Rhodium-catalyzed hydrogenation reaction conditions

Becalski et al.98b found that the (E)/(Z) ratio of imines under rhodium-catalyzed hydrogenation conditions stayed essentially constant during the course of the reaction. This observation was explained in terms of two possibilities (1) the rates of hydrogenation of the two isomers were identical or, more likely, (2) the rate of interconversion of the two isomers was faster than the rate of hydrogenation, leaving the (E)/(Z) ratio of the substrate constant. 

Wittig yhdes have been shown to be compatible with hydroformylation conditions, and may thus be used in a domino reaction sequence such as from 16a to 38 (Scheme 5.15) [20]. When an a-unsubstituted ylide is employed, the resulting alkene undergoes in-situ rhodium-catalyzed hydrogenation in a triple tandem reaction to convert 10 a to 39. Several other examples were reported establishing the generality of this domino reaction sequence. 

Fig. 5. Plot of log(rates) vs. log(pressure) for rhodium-catalyzed CO hydrogenation. Reaction conditions 75 ml sulfolane, 3 mmol Rh, 1.25 mmol pyridine, H2/CO = 1, 240 C, 4 hr (96). Total rate includes rates to methanol, methyl formate, ethanol, ethylene glycol monoformate, and propylene glycol ( ) total ( ) methanol ( ) ethylene glycol. Open figures are for an experiment with H2/CO = 0.67.

Compared to the rhodium-catalyzed stereoselective reactions, studies on the iridium-catalyzed reactions have been limited until recently. Usually lower selectivities have been observed in the Ir(i)-catalyzed reactions.459,460 The asymmetric hydrosilylation of imines affords optically active secondary amines. These are very valuable compounds, but the studies on that reaction are quite limited.461 Close examinations of these reactions revealed that they proceed via a transfer hydrogenation. Other conditions such as the 2-propanol/base system in the presence of an appropriate metal complex have been employed as well, but only low selectivities were obtained.462... 

The influence of the electronic properties of the alkene on the insertion rate has been evaluated in the kinetic study of the reaction of a Rh(III) dihydrido complex and / ara-substituted styrenes (Scheme 6.8) [45]. The process is part of the proposed mechanism for the rhodium catalyzed hydrogenation of alkenes. Under pseudo-first order conditions (excess of PR3 and alkene) in benzene the rate law is -d[RhH2Cl(PR3)]/dt = A obs[RhH2Cl(PR3)] with Us = i k[alkene]/ [PR3] + fir [alkene]. The values of K and k determined for different para-substituted styrenes show that more electron withdrawing substituents in the substituted styrene ring increase the value of K (better alkene coordination), but decrease k (slower insertion). These opposite trends tend to cancel each other and the overall rates of the process vary little for different styrenes. K values are in the range... 

Another important reaction principle in modem organic synthesis is carbon-hydrogen bond activation [159]. Bergman, Ellman, and coworkers have introduced a protocol that allows otherwise extremely sluggish inter- and intramolecular rhodium-catalyzed C-H bond activation to occur efficiently under microwave heating conditions. In their investigations, these authors found that heating of alkene-tethered benzimidazoles in a mixture of 1,2-dichlorobenzene and acetone in the presence of di-//-... 

Morken and co-workers have reported the highly enantioselective version of this reaction, albeit with low efficacy in the aldol-type coupling [8d, e]. Unfortunately, we obtain low enantioselectivity ee 2-4%) using chiral rhodium complexes under our reaction conditions. An intramolecular adaptation has led to new opportunities in cobalt-catalyzed carbocyclizations, wherein the use of PhSiHs was essential for smooth ring formation (Eq. 4) [9]. The identical products were also formed by a combination of [Rh(COD)2]OTf/(p-CE3Ph)3P and molecular hydrogen [10]. 

Scheme 10.8 outlines the application of rhodium-catalyzed allyhc amination to the preparation of (il)-homophenylalanine (J )-38, a component of numerous biologically active agents [36]. The enantiospecific rhodium-catalyzed allylic amination of (l )-35 with the lithium anion of N-benzyl-2-nitrobenzenesulfonamide furmshed aUylamine (R)-36 in 87% yield (2° 1° = 55 1 >99% cee) [37]. The N-2-nitrobenzenesulfonamide was employed to facilitate its removal under mild reaction conditions. Hence, oxidative cleavage of the alkene (R)-36 followed by deprotection furnished the amino ester R)-37 [37, 38]. Hydrogenation of the hydrochloride salt of (l )-37 followed by acid-catalyzed hydrolysis of the ester afforded (i )-homophenylalanine (R)-3S in 97% overall yield. 

A surprising variation on the silylformylation reaction has been reported by Zhou and Alper. A zwitterionic rhodium(I) complex, (l,5-COD)Rh+(Tj6-PhBPh3), catalyzes the silylformylation of alkynes under normal reaction conditions. However, if H2 is added to the system, the reaction may proceed to yield silylalkenals of a different structure. Interestingly, although the H2 must play a key role in the reaction, it is not incorporated in the product. At this time, the mechanistic role of the hydrogen remains unclear. The authors term this reaction a silylhydroformylation [Eq. (51)].126... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

The reaction of alkenes (and alkynes) with synthesis gas (CO + H2) to produce aldehydes, catalyzed by a number of transition metal complexes, is most often referred to as a hydroformylation reaction or the oxo process. The discovery was made using a cobalt catalyst, and although rhodium-based catalysts have received increased attention because of their increased selectivity under mild reaction conditions, cobalt is still the most used catalyst on an industrial basis. The most industrially important hydrocarbonylation reaction is the synthesis of n-butanal from propene (equation 3). Some of the butanal is hydrogenated to butanol, but most is converted to 2-ethylhexanol via aldol and hydrogenation sequences. 

Another important example of COj-hydrogenation is the synthesis of form-amides. In 1970, Haynes c/tf/. of Shell Development Co. discovered the reaction of carbon dioxide, hydrogen and certain amines, when catalyzed under mild conditions by cobalt, rhodium, iridium and palladium complexes [ I70. ... 

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