Rhodium hydride

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...

An especially important case is the enantioselective hydrogenation of a-amidoacrylic acids, which leads to a-aminoacids.29 A particularly detailed study has been carried out on the mechanism of reduction of methyl Z-a-acetamidocinnamate by a rhodium catalyst with a chiral diphosphine ligand DIPAMP.30 It has been concluded that the reactant can bind reversibly to the catalyst to give either of two complexes. Addition of hydrogen at rhodium then leads to a reactive rhodium hydride and eventually to product. Interestingly, the addition of hydrogen occurs most rapidly in the minor isomeric complex, and the enantioselectivity is due to this kinetic preference. 

A thorough computational study of this process has been carried out using B3LYP/ONIOM calculations.31 The rate-determining step is found to be the formation of the rhodium hydride intermediate. The barrier for this step is smaller for the minor complex than for the major one. Additional details on this study can be found at ... 

Rhodium hydride addition to the enone s-cis conformer through a six-centered transition structure accounts for stereospecific Z(0)-enolate forma-... 

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

Rhin(bpy)3]3+ and its derivatives are able to reduce selectively NAD+ to 1,4-NADH in aqueous buffer.48-50 It is likely that a rhodium-hydride intermediate, e.g., [Rhni(bpy)2(H20)(H)]2+, acts as a hydride transfer agent in this catalytic process. This system has been coupled internally to the enzymatic reduction of carbonyl compounds using an alcohol dehydrogenase (HLADH) as an NADH-dependent enzyme (Scheme 4). The [Rhin(bpy)3]3+ derivative containing 2,2 -bipyridine-5-sulfonic acid as ligand gave the best results in terms of turnover number (46 turnovers for the metal catalyst, 101 for the cofactor), but was handicapped by slow reaction kinetics, with a maximum of five turnovers per day.50... 

In the reductive aldol condensation of an ,/J-unsaturated ester and an aldehyde shown in Eq. 291, the initial step is believed to be the addition of an in situ formed rhodium hydride to the a,/Tunsaturated ester, followed by reaction of the resulting rhodium enolate with the aldehyde.470 The reaction has been carried out both inter-470 and intramolecularly471,472 as well as in an asymmetric fashion (Eq. 291). 

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. 

Preparation of a Rhodium Hydride Catalyst Precursor from Spent Catalyst. If, for whatever reason, none of the reactivation procedures above are suitable, one is faced with the option of returning the catalyst concentrate to a vendor for conversion into a catalyst precursor. If the recovery involves reduction of the catalyst concentrate to a rhodium ash, significant expense is involved. Procedures that avoid rhodium ashing may be more economic. 

It is evident that the silica support influences the catalytic performance and it is important to understand the details of the processes involved. For the sol-gel material it was shown by 31P NMR spectroscopy that the immobilised cationic complex completely transforms to the neutral rhodium-hydride species under a CO/H2 atmosphere (Scheme 3.3). On dried silica, however, this conversion might not be complete since the dried support is more acidic [32], It is therefore very likely that the neutral and cationic rhodium complexes co-exist on the silica support. 31P NMR measurements on homogeneous rhodium complexes have shown that a simple protonation indeed converts the neutral rhodium hydride species into the cationic complex. 

For several silica-supported catalysts in condensed phase, including the SAPC system, the rates are disappointing. This can be assigned to slow mass transfer, and perhaps to incomplete rhodium hydride formation as we have discussed and observed. The sol-gel catalyst is relatively fast and is sometimes only a few times slower than the homogeneous one. Since only limited ways of preparation were tested, there is probably more scope for sol-gel catalysts. Space-time yields are promising at the present state of affairs. 

This proposed mechanism involves a rhodium hydride species (Scheme 12). 

Merry-go-round processes involving multiple C-H functionalizations and carbometallations have been described. In the example depicted, a rhodium hydride continues to be formed until steric factors prevail. The 2-norbornene moiety acts as a directing group (Equation (148)).125... 

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)). 

Detailed aspects of the catalytic mechanism remain unclear. However, influence of basic additives on the partitioning of the conventional hydrogenation and reductive cyclization manifolds coupled with the requirement of cationic rhodium pre-catalysts suggests deprotonation of a cationic rhodium(m) dihydride intermediate. Cationic rhodium hydrides are more acidic than their neutral counterparts and, in the context of hydrogenation, their deprotonation is believed to give rise to monohydride-based catalytic cycles.98,98a,98b Predicated on this... 

More recently, during research aimed at supporting the highly linear selective hydroformylation catalyst [Rh(H)(Xantphos)(CO)2] onto a silica support, the presence of a cationic rhodium precursor in equilibrium with the desired rhodium hydride hydroformylation catalyst was observed. The presence of this complex gave the resulting catalyst considerable hydrogenation activity such that high yields of linear nonanol could be obtained from oct-1-ene by domino hy-droformylation-reduction reaction [75]. 

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). 

In the case of BDPP with a bite angle of 90°, the high-pressure NMR and high-pressure IR studies showed the structures of the hydrido dicarbonyl diphosphine resting state as an axial-equatorial BPT. Similar behavior was observed for the furanoside diphosphines. Dinuclear rhodium species in equilibrium with the mononuclear pentacoordinate rhodium hydride carbonyl diphosphines have been found for these ligands. The position of this equilibrium depends on the hydrogen concentration and the ligands. The rate... 

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... 

We estimate a rate constant of 10 -10 M s for the electron-transfer reaction and an E° for the rhodium-hydride couple that is similar to, or slightly less negative than, the E° value for the substrate. Our mechanism is summarized in Scheme I. 

The mechanism of the shift reaction in this catalyst system involves the attack of hydroxide anion at coordinated carbon monoxide, forming a metallacarboxylic acid. Elimination of C02 gives a rhodium hydride species that can react with the proton stemming from water to give dihydrogen. Rhodium may be either Rh(I) or Rh(III) as the valence of the metal does not change during this process. 

In an effort to assign the bands to ee and ae isomers the (thixantphos)Rh(CO)2D complex was measured for comparison. Upon H/D exchange, only Vi and v3 shift to lower wavenumbers (respectively 18 and 14 cm-1), and therefore, these two bands are assigned to the carbonyl frequencies of the ee complex. The two bands that do not shift, v2 and v4, belong to the ae complex. From the disappearance of a low-frequency shoulder upon H/D exchange, it can be concluded that one of the rhodium hydride vibrations is partly hidden under v4. 

Recently, Rh(dppe)2Cl (dppe = l,2-bis(di-phenylphosphino)ethane) has been shown to catalyze the electroreduction of CO2 in an acetonitrile solution. The Rh complex can be electrochemically reduced to the corresponding rhodium hydride. 

Fig. 2.8 A d irect route to vinylboranes in rhodium-catalyzed hydroborations with phos-phine-free catalysts (including oxidative degradation of a rhodium phosphine). The key intermediate is a rhodium hydride, capable of reversible insertion into the alkene (step A), followed by addition of borane in step B. This leads to reductive elimination of RH in step C followed by boryl migration in step D. A further...

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