Diphosphine catalysts

For example, the hydrogenation of methyl (Z)-a-acetamidocinnamate gives a chiral product when conducted in the presence of a chiral diphosphine catalyst. The enantiomeric excess data for micro-reactor and batch operation are in line when performed imder similar conditions [169]. A very high reproducibility of determining data on enantiomeric excess was reported [170]. In addition, the ee distribution was quite narrow 90% of aU ee data were within 40-48% [170]. 

Acetylenic esters react with arylboron reagents in the presence of rhodium diphosphine catalyst to give cyclic ketones.409 Equation (61) shows an example which may involve ortfe-metallation and ketone formation. A catalytic, enantioselective reaction was also achieved (Equation (62)). These processes presumably involve unprecedented addition of organorhodium species to the ester carbonyl group. 

In the hydrogenation of diketones by Ru-binap-type catalysts, the degree of anti-selectivity is different between a-diketones and / -diketones [Eqs (13) and (14)]. A variety of /1-diketones are reduced by Ru-atropisomeric diphosphine catalysts to indicate admirable anti-selectivity, and the enantiopurity of the obtained anti-diol is almost 100% (Table 21.17) [105, 106, 110-112]. In this two-step consecutive hydrogenation of diketones, the overall stereochemical outcome is determined by both the efficiency of the chirality transfer by the catalyst (catalyst-control) and the structure of the initially formed hydroxyketones having a stereogenic center (substrate-control). The hydrogenation of monohydrogenated product ((R)-hydroxy ketone) with the antipode catalyst ((S)-binap catalyst) (mis-... 

Kinetic resolution results of ketone and imine derivatives are indicated in Table 21.19. In the kinetic resolution of cyclic ketones or keto esters, ruthenium atrop-isomeric diphosphine catalysts 25 induced high enantiomer-discriminating ability, and high enantiopurity is realized at near 50% conversion [116, 117]. In the case of a bicyclic keto ester, the presence of hydrogen chloride in methanol served to raise the enantiomer-discriminating ability of the Ru-binap catalyst (entry 1) [116]. 

The DuPHOS/potassium tert-butoxide system was also used to hydrogenate these substrates, in addition to substrates 19 and 22 (Fig. 30.6 Table 30.4) [6], Under normal conditions, ruthenium-diphosphine catalysts are known to be unreactive with styrenes however, in the presence of potassium tert-butoxide, the substrates 15-22 were hydrogenated with high conversion. 

A range of other terminal alkenes has been hydrogenated with ruthenium-diphosphine catalysts. The first set of substrates (Fig. 30.7 Table 30.5) was hydrogenated with Ru-BINAP in dichloromethane (DCM) at 30°C. Products of double bond migration were also detected [5]. 

Cyclic imines do not have the problem of syn/anti isomerism and therefore, in principle, higher enantioselectivities can be expected (Fig. 34.8). Several cyclic model substrates 6 were hydrogenated using the Ti-ebthi catalyst, with ee-val-ues up to 99% (Table 34.5 entry 5.1), whereas enantioselectivities for acyclic imines were <90% [20, 21]. Unfortunately, these very selective catalysts operate at low SCRs and exhibit TOFs <3 h-1. In this respect, iridium-diphosphine catalysts, in the presence of various additives, seem more promising because they show higher activities. With several different ligands such as josiphos, bicp, bi-... 

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. 

The rhodium-diphosphine catalysts are generally sensitive to oxygen, hence the reactions have to be carried out under strictly inert atmospheric conditions. A decrease in the yield or the enantiomeric excess can be due to a lack of sufficient precaution during the procedure or to the inactivation of the catalyst when exposed to oxygen. However, the reactions using rhodium complexes as catalysts give very good results which correlate well with the published material. 

Figure 19. Immobilization of chiral rhodium diphosphine catalysts (5) to A1-SBA-15/A1-MCM-41 [85].

Fig. 2.10 Applications of asymmetric hydroboration with diphosphine catalysts to meso-symmetric alkenes. For 10(a), iridium complexes reverse the sense of enantioselectivity in up to 54% enantiomeric excess.

Finally, NMR evidence has been provided for the occurrence of catalyst degradation by intramolecular phosphine oxidation of the Amatore type in alkene/CO copolymerisation catalysed in organic solvents by Pd"-diphosphine catalysts containing oxygen co-ligands (for example acetate, as exemplified in a dppp complex. Scheme 7.29) [56],... 

FIGURE 3. Representative examples of conjugate addition products using Cu/ferrocenyl diphosphine catalysts. Adapted with permission from Acc. Chem. Res., 40, 179-188 (2007). Copyright 2007 American Chemical Society... 

L-Dopa was produced industrially by Hoffrnann-LaRoche, using a modification of the Erlenmeyer synthesis for amino acids. In the 1960s, research at Monsanto focused on increasing the L-Dopa form rather than producing the racemic mixture. A team led by William S. Knowles (1917—) was successful in producing a rhodium-diphosphine catalyst called DiPamp that resulted in a 97.5% yield of L-Dopa when used in the Hoffrnann-LaRoche process. Knowles s work produced the first industrial asymmetric synthesis of a compound. Knowles was awarded the 2001 Nobel Prize in chemistry for his work. Work in the last decade has led to green chemistry synthesis processes of L-Dopa using benzene and catechol. 

Figure 1.13 Generation of rhodium-based supramolecularcatalysts by assembly of pyridine/hydroxypyridine pairs (a) Self-assembly modes of pyridine-based phosphines, (b) Alkene hydroformylation with supramolecular rhodium-diphosphine catalysts (c) CAChe minimized 3D structure ofthe rhodium-diphosphine complex (other ligands from the metal omitted for clarity).

The mono(diphosphine) complexes, [Rh(dppp)]BF4 and RhCl-(dppp), are less effective than [Rh(dppp)2] + but are still more active than RhCl(PPh3)3. The mono(diphosphine) catalysts also decompose slowly under the reaction conditions, which renders them less useful than the bis(diphosphine) catalysts. The slower rate of decarbonyla-tion observed with the mono(diphosphine) catalysts compared with the bis(diphosphine) catalysts presumably is due to the lower basicity of the former which retards the rate of oxidative addition (vide infra). Consistent with this is the observation that [Rh(COD)(dppp)]BF4 (COD = 1,5-cyclooctadiene) shows a higher rate for catalytic de-carbonylation of benzaldehyde than does [Rh(dppp)]BF4 (22). An additional observation is that the type of anion, Cl or BF4 , has no apparent effect on decarbonylation rates for the bis(diphosphine) catalysts however, for the mono(diphosphine) complexes the chloride salts show slightly lower rates than their tetrafluoroborate analogues. 

An important characteristic of the bis(diphosphine) catalysts is the remarkable selectivity observed in the decarbonylation products. Recall that 1-heptanal is converted into 86% hexane and 14% 1-hexene by RhCl(PPh3)3 (25°C, stoichiometric reaction) (4). In contrast, using [Rh(dppp)2]+ as the catalyst, the only volatile product is hexane,... 

P-31 NMR Comparisons of the Crystalline and Solution States of Rhodium(I) Diphosphine Catalysts... 

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