Olefin hydrogenation enantioselective

The tetradentate ligands (340) and (341) form 1 1 metakligand complexes with [IrCl(cod)]2.548 The complexes were tested in the asymmetric hydrogenation of prochiral olefins, providing enantioselectivities up to 36%. The multitopic ligands L, (342) and (343), bind to Ir1 to form [IrL] species which have been characterized by elemental analysis, mass spectrometry, IR and NMR spectroscopy.549 The complexes show enantioselectivities of up to 30% for the hydrogenation of prochiral olefins under mild reaction conditions. 

For a perspective on the use of chiral monodentate phosphorus ligands in enantioselective olefin hydrogenations see ... 

In catalysis, adsorbed CO may retard some reactions such as olefin hydrogenation, fuel cell conversion, and enantioselective hydrogenation. For instance, Lercher and coworkers observed the deactivation of Pt/Si02 in the liquid-phase hydrogenation of crotonaldehyde, and ascribed this deactivation to the decomposition of crotonaldehyde on platinum surface to adsorbed CO [138]. Blaser and coworkers found that the addition of a small amount of formic acid decreases the rate of liquid-phase hydrogenation of ethyl pyruvate on cinchonidine-modified Pt/Al203 catalyst, which they explained as the decomposition of formic acid on the catalyst to adsorbed CO. Interestingly, the addition of acetic acid does not decrease the reaction rate, but whether acetic acid decomposes on the catalyst as formic acid does was not mentioned [139]. 

As shown in Scheme 1.30, the chiral titanocene catalyst 34 hydrogenates unfunctionalized, disubstituted styrenes under 136 atm of hydrogen at 65°C to give the saturation products with 83 to >99% ee [156]. A high enantioselectivity is now realized only with aryl-substituted olefins. The enantioselectivity of 41% ee attained 2-ethyl-1-hexene and 34 as catalyst is the highest for hydrogenation of non-aromatic olefins. 

Alkali metal in ammonia reductions of pyrrolobenzodiazepine-5,11-diones give trans-2-aminocyclohexanecarboxylic acid derivatives (e.g., 4) in enantiomerically pure form.2 3 A method for preparation of cis-2-aminocyclohexanecarboxylic acids related to 4 is based on the enantioselective hydrolysis of symmetrical diesters with pig liver esterase.4 cis-2-Aminocyclohexane derivatives have been used for syntheses of aminocyclitol antibiotics.4 5 6-Alkyl-cis-2-aminocyclohexanecarboxylic acids can be prepared by alkali metal in ammonia reduction of pyrrolobenzodiazepine-5,11-diones followed by olefin hydrogenation the cis-decahydroquinoline alkaloid (+)-pumiliotoxin C has been prepared by this methodology.2... 

The N,P phosphine-oxazoline chelate (59) is chiral, and complexes can act as homogeneous catalysts for asymmetric synthesis the Ir(l) and Pd(II) complexes promote enantioselective olefin hydrogenation and allylic substitution respectively. An N,P analog of the N,N didentate ligand 2,2 -bipyridine is (60), the soft P donor helping to stabilize low-valent metals. Further, 2,2-bipyridine derivatives such as (61) can bind metals such as Ir and Ru as N,C chelates with one pyridine nitrogen rotated to the opposite side, away from the metal ion. 

It should be finally noted that hydroxyphosphines can be converted under very smooth conditions into sulfonated phosphines by acylation with o-sulfobenzoic anhydride, as shown by Borner et al. (Eq. 5) [26]. With this methodology in hand the severe conditions commonly used for the incorporation of sulphonate groups in phosphines can be avoided. Acid-labile functional groups like acetals survive under these conditions. In comparison to the parent hydroxyphosphines the water solubility of the relevant Rh catalysts was strongly enhanced [27]. In the asymmetric hydrogenation of prochiral olefins, moderate enantioselectivities were achieved. 

The quantitative and selective transformation of the precursor into the dinuclear complex in catalytic conditions, the different catalytic performance of the mononuclear derivative [(BDPBzP)Ru(CH3CN)j]OTf (lower activity and enantioselectivity), the identical catalytic performance of the precursor and of the T12-H2 complex, and the recognized capability of the structurally related Ru(II) complex [(ri2-H2)(dppb)Ru()a.-Cl)3RuCl(dppb)] to maintain the dimeric structure in olefin hydrogenation reactions [35-37] were taken as proofs for the direct involvement of [(BDPzP)(DMSO)Ru((1-C1)3Ru(ti2-H2)(BDPBzP)] in the enantioselective hydrogenation of acetylacetone [68]. 

In comparison to the parent hydroxyphosphines the water solubility of the relevant Rh catalysts was strongly enhanced [23]. In the asymmetric hydrogenation of prochiral olefins, moderate enantioselectivities were achieved. 

Ruthenium catalysts are now widely used for olefin hydrogenation, and many examples of enantioselective ruthenium-catalyzed hydrogenation are discussed in Section 15.7. Here, before addressing the issues of stereoselectivity, the elementary steps of ruthenium-catalyzed hydrogenation are discussed. These catalysts react through monohydride species containing a second anionic ligand. 

As noted in Chapter 14, the Curtin-Hanunett principle applies to this process, and high enantioselectivities are obtained under conditions when the two diastereomeric olefin complexes equilibrate faster than the addition of Because the relative rates for equilibration versus oxidative addition of Hj depend on the concentration of hydrogen, enantioselectivities often depend on the pressure of hydrogen. Enantioselectivities of reactions of MAC have been shown in some cases -to be higher at lower In addition,... 

A versatile and highly enantioselective procedure, based on asymmetric olefin hydrogenation reactions in presence of a chiral Rh-catalyst, has been described by Pizzano et al. for the synthesis of (3-hydroxy-phosphonate (112) (Scheme 38). ... 

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