Hydride complexes rhodium

Finally, selective hydrogenation of the olefinic bond in mesityl oxide is conducted over a fixed-bed catalyst in either the Hquid or vapor phase. In the hquid phase the reaction takes place at 150°C and 0.69 MPa, in the vapor phase the reaction can be conducted at atmospheric pressure and temperatures of 150—170°C. The reaction is highly exothermic and yields 8.37 kJ/mol (65). To prevent temperature mnaways and obtain high selectivity, the conversion per pass is limited in the Hquid phase, and in the vapor phase inert gases often are used to dilute the reactants. The catalysts employed in both vapor- and Hquid-phase processes include nickel (66—76), palladium (77—79), copper (80,81), and rhodium hydride complexes (82). Complete conversion of mesityl oxide can be obtained at selectivities of 95—98%. 

Figure 2.68 Bond lengths in two 5-coordinate rhodium hydride complexes with bulky tertiary...

The separation of rhodium hydride complex from a stream comprising catalyst and high molecular weight aldehyde condensation products using a carboxylic acid functionalized ion exchange resin in illustrated schematically in Figure 2.12. 

The rhodium-catalyzed reaction of pyrrolidine with iodobenzene gives 2-phenylpyrroline in a high yield (Equation (72)). This reaction involves the formation of an imine rhodium hydride complex and phenylation (Equation (73)). 

Systems which fulfil these conditions are tris(2,2 -bipyridyl)rhodium complexes [63] and, more effectively, substituted or unsubstituted (2,2 -bipyridyl) (pentamethylcyclopentadienyl)-rhodium complexes [64], Electrochemical reduction of these complexes at potentials between — 680 mV and — 840 mV vs SCE leads to the formation of rhodium hydride complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolyses are... 

This system fulfills the four above-mentioned conditions, as the active species is a rhodium hydride which acts as efficient hydride transfer agent towards NAD+ and also NADP+. The regioselectivity of the NAD(P)+ reduction by these rhodium-hydride complexes to form almost exclusively the enzymatically active, 1,4-isomer has been explained in the case of the [Rh(III)H(terpy)2]2+ system by a complex formation with the cofactor[65]. The reduction potentials of the complexes mentioned here are less negative than - 900 mV vs SCE. The hydride transfer directly to the carbonyl compounds acting as substrates for the enzymes is always much slower than the transfer to the oxidized cofactors. Therefore, by proper selection of the concentrations of the mediator, the cofactor, the substrate, and the enzyme it is usually no problem to transfer the hydride to the cofactor selectively when the substrate is also present [66]. This is especially the case when the work is performed in the electrochemical enzyme membrane reactor. 

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

The rhodium hydride complex, 18b, was the only rhodium complex observed during the hydroformylation reaction at increased partial hydrogen pressure of 32 bar, as concluded from the absence of carbonyl signals in the IR difference spectra. The spectrum of the rhodium hydride resting state is taken as background at... 

Hydrogenation Catalysis of Olefins by a Rhodium Hydride Complex of PhP(CH2 CH2 CH2 PPh2 )2... 

The catalyst for hydroformylation is a rhodium(I) hydride species, which is clearly distinct from the species that are active for hydrogenation. The hydrogenation catalysts are cationic Rh(I)+ or neutral Rh(I)Cl species. Carbonylation of alcohols also requires an ionic Rh(I) species, e.g. [Rl CO y-- Often rhodium(I) salts are used as the precursor for hydroformylation catalysts. Under the reaction conditions (H2, CO, ligands, temperature >50°C) these salts are converted to a rhodium hydride complex, although there are several papers that seem to invoke cationic rhodium species as the catalysts. Chlorides have a particularly deleterious effect on the activity (i.e. they are not converted into hydrides under mild conditions) and it has been reported that the addition of bases such as amines has a strong promoting effect on such systems ... 

The hydridotetrakis(triphenyl phosphite)rhodium complex described below is the first example of a rhodium hydride complex stabilized by phosphite ligands.2... 

