Mixed rhodium

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). 

For both the dipalladium and palladium-platinum complexes the metal-metal bond is unusually reactive and a number of small molecules undergo an insertion reaction with (9) to give (10 equation 8). The corresponding sulfide-bridged dipalladium dimer can be prepared from the reaction of S8 or MeCHCH2S with (9).83 A mixed rhodium-palladium dimer can also be prepared from (9) (see Scheme 6).84... 

Recently, Severin reported the Kharasch addition of several mixed rhodium(I)-ruthenium(II) and rhodium(III)-ruthenium(II) catalysts (see Part 3, Sect. 8.5). It was also demonstrated that 0.5 mol% of dinuclear rhodium(I) complex 434 having a bridging THDP ligand is an active catalyst for the Kharasch addition of BrCCl3 to simple olefins 433 in water [228]. The yields of adducts 435 ranged from 81 to 92%. 

Recently, Severin et al. studied Kharasch additions of tetrachloromethane to electronically different olefins 383 using several mixed rhodium(I)-ruthenium(II) and rhodium(III)-ruthenium(II) catalysts (Fig. 107). Best results were achieved with catalysts 384 and 385, which gave the addition products 386 in 74—98% yield [470]. With catalyst 385 TOF of 1,200 h 1 and TON of 4,500 were achieved for the... 

However, it can also be argued that the TPP simply enhances the solubility in the organic phase of the mixed rhodium complexes that are formed, just as rhodium complexes with TPPDS or TPPMS instead of TPPTS do. In addition, it has to be considered that the promoter ligand TPP will stay in the crude aldehyde mixture after phase separation and will have to be separated by a distillation step. 

Mixed rhodium-iridium clusters incorporated in zeolites have been formed by thermal decomposition of the appropriate metal carbonyls [286]. 

With these hgands enantioselectivihes in excess of 95% were obtained in the rhodium-catalyzed addition of phenylboronic acid to cyclohexenone. Furthermore, the results of kinetic experiments suggested that catalyst turnover occurred much more rapidly with these mixed rhodium complexes of hybrid ligands compared to the corresponding rhodium complexes of BINAP or cod [76]. 

Two isomers of the cationic rhodium cluster [( /-Cp)2Rh3(CO)(CF3Cs CCF3)(CNR)3]PFe exist in solution. The major isomer is fluxional and its limiting low-temperature NMR spectrum is consistent with an unsymmetrical structure in which the alkyne is a-bonded to one (f/-Cp)Rh —Rh(CNR)3 bond and 7t-bonded to the remaining (/j-CpjRh. The mixed rhodium/ silicon trinuclear cluster [Rh2(//-SiRH)(H)2(CO)2(dppm)2] exhibits exchange between Si —H and Rh—H protons, presumably by facile reductive elimina-tion/oxidative addition of Si—H bonds to the Rh centers (Scheme 28). ... 

The stringency of the conditions employed in the unmodified cobalt 0x0 process leads to formation of heavy trimer esters and acetals (2). Although largely supplanted by low pressure ligand-modified rhodium-catalyzed processes, the unmodified cobalt 0x0 process is stiU employed in some instances for propylene to give a low, eg, - 3.3-3.5 1 isomer ratio product mix, and for low reactivity mixed and/or branched-olefin feedstocks, eg, propylene trimers from the polygas reaction, to produce isodecanol plasticizer alcohol. 

The use of a catalyst such as cadmium oxide increases the yield of dibasic acids to about 51% of theoretical. The composition of the mixed acids is about 75% C-11 and 25% C-12 dibasic acids (73). Reaction of undecylenic acid with carbon monoxide using a triphenylphosphine—rhodium complex as catalyst gives 11-formylundecanoic acid, which, upon reaction with oxygen in the presence of Co(II) salts, gives 1,12-dodecanedioic acid in 70% yield (74). 

Ammonia vapor is mixed with air and converted into nitrogen oxide at an elevated temperature in the presence of a catalyst, which generally contains noble metals such as platinum and rhodium. The optimal gauge temperature is maintained by controlled ammonia and combustion air preheating. The reaction is highly exothermic ... 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

Perhaps the most familiar example of heterogeneous catalysis is the series of reactions that occur in the catalytic converter of an automobile (Figure 11.12). Typically this device contains 1 to 3 g of platinum metal mixed with rhodium. The platinum catalyzes the oxidation of carbon monoxide and unburned hydrocarbons such as benzene, C6H6 ... 

Other compounds with the lantern structure include the acetamidates Rh2(MeCONH)4L2 and the mixed-valence anilinopyridinate Rh2(ap)4Cl (Figure 2.39), which has an unusual ESR spectrum in that the electron is localized on one rhodium [79]. 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

OS 41a] ]R 19] ]P 30] Ten different substrates (C4-C8 alcohols) were reacted with rhodium(I)-tris(fn-sulfophenyl)phosphane [110]. The variance in conversions (ranging from about 1-62%) determined was explained by differences in the solubility of the alcohols in the aqueous catalytic layer and by their different intrinsic activities. Chain length and steric/electronic effects of the different alcohols affected their reactivity in a well-known pattern (Figure 4.63). The results obtained correspond to the conversions achieved in a well-mixed traditional batch reactor (40 cm ). They further agreed with data from mono-phasic processing. 

Hydroformylation represents the most industrially important homogeneous catalysed reaction by volume [2, 3]. The petrochemical, agrochemical and pharmaceutical industries are particularly interested in this transformation. The reaction uses syngas (COiH mix) and a catalyst, commonly rhodium or platinum, to transform an olefin into an aldehyde (Scheme 9.1) [4]. 

Fig. 9.3 Rhodium complexes with mixed bidentate NHC ligands...

In general, of the mixed phosphorus-thioether ligands that have been used in the asymmetric hydrogenation of prochiral olefins, the thioether-phosphinite ligands have provided some of the best results. As an example, a new class of thioether-phosphinite ligands developed by Evans et al. has recently proved to be very efficient for the rhodium-catalysed asymmetric hydrogenation of a... 

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