Rhodium precursor, Rh

Propylene carbonate is a good solvent of the rhodium precursor [Rh(acac) (00)2] and the phosphite ligand BIPHEPHOS and can thus be used as the catalyst phase in the investigation of the isomerizing hydroformylation of trans-4-octene to n-nonanal in a biphasic system [24]. As already mentioned, the reaction products can be extracted with the hydrocarbon dodecane. Instead of an additional extraction after the catalytic reaction, we carried out in-situ extraction experiments, where the products are separated from the catalytic propylene carbonate phase while the reaction is still in progress. Conversion of 96% and selectivity of 72% was achieved under comparably mild conditions (p(CO/H2) = 10 bar, T = 125 °C, 4 h, substrate/Rh = 200 1). 

The rhodium complexes were prepared in situ from the rhodium precursor [Rh(nbd)2](C104) (nbd = 2,5-norbornadiene) and applied in the hydrogenation experiments under an initial hydrogen pressure of 5 bar at 35°C. The dendrimer structure had almost no effect on the activity of the catalyst in the batch-wise rhodium-catalyzed hydrogenation of dimethyl itaconate (Scheme 4). 

Before the start of the continuous reaction, the substrate solution was saturated with hydrogen by stirring under 5 bar of H2 for 16h. Subsequently, a premixed solution of the ligand and the rhodium precursor [Rh(nbd)2](C104) was injected into the membrane autoclave, after which the substrate solution was pumped through the reactor. 

R,R) PhTRAP [36], 85% ee and 75% conversion were attained. Further investigation showed that the rhodium precursor [Rh(nbd)2]SbF6 was superior to Rh(acac)(cod). 

A dimetallic rhodium(l)-rhodium(iii) complex 340 bearing a bis(imino)carbazolide ligand L and an olefin LRh(C2H4) was prepared via a salt metathesis reaction between NaL and the dimeric rhodium precursors [Rh(/t-... 

Mixed imidazolium triazolium salt 24 was readily metalated at the imidazolium heterocycle with either [Pd(OAc)2] or [Rh(OMe)(COD)]2 (COD = 1,5-cyclooctadiene), due to the higher acidity of the imidazolium C2-bound proton compared to the triazolium proton. Extended reaction times for palladation or the use of the acetate analogue of the rhodium precursor, [Rh(OAc)(COD)]2, induced cyclometalation and produced ehelate complexes... 

The monosulfonated PPh derivative, Ph2P(m-C6H4S03K) (DPM) and its rhodium complex, HRh(CO)(DPM)3 have been synthesized and characterized by IR and NMR spectroscopic techniques. The data showed that the structure was similar to [HRh(CO)(PPh3)3]. The catalytic activity and selectivity of [HRh(CO)(DPM)3] in styrene hydroformylation were studied in biphasic catalytic systems.420 421 Rh1 complexes [Rh(acac)(CO)(PR3)] with tpa (131), cyep (132), (126), ompp (133), pmpp (134), tmpp (135), PPh2(pyl), PPh(pyl)2, and P(pyl)3 were characterized with NMR and IR spectra. Complexes with (131), (132), and (126) were catalysts for hydrogenation of C—C and C—O bonds, isomerization of alkenes, and hydroformylation of alkenes.422 Asymmetric hydroformylation of styrene was performed using as catalyst precursor [Rh(//-0 Me)(COD)]2 associated with sodium salts of m-sulfonated diarylphosphines.423... 

The catalytic activity of cationic rhodium precursors of formula [Rh(diene)(di-phosphine)]+ was also explored by Schrock and Osborn [28]. Halpern and coworkers made very detailed mechanistic studies of olefin hydrogenation by [RhS2(diphos)]+ species (diphos = l,2-bis(diphenylphosphino)ethane S = solvent) [31]. Significant differences have been observed in the reaction of the catalyst precursors [Rh(NBD)(PPh3)2]+ and [Rh(NBD)(diphos)]+ in methanol, as shown in Eqs. (8) and (9) ... 

The binuclear precursor (di-,u-chloro-bis-[ /4-2,5-norbomadiene]-rhodium(I)) = [(Rh(NBD)Cl]2 is well suited for the in-situ preparation of a variety of homogeneous hydrogenation catalysts, if tertiary phosphines (here PMe3, PMe2Ph,... 

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]. 

The same rhodium precursor, (S Rh,/ c)-[(Tl -C5Me5)Rh (l )-Prophos (H20)] (SbFg)2, promotes the reaction between the nitrones A-benzylideneaniline A-oxide or 3,4-dihydroisoquinoline A-oxide with other enals different from methacrolein (Scheme 10). The cycloadducts were prepared with excellent regioselec-tivity, perfect endo selectivity, and enantiomeric excesses up to 94% [35]. 

A series of rhodium-modified heterogeneous materials has been prepared [36a] using the silicon oxides Aerosil 200 and SBA-15 and dinuclear rhodium(l) siloxide precursors [ Rh( x-OSiMe3)(cod) 2], [ Rh( x-OSiMe3)(nbd) 2], [ Rh( x-OSiMe3)(tfb) 2]... 

These NMR experiments provided great insight into the catalytic reaction. Whereas the catalytic reaction requires a high reachon temperature of 100 °C, aU three transformations in Scheme 3.5 proceed at 25 °C. In the same paper [16], Hayashi answered the question of why the catalytic reaction does not take place at the lower temperature. An outline of the reason is illustrated in Scheme 3.6. In the catalytic reaction, Rh(acac)(BINAP) is involved as a significant intermediate, because Rh(acac)(C2H4)2 is used as the rhodium precursor. It was confirmed that the hydroxo-rhodium complex is immediately converted into Rh(acac) (BINAP) by the reaction with 1 equiv. acetylacetone at 25 °C, in which the transmetallation from boron to rhodium is very slow at the same temperature. Thus, the acetylacetonato Hgand inhibits the catalytic reaction (Scheme 3.6, path a). 

The new diphosphines 39/42-44 react with various common rhodium and palladium precursors [Rh(nor)+, Rh(acac), PdMeCl] to give both cis and trans complexes [88]. 

The tetrasulfonated chiraphos reacts in water with [Rh(COD)Cl]2 in a ratio of 2 1 to give a rhodium complex [Rh(COD)(ChiraphosTS)]Cl1 [31P(DzO) S ppm 57.4 (d, JRh P = 148 Hz)] which catalyzes the enantioselec-tive reduction of amino acid precursors in a two-phase system of water-ethyl acetate with ee in the range 81-88%. 

Step (1) involves the formation of methyl iodide, which then reacts with the rhodium complex Rh(I)L by oxidative addition in a rate-determining step (2) to form a methylrhodium(III) complex. Carbon monoxide is incorporated into the coordination sphere in step (3) and via an insertion reaction a rhodium acyl complex is formed in step (4). The final step involves hydrolysis of the acyl complex to form acetic acid and regeneration of the original rhodium complex Rh(I)L and HI. Typical rhodium compounds which are active precursors for this reaction include RhCl3, Rh203, RhCl(CO)(PPh3)2, and Rh(CO)2Cl2. 

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