Rhodium-phosphine system

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)]. ... 

A method for the potential commercial synthesis of isovaleraldehyde is hydroformylation of isobutene, which was considered by Solodar and co-workers at Monsanto. However, the low production volume excluded the possibility of a continuous process, and it was felt that catalyst-handling losses would make a traditional rhodium phosphine system too costly in the batch mode. This dilemma was solved by developing a batch process for the hydroformylation of isobutene, which employed a non-phosphine Rh(CO)2-acetylacetonate system at such low levels that Rh could be considered a throwaway ingredient in the process. 

Hydroaminomethylation of alkenes occurred to give both n- and /. so aliphatic amines catalyzed by [Rh(cod)Cl]2 and [Ir(cod)Cl]2 with TPPTS in aqueous NH3 with CO/H2 in an autoclave. The ratio of n-and /.soprimary amines ranged from 96 4 to 84 16.178 The catalytic hydroaminomethylation of long-chain alkenes with dimethylamine can be catalyzed by a water-soluble rhodium-phosphine complex, RhCl(CO) (Tppts)2 [TPPTS P(m-C6H4S03Na)3], in an aqueous-organic two-phase system in the presence of the cationic surfactant cetyltrimethy-lammonium bromide (CTAB) (Eq. 3.43). The addition of the cationic surfactant CTAB accelerated the reaction due to the micelle effect.179... 

In 1968 Wilkinson discovered that phosphine-modified rhodium complexes display a significantly higher activity and chemoselectivity compared to the first generation cobalt catalyst [29]. Since this time ligand modification of the rhodium catalyst system has been the method of choice in order to influence catalyst activity and selectivity [10]. 

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

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-... 

After the discovery of the high ee provided by rhodium/diphosphite and rhodium/phosphine-phosphite complexes, with total conversion in aldehydes and high regioselectivities, rhodium systems became the catalysts of choice for asymmetric hydroformylation. Important breakthroughs in this area have been the use of rhodium systems with chiral diphosphites derived from... 

This chapter mainly focuses on the latest achievements and recent developments in asymmetric hydroformylation. Since several reviews have been made in the last decade [9,14-16], the chapter discusses the contributions reported between 2000-2005 in particular, although the main diphoshites and phosphine-phosphite rhodium catalytic systems discovered since 1995 are also considered because of their significance in the subject. Particular attention is paid to mechanistic aspects and characterization of intermediates in the case of the hydroformylation of vinyl arenes because this is one of the most important breakthroughs in the area. The application of this catalytic reaction to different type of substrates, in particular dihydrofurans and unsaturated nitriles is the other main subject of this chapter because of their interest in organic synthesis and their industrial relevance. 

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. 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

Using the catalyst system described above in combination with a rhodium phosphine catalyst Lebel reported the de novo synthesis of alkenes from alcohols [100]. They developed a one-pot process, avoiding the isolation and purification of the potentially instable aldehyde intermediate. They combined the oxidation of alcohols developed by Sigman [89] with their rhodium-catalyzed methylenation of carbonyl derivatives. The cascade process is compatible with primary and secondary aliphatic as well as benzyUc alcohols in good yields. They even added another reaction catalyzed by a NHC complex, the metathesis reaction, which has not been addressed in this review as there are many good reviews, which exclusively and in great depth describe all aspects of the reaction. 

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-... 

These phosphine-rhodium catalyst systems are quite sensitive to variables and thus extreme caution must be used in comparing one ligand... 

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. 

The rhodium-trimethylphosphine system is remarkable because it catalyzes the dimerization of aryl substituted acetylenes, yielding the scarce branched head-to-tail coupling product [12], The iridium species generated from [Ir(COD)Cl]2 and phosphine selectively yields linear ( ) or (Z) enynes from silylalkynes, depending on whether triaryl or tripropylphosphines, respectively, are used [16]. 

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 ... 

We previously prepared surface-bonded rhodium phosphine complexes in Al-MCM-41. In a solution of dichloromethane, [Rh(acac)(chiraphos)] and Al-MCM-41 react to a surface bonded [(Os)x-Rh(chiraphos)] complex due to an exchange reaction of the acetylacetonato ligand and surface oxygens of the acidic support12. Here we present a heterogeneous Rhodium diphosphine catalyst and its application in the enantioselective hydrogenation of dimethylitaconate. The results indicate the localisation of the complex inside of the mesoporous channel system. 

---