Rhodium catalysts olefin hydrogenation

Olefin hydrogenation has been known since 1966, when Wilkinson and cowork-ers reported the homogeneous hydrogenation of olefins by rhodium catalysts (see... 

Process Technology. In a typical oxo process, primary alcohols are produced from monoolefins in two steps. In the first stage, the olefin, hydrogen, and carbon monoxide [630-08-0] react in the presence of a cobalt or rhodium catalyst to form aldehydes, which are hydrogenated in the second step to the alcohols. 

Efficient enantioselective asymmetric hydrogenation of prochiral ketones and olefins has been accompHshed under mild reaction conditions at low (0.01— 0.001 mol %) catalyst concentrations using rhodium catalysts containing chiral ligands (140,141). Practical synthesis of several optically active natural... 

This is an ion-exchanger like the sulfonated polymer. The siUca surface can also be functionalized with phosphine complexes when combined with rhodium, these give anchored complexes that behave like their soluble and polymer-supported analogues as catalysts for olefin hydrogenation and other reactions ... 

The use of rhodium catalysts for the synthesis of a-amino acids by asymmetric hydrogenation of V-acyl dehydro amino acids, frequently in combination with the use of a biocatalyst to upgrade the enantioselectivity and cleave the acyl group which acts as a secondary binding site for the catalyst, has been well-documented. While DuPhos and BPE derived catalysts are suitable for a broad array of dehydroamino acid substrates, a particular challenge posed by a hydrogenation approach to 3,3-diphenylalanine is that the olefin substrate is tetra-substituted and therefore would be expected to have a much lower activity compared to substrates which have been previously examined. 

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

The catalytic activity of cationic rhodium precursors of formula [Rh(diene)(di-phosphine)]+ was also explored by Schrock and Osborn [28]. Halpern and coworkers made very detailed mechanistic studies of olefin hydrogenation by [RhS2(diphos)]+ species (diphos = l,2-bis(diphenylphosphino)ethane S = solvent) [31]. Significant differences have been observed in the reaction of the catalyst precursors [Rh(NBD)(PPh3)2]+ and [Rh(NBD)(diphos)]+ in methanol, as shown in Eqs. (8) and (9) ... 

In entries 10-13 (Table 21.8) of trisubstituted alkenes, very high diastereo-selectivity is realized by the use of a cationic rhodium catalyst under high hydrogen pressure, and the 1,3-syn- or 1,3-anti-configuration naturally corresponds to the ( )- or (Z)-geometry of the trisubstituted olefin unit [48, 49]. The facial selectivity is rationalized to be controlled by the A(l,3)-allylic strain at the intermediary complex stage (Scheme 21.2) [48]. 

In the case of tri-substituted alkenes, the 1,3-syn products are formed in moderate to high diastereoselectivities (Table 21.10, entries 6—12). The stereochemistry of hydrogenation of homoallylic alcohols with a trisubstituted olefin unit is governed by the stereochemistry of the homoallylic hydroxy group, the stereogenic center at the allyl position, and the geometry of the double bond (Scheme 21.4). In entries 8 to 10 of Table 21.10, the product of 1,3-syn structure is formed in more than 90% d.e. with a cationic rhodium catalyst. The stereochemistry of the products in entries 10 to 12 shows that it is the stereogenic center at the allylic position which dictates the sense of asymmetric induction... 

Because the thermal separation of products has been substituted by a liquid-liquid separation, the two phase technology should be best suited for hydroformylation of longer chain olefins. But with rising chain length of the olefins the solubility in the aqueous catalyst phase drops dramatically and as a consequence the reaction rate. Only the hydroformylation of 1-butene proceeds with bearable space-time yield. This is applied on a small scale for production of valeraldehyde starting from raffinate II. Because the sulfonated triphenylphosphane/rhodium catalyst exhibits only slow isomerization and virtually no hydroformylation of internal double bonds, only 1-butene is converted. The remaining raffinate III, with some unconverted 1-butene and the unconverted 2-butene, is used in a subsequent hydroformy-lation/hydrogenation for production of technical amylalcohol, a mixture of linear and branched C5-alcohols. 

Secondary and tertiary amines can be obtained if the hydroformylation of olefins is conducted in the presence of primary and secondary amines under elevated hydrogen partial pressures. Here the rhodium catalyst is involved in both steps, the hydroformylation of an olefin as well as the hydrogenation of the imine or enamine resulting from a condensation of the oxo-aldehyde with the amine (Scheme 14). This combination of hydroformylation and reductive amination is also known as hydroaminomethylation and has been applied to the synthesis of various substrates of pharmaceutical interest [55-57] as well as to the synthesis of macrocycles [60-63] and dendrimers [64,65]. 

The process requires two conversion steps. In the first, an olefin plus synthesis gas (carbon monoxide and hydrogen) are reacted over a cobalt or rhodium catalyst to produce two aldehydes, with one being an isomer of the other. 

Rhodium catalysts generated from the sulfonated phosphine 23 (Table 2) were effective in the hydrogenation of olefins in an aqueous/organic or in a homogeneous methanol system, substantially higher rates being observed in the latter system.131 For example, the TOF observed in the hydrogenation of 1-hexene in the biphasic system was 220 h 1 compared to 7860 h 1 in methanol.131... 

Rhodium catalysts modified with carboxylated phosphines 45 (Table 3 n=5, n=7)229 and phosphonium phosphines 103 (Table 5 n=2,3,6,10)255 form very active catalytic systems for the hydrogenation of olefins in aqueous/organic two phase systems. 

Enantioselective Hydrogenation of Prochiral Olefins - The presence of SDS increased both the rate and the enantioselectivity of hydrogenation of prochiral dehydroaminoacid derivatives using rhodium catalysts modified with the diphosphine 75a (Table 4) in aqueous media.500 For example, addition of 0.13 mmol of SDS to the Rh/75a catalyst shortened the reaction half time (tj/2) from... 

Hanson et a/.149 hydrogenated the prochiral olefin methyl a-acetamidocinna-mate using rhodium catalysts modified with the tenside chiral sulfonated diphosphine 34 (Table 2) in an ethylacetate/H20 micellar system at 25° C and 1 bar H2. The yield (100%) and enantiomeric excess (69%) were considerably higher than with the tetrasulfonated diphosphine 31 (Table 2 m=0, n=0) which gave 32% yield and 20% e.e. and the reaction time was shorter (1.5 versus 20h). Rh/34 and Rh/31 (m=0, n=0) gave nearly the same results (100% yield and 72-75% e.e. within < lh) in homogeneous methanol solutions.149... 

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