Diphosphite ligand

Inspired by the excellent early results obtained with the Union Carbide type ligands lOa-c, other research groups have studied different modifications in these types of 

The following were observed when using biphenyl-based ligands lS-20a-d (Table 3.3) in Rh-catalyzed asymmetric hydrogenation of vinyl arenes  

In summary, both S- and J -enantiomers of the product can be obtained with excellent regio- and enantioselectivity. These results are among the best ever reported for the asymmetric hydroformylation of vinyl arenes [1]. 

The characterization of rhodium complexes formed under hydroformylation conditions by NMR techniques and in situ IR spectroscopy showed that there is a relationship between the structure of [HRh(CO)2(P-P)] (P-P = 15-20) species and their enantiodiscriminating performance. In general, enantioselectivities were highest with ligands with a strong ee coordination preference, while an equilibrium of species with ee and ea coordination modes considerably reduced the ee s [20]. 

Ligand 18a was also successfully applied in the Rh-catalyzed asymmetric hydroformylation of 2,5-dihydrofuran and 2,3-dihydrofuran. Good enantioselectivities (up to 75% ee) and excellent regioselectivities (up to 99%) were achieved. Note that both enantiomers of tetrahydrofuran-3-carbaldehyde can be obtained using this ligand by simple substrate change from 2,5-dihydrofuran to 2,3-dihydrofuran [21]. 

The selectivity to the linear product nonanal vas strongly dependent on the CO pressure (see Table 6.4). The linear to branched ratio drops from 29 at Pco = 5 bar to 4 at Pco = 40 bar. Part of this selectivity change can be ascribed to enhanced isomerization at lower CO pressure, but faster P-hydride elimination cannot account completely for the increased formation of the branched aldehyde. The reduced selectivity was attributed to partial ligand dissociation resulting in less selective rhodium monophosphites and/or ligand-free complexes. 

This implies that the three frequencies in the hydrido complex are a combination of two CO stretching frequencies and one rhodium hydrido stretching frequency. 

The rhodium-hydride vibration disappears upon deuteration of the complex as the rhodium-deuteride vibration appears in the fingerprint region. The large frequency shift of the highest energy absorption is indicative of a trans-CO geometry [40]. In solution IR, the rhodium hydride vibration and the lowest energy CO vibration overlap, which results in only two absorptions. 

Chart 6.5 Chiral diphosphites forming exclusively ea, 14, or ee, 15, rhodium complexes. 

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Remarkably, Claver et al. showed that in a square planar rhodium carbonyl chloride complex, two bulky phosphite ligands (65) were able to coordinate in a trans orientation.214 Diphosphite ligands having a high selectivity for linear aldehyde were introduced by Bryant and co-workers. Typical examples are (67)-(70).215,216... 

Several types of chiral diphosphite ligand have been synthesized and tested in Pt-catalyzed asymmetric hydroformylation.338-341... 

The first reports on asymmetric hydroformylation using diphosphite ligands revealed no asymmetric induction. In 1992, Takaya et al. published the results of the asymmetric hydroformylation of vinyl acetate (ee = 50%) with chiral diphosphites.358... 

The first report to use diphosphite ligands in the asymmetric hydroformylation of vinyl arenes revealed no asymmetric induction [46]. An important breakthrough came in 1992 when Babin and Whiteker at Union Carbide patented the asymmetric hydroformylation of various alkenes with ee s up to 90%, using bulky diphosphites 2a-c derived from homochiral (2R, 4R)-pentane-2,4-diol (Scheme 4) [17]. Their early results showed that (a) bulky substituents are required at the ortho positions of the biphenyl moieties for good regio- and enantio-selectivity and (b) methoxy substituents in the para positions of the biphenyl moieties always produced better enantio-selectivities than those observed for the corresponding ferf-butyl-substituted analogues. 

Interestingly, the hydroformylation results obtained with ligands 2b and 2d, which have conformationally flexible axially chiral biphenyl moieties, are similar to those obtained with ligand 2m, which have conformationally rigid binaphthyl moieties. This indicates that diphosphite ligands that contain the conformationally flexible axially chiral biphenyl moieties predominantly exist as single atropoisomer in the [RhH(CO)2 (diphosphite)] complexes... 

Van Leeuwen s studies using diphosphite ligands 2 and 18 indicated that the stability and catalytic performance of the [RhH(CO)2(diphosphite)] species depends strongly on the configuration of the 2,4-pentanediol ligand backbone and the chiral biaryl phosphite moieties. Thus, for example, ligands 2b, 2d and 2m, which form well defined stable bis-equatorial (ee)... 

Fig. 4 Bis-equatorial (ee) and equatorial-axial (ea) coordination modes of diphosphite ligands in the [RhH(CO)2(diphosphite)] complexes...

Over the years, several authors have developed new diphosphite ligands with binaphthyl, spiro, pyranoside, and macrocyclic backbones for asymmetric hydroformylation of vinyl arenes with low-to moderate success (ee s from 36% to 76%) [48-58]. 

For successful applications with diphosphite ligands see the review Didguez M, Pamies O, Claver C (2004) Tetrahedron Asymmetry 15 2113... 

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],... 

Optically active diols are useful building blocks for the synthesis of chiral diphosphite ligands. Chiral diphosphites based on commercially available optically active 1,2 and 1,4-diols, l,2 5,5-diisopropylidene-D-mannitol, L-a,a,a,a-tetramethyl-l,3-dioxalan-4,5-dimethanol and L-diethyl tartrate, were first used in the asymmetric hydroformylation of styrene [75],... 

Fig. 14 Summary of the best results obtained in the Ir-catalyzed hydrogenation using nanoparticles stabilized with furanoside diphosphite ligands 14 and 15...

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. ... 

A modified 14 with perfluoralkylphenyl groups increased the selectivity up to 96%.280 Both high enantioselectivity (91%) and high regioselectivity (98.8%) were achieved under mild conditions with a new chiral diphosphite ligand derived from D-(+)-glucose.281... 

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