Rh-diphosphine ligands

Manufacture of rhodium precatalysts for asymmetric hydrogenation. Established literature methods used to make the Rh-DuPhos complexes consisted of converting (1,5-cyclooctadiene) acetylacetonato Rh(l) into the sparingly soluble bis(l,5-cyclooctadiene) Rh(l) tetrafluoroborate complex which then reacts with the diphosphine ligand to provide the precatalyst complex in solution. Addition of an anti-solvent results in precipitation of the desired product. Although this method worked well with a variety of diphosphines, yields were modest and more importantly the product form was variable. The different physical forms performed equally as well in hydrogenation reactions but had different shelf-life and air stability. 

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

In contrast, synthesis of 3,4-diphosphorylthiophenes requires more elaboration because of low reactivity of 3,4-positions of thiophene and unavailability of 3,4-dihalo or dimetallated thiophenes. Minami et al. synthesized 3,4-diphosphoryl thiophenes 16 as shown in Scheme 24 [46], Bis(phosphoryl)butadiene 17 was synthesized from 2-butyne-l,4-diol. Double addition of sodium sulfide to 17 gave tetrahydrothiophene 18. Oxidation of 18 to the corresponding sulfoxide 19 followed by dehydration gave dihydrothiophene 20. Final oxidation of 20 afforded 3,4-diphosphorylthiophene 16. 3,4-Diphosphorylthiophene derivative 21 was also synthesized by Pd catalyzed phosphorylation of 2,5-disubstituted-3,4-dihalothiophene and converted to diphosphine ligand for Rh catalysts for asymmetric hydrogenation (Scheme 25) [47],... 

In this reaction, a rhodium atom complexed to a chiral diphosphine ligand ( P—P ) catalyzes the hydrogenation of a prochiral enamide, with essentially complete enan-tioselectivity and limiting kinetic rates exceeding hundreds of catalyst turnovers per second. While precious metals such as Ru, Rh, and Ir are notably effective for catalysis of hydrogenation reactions, many other transition-metal and lanthanide complexes exhibit similar potency. 

Enantioselectivities >90% were reported for a Ti-ebthi catalyst (Table 34.4 entry 4.1) and for some Rh-diphosphine complexes (entries 4.2-4.4). Interestingly, the highest ee-values were obtained using sulfonated diphosphines (bdppsuif) in an aqueous biphasic medium (entry 4.3). The degree of sulfonation strongly affected the enantioselectivity the Rh-mono-sulfonated bdpp gave 94% ee, compared to 65% ee with Rh-bdpp in MeOH, and almost racemic product with bis-or tris-sulfonated ligands. In addition, the activity of the mono-sulfonated cata-... 

Rhodium diphosphine catalysts can be easily prepared from [Rh(nbd)Cl]2 and a chiral diphosphine, and are suitable for the hydrogenation of imines and N-acyl hydrazones. However, with most imine substrates they exhibit lower activities than the analogous Ir catalysts. The most selective diphosphine ligand is bdppsuif, which is not easily available. Rh-duphos is very selective for the hydrogenation of N-acyl hydrazones and with TOFs up to 1000 h-1 would be active enough for a technical application. Rh-josiphos complexes are the catalysts of choice for the hydrogenation of phosphinyl imines. Recently developed (penta-methylcyclopentyl) Rh-tosylated diamine or amino alcohol complexes are active for the transfer hydrogenation for a variety of C = N functions, and can be an attractive alternative for specific applications. 

Salzer et al. prepared a set of planar-chiral diphosphine ligands based on the arene chromium tricarbonyl backbone (Fig. 36.3) [21]. The straightforward four-step synthetic route allowed the preparation of 20 ligands of this family. These ligands were tested in Ru- and Rh-catalyzed enantioselective hydrogenation of various substrates, including the standard C=C substrates (dimethyl itaconate, methyl-2-acetamidocinnamate, methyl-2-acetamidoacrylate) as well as MEA-imine (l-(methoxymethyl)ethylidene-methylethylaniline) and ethyl pyruvate. Moderate conversions and ee-values were obtained. 

