Rhodium equilibration

CO oxidation, an important step in automotive exhaust catalysis, is relatively simple and has been the subject of numerous fundamental studies. The reaction is catalyzed by noble metals such as platinum, palladium, rhodium, iridium, and even by gold, provided the gold particles are very small. We will assume that the oxidation on such catalysts proceeds through a mechanism in which adsorbed CO, O and CO2 are equilibrated with the gas phase, i.e. that we can use the quasi-equilibrium approximation. 

The mechanistic and synthetic puzzle of alkyne hydrosilylation opened more fully with the discovery that rhodium will catalyze the /r.mr-hydrosilylation of terminal alkynes.22 There is much work extant in this area, and good summaries of the various catalytic systems exist.11 A trans-addition process to give (Z)-j3-silane products G is well precedented with trialkylsilanes (Table 3), for both rhodium and mixed rhodium-cobalt complexes (entry 4).22,26 However, the selectivity erodes significantly upon switching to Me2PhSiH (entry 5), and, due to the mechanistic requirements for equilibration of the /3-silyl vinylrhodium intermediate, electron-poor silanes react exclusively to give CE)-/3-silane products B (see entries 6 and 7). 

Most recently, we have investigated the use of iterative oxonium ylide [1,2]- or [2,31-shifts as a convenient approach to the polypyran domains often found in the marine polyether ladder toxins (Scheme 18.8) [21]. Initial studies indicated that [l,2]-shifts of O-benzyl oxonium ylides such as 19 a or 19 b were inefficient. Alternative metallocarbene processes including C-H insertion and dimerization were found to predominate in these cases, again suggesting that carbene-ylide equilibration may occur [21b]. On the rationale that concerted [2,3]-shifts of the corresponding O-allyl oxonium ylides might occur more readily, the allyl ethers 19 c, 19 d were then examined. These examples were much more effective, especially in conjunction with the optimized catalyst Cu(tfacac)2 [21a]. However, rhodium(II) triphenylacetate (Rh2(tpa)4) [22] was found to... 

The automatic procedure for reference spectra generation was first demonstrated for the start-up of a homogeneous catalyzed rhodium hydroformylation of cyclo-octene using Rh4(CO)i2 as precursor, n-hexane as solvent and FTIR as the in situ spectroscopy at 298 K [63]. The first n spectra were (i) empty spectrometer compartment (background), (ii) n-hexane at 0.2 MPa in a high pressure thermostatically controlled cell fitted with Cap2 windows (iii) system equilibrated with 2.0 MPa CO, (iv) system upon addition of cyclo-octene, and (v) system upon addition of Rh4(CO)i2. The n=l reference spectrum, which contained atmospheric... 

With respect to the formation of mono- and dihydride sites, Schrock and Osborn (16) observed an interesting equilibration between a rhodium monohydride complex and a rhodium dihydride complex as expressed by Eqs. (7) and (8). 

A three-component reaction of aryl diazoacetates, alcohols, and araldehydes (or araldimines) has been investigated, using a rhodium(II) catalyst.283 The first two components combine in the presence of catalyst to produce a zwitterion (72a). Evidence for equilibration with an alcoholic oxonium ylide intermediate (72b) is presented. It is proposed that this species is trapped by electron-deficient araldehyde (or imine) to give new C-C bond formation. 

In these experiments the isomerization was also recorded and was found to be rather low (6%). The low selectivity to isomerized products has consequences with regard to the mechanism. Let us return the kinetic expression (eqn. 1) which states that the reaction is first order in rhodium and H2, zeroth order in alkene, and minus one order in CO. In the extreme case of rapid pre-equilibration up to reaction 6 one would expect the system to go back and forth very fast between species 1 and 5. As we have learned from the work by Lazzaroni this would implicate that the isoalkyl species 3i would regenerate alkene complex 2 now containing the isomerized alkene. The isomerized alkene, however, is not observed, or only in very minor quantities. This means that backward reactions 3 and 8i do not occur. The first reactions of the cycle determining the regio- and chemoselectivity are therefore irreversible while reaction 6 is still rate-determining. 