Polar apiotic solvents, such as HMPA, and long reaction times were effective. The reaction of rhodium hydride complexes with CO3 in the presence of II3O was found to form the dihydrido bicarbonato complexes RhH3(OjCOH)L2 which reduce CO to form the corresponding Rh(l)[Pg.203]

Enolates can also be prepared by rhodinm-catalyzed isomerization of allylic Uthinm alcoholates, such as 14 (equation 5)". Subsequent treatment of the intermediately formed rhodium hydride complexes (15 and 16) with an electrophile led to the formation of various products. For example, alkyl halides gave a-alkylated ketones (17) in good yields, as shown in Table 4. Interestingly, addition of benzaldehyde under kinetically controlled... 

Reactions of soluble metal complexes, whose mechanisms of catalysis appear to be reasonably well known, can serve as a guide to the main reaction paths followed on heterogeneous catalysts. Mononuclear complexes catalyze syn addition of H2 to alkynes to yield initially only cis isomers, as in equation (25). 5 More recently, Muetterties and coworkers showed that the dinuclear rhodium hydride complex ( yi-H)Rh[P(OPr )3]2 2 (38) converts alkynes to trans isomers as initial products (equation 26). The alkyne addition compound (39) was isolated its structure shows the vinyl group bonded to one rhodium atom by a a-bond and to the other by a ir-bond, while the substituents on the vinyl group are trans to one another. This structure resembles ones hypothesized earlier to explain the formation of trans isomers and alkanes. Hydrogenations of alkynes which are catalyzed by the dinuclear rhodium hydride are much slower than the hydrogenation of an alkene catalyzed by the dinuclear tetrahydride (40), which is formed rapidly from (38) in the presence of H2 (equation 11). ... 

Carbonyl groups are also utilized in catalytic C-C bond cleaving reactions. Under catalytic conditions, 8-quinolyl phenyl ketone 85 reacts with ethylene to give 8-quinolyl ethyl ketone 86 and styrene in quantitative yield [105]. Styrene is formed by cleavage of the phenyl-carbonyl bond, followed by ethylene insertion into the resultant phenyl-rhodium bond, and (3-hydride elimination. The accompanying formation of a rhodium-hydride complex is followed by incorporation of ethylene to furnish the ethyl ketone 86. 

Operation of the insertion-elimination mechanism has been demonstrated in the reaction of rhodium hydride complex, RhHL4 (L=PPh3), with two isomeric allyl phenyl carbonates [56]. Unbranched 2-butenyl phenyl carbonate was found to give branched allylic phenyl ether exclusively, whereas the decarboxylation of the branched l-methyl-2-propenyl phenyl carbonate afforded unbranched 2-butenyl phenyl ether. These results can be accounted for by assuming a precata-lytic and catalytic insertion-elimination process as shown in Scheme 7. 

Besides acting as a hydride relay, the Lewis acid fragment can also participate in substrate activation. This possibility was first illustrated by Owen and co-workers while investigating the reactivity of the Rh complex 44b with sterically demanding phosphines (Scheme 14). No reaction is observed in the absence of Hj, but under 2.5bar of Hj, rhodium hydride complexes 43c,d are formed along with tricy-clo[2.2.1.0 ]heptane. This transformation involves the addition of Hj, the cleavage of the Rh B interaction and the formation of a C—H bond (presumably via reductive... 

Sometimes thermal and photochemical activation can result in the formation of different products. For example, heating the solution of the ethylrhodium carbonyl complex RhHBPz 3(CO)(C2H4) (Pz = 3,5-dimethylpyrazole) in benzene entails the elimination of the ethylene 7t-ligand and the formation of phenyl-rhodium hydride (complex IV-3 in Scheme IV. 11) [20a]. On irradiation of this solution, the hydride ligand adds not to the metal atom but to the ethylene molecule, which results in the appearance of an o-ethyl group in the complex IV-4 [20b]. 

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