The search for a commercially viable process took many years [126], Several approaches with Rh or Ir complexes using commercially available diphosphine ligands were not successful. A critical breakthrough was achieved when Ir complexes with a new class of ferrocenyl-based ligands (now called Solvias Josiphos) were used. Extremely active and productive catalysts were obtained, especially in the presence of acid and iodide ions. Different Josiphos ligands were tested and a selection of the best results obtained is shown in Table 37.5. 

When an appropriate chiral phosphine ligand and proper reaction conditions are chosen, high enantioselectivity is achievable. If a diphosphine ligand with C2 symmetry is used, two diastereomers for the enamide-coordinated complex can be formed because the olefin can interact with the metal from either the Re- or Sf-face. Therefore, enantioselectivity is determined by the relative concentrations and reactivities of the diastereomeric substrate-Rh complexes. It should be mentioned that in most cases it is not the preferred mode of initial binding of the prochiral olefinic substrate to the catalyst that dictates the final stereoselectivity of these catalyst systems. The determining factor is the differ-... 

In 1971, Kagan published his ground-breaking research in the field.141 He demonstrated that high enantioselectivities could be obtained in the Rh-catalyzed hydrogenation of functionalized olefins, such as the dehydroamino acid derivatives 3 and 4, using a novel diphosphine ligand which he called DIOP 2 (Scheme 2). 

Recently Togni et al. [19] focussed on the preparation of asymmetric dendrimer catalysts derived from ferrocenyl diphosphine ligands anchored to dendritic backbones constructed from benzene-1,3,5-tricarboxylic acid trichloride and adamantane-l,3,5,7-tetracarboxylic acid tetrachloride (e.g. 11, Scheme 11). In situ catalyst preparation by treatment of the dendritic ligands with [Rh(COD)2]BF4 afforded the cationic Rh-dendrimer, which was then used as a homogeneous catalyst in the hydrogenation reaction of, for example, dimethyl itaconate in MeOH. In all cases the measured enantioselectivity (98.0-98.7%) was nearly the same as observed for the ferrocenyl diphosphine (Josiphos) model compound (see Scheme 11). 

We applied the QM/MM IMOMM method [41] to Rh-diphosphine catalyzed hydroformylation, to provide a quantitative theoretical characterization of the origin of regioselectivity in Rh-diphosphine systems. We focused on the experimentally characterized xantphos ligands, for which variation in electronic properties is minimal. Using the IMOMM method, which only accounts for the steric properties of ligands, was fully justified. 

A water-soluble diphosphine ligand with large bite angle was prepared by controlled sulfonation of XANTHPHOS. The rhodium complex of the resulting (2,7-bis(S03Na)-XANTHPHOS (51) showed a catalytic activity in propene hydroformylation comparable to Rh/TPPTS (TOF 310 vs 500 h" at 120 °C, 9 bar propene and 10 bar CO/H2 = 1/1) [70]. The regioselectivity... 

A family of wide bite angle diphosphine ligands based upon xantphos (8) has been developed and tested in rhodium catalysed hydroformylation. van Leeuwen and co-workers conducted HP IR measurements on a range of Rh/thixantphos... 

Figure 5.2-5 Recycling experiments - Rh-catalyzed, biphasic 1-octene hydroformylation in [BMIMJfPFg] with a guanidinium-modified diphosphine ligand with xanthene backbone.

Sturm, T., Weissensteiner, W. and Spindler, F. A Novel Class of Eerrocenyl-aryl-based Diphosphine Ligands for Rh- and Ru-catalysed Enantioselective Hydrogenation. Adv. Synth. Catal. 2003, 345, 160-164. 

The procedure is fast, inexpensive and very easy to reproduce. The chiral diphosphine ligand can be synthesized on a multigram scale. The ligand has been also successfully used in enantioselective Rh(I)-catalyzed hydrogenations (Table 3.1). 

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