% yield (cf. p. 27). Stereospecific introduction of the stereocentres C24 and C25 was achieved by a chelation controlled addition of an allylstannane to an aldehyde (see p. 66 f.). After some experimentation with Lewis acid catalysts and reaction conditions a single diastereomer of the desired configuration was ob-... 

Raval and Ernst " have particularly studied the adsorption of tartaric acid at the surface of Cu (110) and Ni (110). Knowing the role that tartaric acid (6.4)-2H plays in the enantioselective catalytic hydrogenation of -ketoesters by as well as... 

Reschetilowski, W., Boehmer, U., Wiehl, J. (1994) Pt-containing zeolites for enantioselective catalytic hydrogenation of ethyl-p5Tuvate-eflfeets of zeolite structures, solvent and modifier eoncentration. Stud. Surf. Sci. Catal. 84, 2021-2028. 

Finally, Werner Bonrath, Reinhard Karge and Fehx Roessler from DSM succeeded in reducing the anhydride by enantioselective catalytic hydrogenation. An initial catal) screening boosted the enantioselectivity up to 90 % ee further optimisation of the reaction conditions and the substrate / catalyst ratio to 5,000 1 lead to more than 95 % ee, and recrystaUisation enhanced the optical purity even further to more than 99 % ee. [126]... 

Izumi Y (1971) Methods of asymmetric synthesis. Enantioselective catalytic hydrogenation. Angew Chem Int Ed Engl 10 871-881... 

Nagel, U. and Kinzel, E., Enantioselective catalytic hydrogenation of z-(acetamido)cinnamic acid with a rhodium phosphane complex in aqueous solution, Chem. Bet, 1986,119, 1731. 

The metal-free highly enantioselective catalytic hydrogenation of pyridines, especially 2,6-disubstituted pyridines, which are inert to hydrogenation, has been achieved with H2 in toluene by using catalysts generated in situ by hydroboration of alkenes with HB(C6F5)2 a broad range of piperidines has been formed in 44-99% yields with 90 10 0 to 99 1 cis trans ratio. 0... 

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