Diphosphite catalysts

Chiral diphosphites based on (2R,3R)-butane-2,3-diol, (2R,4R)-pentane-2,4-diol, (25, 5S)-hexane-2,5-diol, (lS -diphenylpropane-hS-diol, and tV-benzyltartarimide as chiral bridges have been used in the Rh-catalyzed asymmetric hydroformylation of styrene. Enantioselectivities up to 76%, at 50% conversion, have been obtained with stable hydridorhodium diphosphite catalysts. The solution structures of [RhH(L)(CO)2] complexes have been studied NMR and IR spectroscopic data revealed fluxional behavior. Depending on the structure of the bridge, the diphosphite adopts equatorial-equatorial or equatorial-axial coordination to the rhodium. The structure and the stability of the catalysts play a role in the asymmetric induction.218... 

The first catalytic 1,4-addition of diethylzinc to 2-cyclopentenone with over 90% ee was described by Pfaltz and Escher, who used phosphite 54 with biaryl groups at the 3,3 -positions of the BINOL backbone.46 Chan and co-workers achieved high enantioselectivity in the same reaction (up to 94% ee) by using chiral copper diphosphite catalyst (R,R,R)-41 48,48a 48d Hoveyda and co-workers used ligand 46 to realize excellent enantiocontrol (97% ee) in the 1,4-additions of 2-cyclopentenones,52 which may be used in the practical asymmetric synthesis of some substituted cyclopentanes (including prostaglandins). 

The phosphite complex, a in Figure 6.2, is a typical example of the new generation catalysts (UCC has commercialised a process using a diphosphite catalyst). The NMR characteristics are of course the same for a and b ... 

Table 2. Hydroformylation using rhodium bulky diphosphite catalysts ...

In contrast to bulky monophosphite modified systems, rhodium diphosphite catalysts showed very low activity for internal alkenes like cyclohexene. The required monodentate coordination of the ligand is... 

For lower alkenes such as 2-butenes UCC has achieved high contents of linear products (see Table 2). Bryant reported 74% selectivity for the formation of linear pentanal by hydrofonnylation of 2-butene using the rhodium bulky diphosphite catalyst [22, 23]. 

Table 1. Hydroformylation of styrene with chiral rhodium diphosphite catalysts...

As pointed out in Chapters 3 and 4 the intermediate alkene adducts of the diphosphite catalysts probably contain bis-equatorial bidentate ligands and the intermediates of BINAPHOS contain an equatorial-apical phosphine-phosphite ligand. Thus the structures involved in the migratory insertion are different for the two systems. 

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]. 

Recent patent activity suggests that DuPont is developing a new generation of chelating diphosphite—nickel catalysts for this technology which are significantly more active than the monodentate phosphite based catalyst system used for the last two decades (61—64). 

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 ees up to 90%, using bulky diphosphites derived from homochiral (2i ,4R)-pentane-2, 4-diol, UC-PP (1 19).359 360 van Leeuwen et al. studied these systems extensively. The influence of the bridge length, of the bulky substituents and the cooperativity of chiral centers on the performance of the catalyst has been reported.217 218 221 361-363... 

Bakos et al. reported a series of diastereomeric diphosphites that were used in the Pt- and Rh-catalyzed asymmetric hydroformylation of styrene. Systematic variation in chirality at both the chelate backbone and the terminal groups revealed a remarkable effect on the enantioselect-ivity of the catalysts. These systems have been described in Section 9.3.3.5. 

A chiral diphosphite based on binaphthol, coordinated with rhodium (I) forming a nine-member ed ring, led to an efficient hydroformylation of vinylarenes, although moderate ees were obtained (up to 46%) at mild pressure and temperature reaction conditions.364 Chiral diphosphites and phosphinite-phosphites derived from spiro[4.4]nonane-l,6-diol were synthesized. Using these catalysts in the asymmetric hydroformylation of styrene, high regioselectivity (97%) and... 

However, platinum catalysts have several disadvantages they have low reaction rates, they hydrogenate the substrate and their regioselectivity to the branched aldehyde is low. The selectivity of Pt-diphosphite/SnCl2 systems is also low. When the appropriate diphosphite is used, ee s can be as high as 90% [13]. In the early 90s, several reports were published which described the state of the art in hydroformylation with both rhodium and platinum systems [14-16]. 

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... 

Asymmetric rhodium catalysts are discussed in section 8.6. The most interesting ligand discovered for asymmetric hydroformylation is undoubtedly BINAPHOS, introduced by Takaya [18], but certain diphosphites also give high enantioselectivities [19,20],... 

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 asymmetric catalytic Pauson-Khand reaction met success in the late 1990s. Not only the conventional Co catalyst but also other metal complexes, such as Ti, Rh, and Ir, are applicable to the reaction. Asymmetric hydrocyanation of vinylar-enes is accomplished using Ni complex of chiral diphosphite. Further studies on the scope and limitation are expected. 

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