Equilibration with carbon monoxide at room temperature and low pressure (a few torr ) yielded the rhodium(I)-dicarbonyl compound (13) in addition to the Rh(I)(C0) paramagnetic complexe (11). The structure of this complex was elucidated by ESCA and UV measurements (13) which showed that the trivalent rhodium was indeed reduced to the monovalent state and by infrared spectroscopy which provided evidence for a gem dicarbonyl (14). Use of 1 1 C0 ... 

The cluster [CpRhFe2(CO)6(n3-Te)2] (213) exists in solution as an equilibrating mixture of two isomers, one in which the rhodium atom is in a basal site of the Fe2RhTe2 square pyramid, and another in which it occupies the apical site.131 In several other mixed-metal clusters that also exhibit a... 

In situ IR and NMR spectra showed that, indeed, the assembled diphosphine coordinates preferentially in the bis-equatorial, wide-bite-angle fashion to the penta-coordinated rhodium carbonyl hydride. Both homo- and heterocombinations of monophosphine ligands can be formed in situ by mixing the components several heterocombinations perform better as catalysts than the respective homocombinations. However, in contrast to the catalysts prepared by Takacs, the present complexes do not equilibrate to afford solely the heterocombination. 

Many studies have been done on the CO/H2 reaction over rhodium. In particular, SSITKA experiments have been performed at 220"C over 5% Rh/AFOs for a steady conversion of 1.5% 194). After the change from C0/H2 to - C0/H2, the shape of the CO response curve does not agree with that predicted 17) for a fast, continuously equilibrated exchange with the surface CO. For instance, over Ni the reverse exchange rate divided by the net forward rate b) can be assumed to be infinite 17). Using the data over Rh, the data arc best fit with j3 = 2.2 194). From these data and the response of the CH4 curve, it is determined that the coverage of the active carbon, Q, is only about 0.03. From a series of experiments, it has been possible to estimate the surface coverages of C , Q, CO, and a formate species, COOH, the latter present on the support. 

To evaluate the influence of the support on the equilibration reaction on the metal particle a wide number of oxides supported rhodium catalysts have been studied (Table 7.7). 

Tabic 7.7. O2 " 02 equilibration over Rhodium catalysts at 300X- Suppofi effects... 

This approach should be useful in determining the direction of hydrogenation for molecules in which the carbinol group is replaced by carbon-carbon or carbon-nitrogen double bonds. With an alkene, though, the simple conformational model would have to be used and the hydrogenation should be run under conditions that do not promote double bond isomerization, that is, not with palladium or nickel catalysts. With carbonyl compounds the preferred eonditions for selective reaction involve platinum, rhodium or ruthenium catalysts imder non-diffusion control conditions. The use of nickel catalysts, especially Raney nickel, with its basic components, can cause an equilibration of the alcohol product. 

Table 7.7. 02 + 02 equilibration over Rhodium catalysts at 300°C- Support effects [61.71]. 

In general, Pt catalysts are much more sensitive to particle size variations than Rh catalysts. It clearly appears that the + 02 equilibration reaction is only slightly sensitive to the Rh particles size. In fact the rate of equilibration varies by a factor of only 1 to 2 when the dispersion decreases by a factor of 6. On platinum the variations are much more important with a factor of 20 on the rate of equilibration when the dispersion of the metallic phase varies by a factor of 21. The reaction is faster on larger Pt particles [33,69,72] while the opposite occurs with rhodium [33,69,72,73]. 

While mononuclear rhodium complexes give a-branched aldehydes 6, the hydroformylalions of a- and /3-pinene in the presence of Rh6(CO)1(, both give (/J)-10-formylpinane (7) as the major product (d.r. up to 83 16)18. Under the reaction conditions an equilibration of a- to /J-pinene is established (a//J 8 1). 

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