Diphosphite ligand catalysts

The first reports on asymmetric hydroformylation using diphosphite ligands revealed no asymmetric induction [71], In 1992, Takaya et al. published the results of the asymmetric hydroformylation of vinyl acetate (e.e.=50%) with chiral diphosphites [72], In 1992, an important breakthrough appeared in the patent literature when Babin and Whiteker at Union Carbide reported the asymmetric hydroformylation of various alkenes with e.e. s up to 90%, using bulky diphosphites derived from homochiral (2R,4R)-pentane-2, 4-diol (see Figure 8.20). Van Leeuwen et al. studied the influence of the bridge length, bulky substituents and cooperativity of chiral centres on the performance of the catalyst [73,74],... 

Since the discovery and development of highly efficient Rh catalysts with chiral diphosphites and phosphine-phosphites in the 1990s, the enantioselectivity of asymmetric hydroformylation has reached the equivalent level to that of asymmetric hydrogenation for several substrates. Nevertheless, there still exist substrates that require even further development of more efficient chiral ligands, catalyst systems, and reaction conditions. Diastereoselective hydroformylation is expected to find many applications in the total synthesis of complex natural products as well as the syntheses of biologically active compounds of medicinal and agrochemical interests in the near future. Advances in asymmetric hydrocarboxylation has been much slower than that of asymmetric hydroformylation in spite of its high potential in the syntheses of fine chemicals. 

The combination of rhodium dicarbonyl acetylacetonate complex (Rh(acac)(CO)2) and a diphosphite ligand, (2,2 -bis[(biphenyl-2,2 -dioxy)phosphinoxy]-3,3 -di-/i t/-butyl-5,5 -dimethoxy-l,T-biphenyl (BIPHEPHOS), is an excellent catalyst system for the linear-selective hydroformylation of a wide range of alkenes. This catalyst system has been successfully applied to the cyclohydrocarbonylation reactions of alkenamides and alkenylamines, which are employed as key steps for the syntheses of piperidine,indolizidine, and pyrrolizidine alkaloids. ... 

Chemists have investigated asymmetric P-K reactions, using chiral diphosphine and diphosphite ligands to induce enantioselectivity. When (,S )-BINAP was added to a catalytic amount of Co4(CO)12, (S)-bicyclic ketone 106 formed in 55% yield with an ee of 88% (equation 12.99). Experimental evidence suggested that the active catalyst was dicobalt complex 107, in which BINAP binds in bidentate... 

Optically-active aldehydes are very important as precursors not only for biologically active compounds but also for new materials. Asymmetric hydroformylation is an attractive catalytic approach to the synthesis of a large number of chiral aldehydes. With the platinum precursor (Pt(PhCN)2Cl2), anhydrous tin(II) chloride was used as cocatalyst (SnCl2/Pt 1), which is essential for catalytic activity. In case of rhodium systems an excess amount (P/Rh = 4) of diphosphite ligand was always added to the catalyst precursor to exclude the formation of HRh(CO)4, which is an active achiral hydroformylation catalyst. 

Effective control of chemo and steric selectivity of a chemical reaction lies in our ability to manipulate nano-environment of active sites. Due to difiGculty in manipulating the active site on the nanoscale and lack of fxmdamental nderstanding of the reaction mechanism, development of chemo- and stereoselective catalysts has relied heavily upon empirical studies. One successful xample of fine-tuning steric environment of the active site is the use of chiral diphosphite ligands to control the selectivity of styrene hydroformylation on Rh omplex catalysts (1-3). 

In 1992, Takaya et al. reported on the use of a chiral diphosphite derived from bisnaphthol in the asymmetric hydroformylation of vinyl acetate [11], but the enantioselectivity achieved was only 50%. They noted that diphosphites led to more stable hydroformylation catalysts than diphosphines. This observation prompted Takaya, Nozaki et al. to synthesize the chiral phosphine-phosphite ligands (R,S)- and (R,R)-BINAPHOS (24, figure 17), which were expected to combine the high enantioselectivity obtained with diphosphines such as BINAP in asymmetric hydrogenation, with the apparently efficient coordination of the phosphite moiety [38]. Indeed, the Rh(I) complex of Ci-symmetric (R,S)-BINAPHOS provided much higher enantioselectivities than either C2 symmetric diphosphine ligands or diphosphite ligands, viz. more than 90% ee for a wide variety of both functionalized and internal aUcenes [38,39,40,41]. 

Two process variants have been described. A type n process [91] resembles the Union Carbide process for butenes hydroformylation (see 8.6.1). Rapid oxidation of the diphosphite ligands by air ingress in vacuum distillation columns is mitigated by addition of an excess of sacrificial ligand (tri-orthotolylphosphine). In a type IVA process, very much like the Kuraray process (see 8.6.4.1), apolar and polar solvents are applied to effect the desired combinahon of one-phase reachon fohowed by phase separation, with most Rh/diphosphite catalyst in the apolar layer [92,93]. 

For a detailed study dealing with the distribution of products in dependence on the reaction conditions including different bidentate diphosphine and diphosphite ligands as well as a corresponding iridium catalyst, compare ... 

Currently, the workhorse in hydroformylation that converts a range of terminal long-chain olefins with high activity and w-regioselectivity into the corresponding n-aldehydes is a Rh catalyst modified with the commercially available and cheap diphosphite ligand BIPHEPHOS [48] (used for inexpensive floral notes) [49]. 

The double bond in methyl oleate can migrate to the terminus under the effect of Rh catalysts containing a sterically demanding diphosphite ligand, as shown by Behr et al. [26] with BIPHEPHOS (Scheme 6.81, lower part). The subsequent hydroformylation achieved 65% conversion of the substrate and produced methyl 19-oxononadecanoate in 26% yield within 17 h. Approximately 12% of the olefin hydrogenation product was simultaneously observed. 

The enantioseleetive hydroc) nation of cyclohexa-1,3-diene was investigated by Vogt et al using a nickel catalyst generated in situ from Ni(cod)2 and ehiral diphosphite ligand 21 (Scheme 5.16). The reaction resulted in the formation of eyelohex-2-ene-l-carbonitrile, which could arise from both... 

Scheme 5.16 Hydrocyanation of cyclohexa-1,3-diene with an in situ generated nickel catalyst derived from a diphosphite ligand.

Among the rhodium complexes, the catalyst containing the nonsymmetrical atropisomeric phosphine-phosphite chelating ligand (1) gave the best enantioselec-tivity (up to 92% ee) in the hydroformylation of the ibuprofen precursor 4-isobutyl-styrene [2] whereas the use of the bulky diphosphite ligand 2 resulted in a remarkable regioselectivity with a branched/normal ratio of 98.5/1.5 and somewhat lower optical yield (82% ee) [3]. 

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