Homogeneous hydrogenation, catalyzed

The mechanism of homogeneous hydrogenation catalyzed by RhCl(Ph3P)3 ° involves reaction of the catalyst with hydrogen to form a metal hydride (PPh3)2RhH2Cl (43), which rapidly transfers two hydrogen atoms to the alkene. 

A. Homogeneous Hydrogenation Catalyzed by Cobalt Cyanide Complexes. 433... 

Erankel, E. (1970) Conversion of polyunsaturates in vegetable oils to cis Monounsaturates by homogeneous hydrogenation catalyzed with chromium carbonyls./.Am. Oil Chem. Soc., 47, 11-14. 

Table 5.3 Different substrates in the homogeneous hydrogenation catalyzed by in-situ mixtures of Ni(OAc)2 and o-MeO-dppp.a)...

Homogeneous hydrogenation catalyzed by the four-coordinated rhodium complex, Rh[(C6H5)3P]3Cl, has been particularly well investigated. With this catalyst, the first step is formation of the six-coordinated rhodium hydride of known configuration, 16, in which we abbreviate the ligand, triphenylphosphine, (C6H5)3P, as L ... 

In recent years there has been much interest in homogeneous hydrogenations catalyzed by transition metal complexes (7). One facet of research in this area is the search for chiral catalysts (catalysts that are dissymmetric, i.e., optically active) that can be used to produce chiral compounds via asymmetric reactions. In this review, we survey asymmetric homogeneous hydrogenation reactions, that is reactions that create asymmetric carbon atoms by the addition of hydrogen across multiple bonds under the influence of soluble chiral catalysts. 

Abley and McQuillin (44) have reported asymmetric homogeneous hydrogenations catalyzed by rhodium complexes of chiral amides. In initial experiments the catalyst was generated in situ by treating trichlorotripyridylrhodium(III) with sodium borohydride in an optically active amide solvent (Fig. 20). In later work a 5% solution of the amide in diethylene glycol monoethyl ether was used and products with the same optical purities were obtained. This evidence indi-... 

To elucidate the mechanism of homogeneous hydrogenation catalyzed by Fe(CO)s, kinetic studies were carried out with mixtures of unsaturated fatty esters containing a radioactive label. A C-labeled methyl octadecadienoate-Fe(CO)3 complex was prepared to serve as a catalytic intermediate. Hydrogenation of methyl oleate (m-9-octa-decenoate) and palmitoleate (cis-9-hexadecenoate) and of their mixtures with methyl linoleate was also studied to determine the selectivity of this system, the function of the diene-Fe(CO)3 complex, and the mechanism of homogeneous isomerization. Mixtures of reaction intermediates with a label helped achieve unique simulation of the kinetic data with an analog computer. 

Enantioselective homogeneous hydrogenation catalyzed by chiral transition metal complexes is one of the most well established transformations in asymmetric synthesis [1]. Excellent enantioselectivities have been achieved in the hydrogenation of a wide range of substrates, often with very low catalyst loadings. High reliability, mild reaction conditions, and perfect atom economy are further attractive attributes of this method. In particular complexes based on Ru or Rh have found broad application in industrial processes [1] and the impact of these catalysts has been recognized by the Nobel Prize awarded to Ryoji Noyori and William S. Knowles in 2001 [2]. 

Reduction of anhydrides to y-lactones. Carboxylic acid anhydrides are reduced by homogeneous hydrogenation catalyzed by this ruthenium complex... 

Most organopentacyanides are stable towards [Co(CN)jH], with the exception of allyl complexes which react to liberate propylene derivatives (105). This is one of the steps in the homogeneous hydrogenation of butadienes catalyzed by cobalt cyanide complexes (see Section VII,A). 

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

Based on the above reactions, an overall mechanism for the hydrogenation of NBR catalyzed by Wilkinson s catalyst was proposed (see Scheme 19.4), which is also applicable to the kinetic performance of the homogeneous hydrogenation of PB [88] and styrene-butadiene copolymers [89], where K2 and Ks vanish. 

In 1968, Knowles et al. [1] and Horner et al. [2] independently reported the use of a chiral, enantiomerically enriched, monodentate phosphine ligand in the rhodium-catalyzed homogeneous hydrogenation of a prochiral alkene (Scheme 28.1). Although enantioselectivities were low, this demonstrated the transformation of Wilkinson s catalyst, Rh(PPh3)3Cl [3] into an enantioselective homogeneous hydrogenation catalyst [4]. 

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