Phosphine systems

Olivier-Bourbigou s group, for example, has recently shown that phosphite ligands can be used in Rh-catalyzed hydroformylation in ionic liquids as well as the well loiown phosphine systems [81]. Since phosphite ligands are usually unstable in aqueous media, this adds (apart from the much better solubility of higher olefins in... 

If cobalt carbonylpyridine catalyst systems are used, the formation of unbranched carboxylic acids is strongly favored not only by reaction of a-olefins but also by reaction of olefins with internal double bonds ( contrathermo-dynamic double-bond isomerization) [59]. The cobalt carbonylpyridine catalyst of the hydrocarboxylation reaction resembles the cobalt carbonyl-terf-phos-phine catalysts of the hydroformylation reaction. The reactivity of the cobalt-pyridine system in the hydrocarboxylation reaction is remarkable higher than the cobalt-phosphine system in the hydroformylation reaction, especially in the case of olefins with internal double bonds. This reaction had not found an industrial application until now. 

Nickel complexes prove to be the most effective catalysts for hydroboration of thioalkynes. The bidentate phosphine systems [(P-P)NiCl2] (P-P = dppf, l,3-bis(diphenylphosphino)propane(dppp), dppe) all displayed high activity, even with bulky substituents on the alkyne (Equation (4)) 43 44... 

An extension of the research on silver complexes with Lewis base-functionalized mono(A-heterocyclic carbene) ligands has been made toward the better-studied and stronger coordinating phosphine systems. The reaction of a diphenylphosphine-functionalized imidazolium salt with silver oxide in dichloromethane affords a trinuclear silver carbene complex 50, as confirmed by electrospray-ionization mass spectrometry.96,97 Metathesis reaction of 50 in methanol using silver nitrate gives 51 in 33% yield. The crystal structures of 51 were found to be different when different solvents were used during crystallization (Scheme 12).97 One NO3- anion was found to be chelated to... 

Other phosphine systems have been reported in which four phenyl groups are oriented around a rhodium center (249-254). They all hydrogenate Z-enamides efficiently, and intermediates with a conformation of edge-face phenyls seem plausible in each case. The 2S,4S-4-diphenyl-... 

Toward the Reduction of Simple Ketones, Nitriles, Esters and Aromatics with Monodentate Phosphine Systems... 

In a subsequent detailed study, other iridium phosphine systems prepared in situ were used. Depending on the steric properties of the phosphine employed and... 

Spogliarich et al. published the details of a study [15] on the electronic effect implicated in reactions catalyzed by iridium/phosphine systems, and found there to be a slight dependence on the charge distribution around the carbonyl group of the substrates, while electron-withdrawing groups enhance the reduction rate... 

Rhodium(I) complexes with l,3-dimethylimidazolin-2-ylidene ligands were used in the hydroformylation of olefins. However, the activity and selectivity toward formation of branched versus linear aldehyde cannot compete with rhodium-phosphine systems. " Similar catalyst systems with the sterically more demanding l,3-dimesitylimidazolin-2-ylidene give higher branched/linear ratios for vinyl arenes (95 5), but the turnover frequency is still low compared to established systems [Eq. (52)]. ... 

Rhodium(I) complexes have also been shown to promote metallo-ene type reactions efficiently (Scheme 7.14) [26]. Typically, the reaction of 2,7-octadienyl-l-carbonate 27 is carried out using the RhH(PPh3)4-tris(2,4,6-trimethoxyphenyl)phosphine system as the catalyst in acetic acid at 80 °C for 1-1.5 h, to give the corresponding l-exo-methylene-2-ethenylcyclopentane 28 in high yield. 

A variety of palladium(O) or palladium(ii)/phosphine systems have been used as catalyst precursors (Figure 9). Triphenylphosphine was usually the ligand of choice until Farina showed in 1991 that the use tri-(2-furyl)phosphine enhanced reaction rates. The positive effects of additives such as copper salts and diethylamine ... 

Wilkinson and co-workers (3) showed that the maximum activity of the tertiary phosphine rhodium(I) chloride catalysts occurred at a ligand. -rhodium ratio of about 2. This ratio was used in the systems studied for the effects of hydrogen pressure (Table I). In the triphenyl-phosphine system (abbreviated as L°), the rate of hydrogenation increased with pressure in the accessible pressure range, in accord with previous observations (2) by Wilkinson and co-workers. However, with the p-dimethylamino substituted tertiary phosphines L1 and L2 the rates of hydrogenation were essentially independent of the hydrogen pressure within the experimental errors. For tris (p-dimethylaminophenyl) phos-... 

Catalyst cycle of Rh(I)-phosphine system. Most mechanistic studies on ligand-modified rhodium catalysts have been performed with HRh(CO)(PPh3)3. Extensive mechanistic studies have revealed that HRh(CO)2(PPh3)2 (18-electron species) is a key active catalyst species, which readily reacts with ethylene at 25°C [43]. Two mechanisms, an associative pathway and a dissociative pathway, were proposed [43-46], depending on the concentration of the catalyst. 

Rhodium (I)-phosphine systems lead to catalytic tetramerization. For example, the system [RhCl H with 1 to 2 moles of PPh3 is effective in the selective formation of an interesting spiro compound (XV) (152) free from other isomers. Although the detailed reaction path is unknown due to the inaccessibility of the intermediate complexes, the formation of (XV) may be visualized from a tetramer complex as follows ... 

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