Mechanism, of Rh-catalyzed

Scheme 3. The mechanism of Rh-catalyzed hydrogenation of functionalized olefins in the presence of a bidentate P,P-ligand (S = solvent, P = phosphine unit, R = alkyl).

For a good discussion of the mechanism of Rh-catalyzed carbonylation, see articles written by chemists who were instrumental in developing the process at Monsanto D. Forster, Adv. Organometal. Chem., 1979,17, 255 and D. Forster and T. W. Dekleva, J. Chem. Ed., 1986, 63, 204, and references therein. 

Scheme 54 Proposed mechanism of Rh-catalyzed conjugate addition to enones...

Scheme 6 The proposed mechanism of Rh-catalyzed [(5+2)+l] cycloaddition of VCPs...

Figure 4.12 Proposed mechanism of Rh-catalyzed water gas shift reaction.

Scheme 8.9 Proposed mechanism of Rh-catalyzed hydrophosphorylation of alkynes...

The mechanism of IPT-catalyzed conversion of alkanes (RH) involves the photoinduced charge transfer in the photoactive WVI = O group in the octahedral moiety (A) leading to the formation of a reactive electron-deficient radicallike specie (B) capable of abstracting H atom from organic substrates ( denotes an excited state) ... 

Scheme 9.5 Proposed mechanism of Rh (I)-catalyzed N-propargyl enamine cycloisomerization.

Despite some differences, the mechanism of rhodium-catalyzed hydroformylation is very similar to that of the cobalt-catalyzed process.39-42 Scheme 7.1 depicts the so-called associative route which is operative when the ligand is in excess. Rhodium metal and many Rh(I) compounds serve as precursor to form21,22—in the presence of triphenylphosphine, CO and H2—the active species [RhH(CO)(PPh3)3] (5). At high CO partial pressure and low catalyst concentration without added PPh3, the [RhH(CO)2(PPh3)] monotriphenylphosphine complex instead of 6 coordinates the alkene and participates in the so-called dissociative route.21,39... 

Unlike the Rh-based hydrogenation of a-(acylamino)acrylates, the corresponding Ru chemistry has not been studied extensively. Ru complexes of (S)-BINAP and (S,S)-CHIRAPHOS catalyze the hydrogenation of (Z)-a-(acylamino)cinnamates to give the protected ( -phenylalanine with 92% ee [74] and 97% ee [75], respectively. It is interesting that the Rh and Ru complexes with the same chiral diphosphines exhibit an opposite sense of asymmetric induction (Scheme 1.6) [13,15,56,74,75]. This condition is due primarily to the difference in the mechanisms the Rh-catalyzed hydrogenation proceeds via Rh dihydride species [76], whereas the Ru-catalyzed reaction takes place via Ru monohydride intermediate [77]. The Rh-catalyzed reaction has been studied in more detail by kinetic measurement [78], isotope tracer experiments [79], NMR studies [80], and MO calculations [81]. The stereochemical outcome is understandable by considering the thermodynamic stability and reactivity of the catalyst-enamide complexes. 

Ab Initio MO calculations of a model complex Rh(PH3)2(NH3)(CH2=CHCH2NH2) were earned out to shed light to the detailed mechanism of Rh(l)-catalyzed isomerization of allylic amines to enamines.5 This study suggests that the square-planar [RhiPHjyNHjXCf CHCHjNHj) complex is transformed to [Rh(PH3)2(NH3)(( )-CH3CH=CHNH2)]+ via intramolecular oxidative addition of the C(l)-H bond to the Rh(I) center, giving a distorted-octahedral Rh(lll) hydride intermediate, followed by reductive elimination accompanied by allylic transposition. 

Rhodium Catalysts Rhodium catalysts have been widely used in C—N bond formation via the Rh-nitrene intermediates. Du Bois and co-workers have developed various types of Rh-catalyzed intramolecular C—N bond-forming reactions (Equation 11.18) [44] and intermolecular C—N bond-forming reactions (Equation 11.19) [45]. The mechanism of these C—H amidation reactions proceeds via a concerted asynchronous two-electron oxidation pathway. 

Figure 2 Mechanism of Rh-diphosphine catalyzed hydrogenation ofenamide 9...

Scheme 5 Proposed mechanism for Rh-catalyzed hydrophosphinylation of an alkyne...

The basic steps of the mechanism of hydroformylation catalyzed by the combination of Rh and PPhj are similar to those of hydroformylation catalyzed by HCo(CO). However, the number of possible intermediates is much larger because many combinations of the number of phosphines and geometric orientation are possible in each intermediate complex. For example, phosphines in a five-coordinate complex can occupy the apical or equatorial positions, and these complexes are typically stereochemically nonrigid. Similarly, two phosphines in a four-coordinate intermediate can be located cis or trans to each other. A five-coordinate hydridorhodium-alkene complex containing diequatorial phosphines might be expected to convert to a four-coordinate Rh alkyl complex containing trans phosphines, but it could also lead to an alkyl complex containing cis phosphines. 

Sanford MS, Ulman M, Grubbs RH. New insights into the mechanism of ruthenium-catalyzed olefin metathesis reactions. J Am Chem Soc. 2001 123(4) 749—750. 

At the early stage of the mechanistic studies of Rh-catalyzed asymmetric hydrogenation, this mechanism has been accepted on the basis of low-temperature hydrogenation experiments clearly demonstrating higher reactivity of the catalyst-substrate complexes with "proper" mode of coordination of the double bond toward ... 

Scheme 5.15 Mechanism of Rh(dppe)(ti -BPh4)-catalyzed formation of 4,6-dimethyl-2-pyrone...

M. Brookhart et al. - Implication of Three-Center, Two-Electron M-H-C Bonding for Related Alkyl Migration Reactions Design and Study of an Ethylene Polymerization Catalyst, /. Am. Chem. Soc. 107, 1443,1985 Mechanism of Rh(III) Catalyzed Methyl Acrylate Dimerization, ibid. 114,4437,1992. 

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