Monophosphites

The susceptibility of phosphites to hydrolysis limits their application as ligands for homogeneous catalysis. The fused tricyclic monophosphites derived from ca 1 i x[4]arenes260-262 form coordination complexes with palladium.263... 

It thus came as a surprise that in the year 2000, three groups independently reported the use of three new classes of monodentate ligands (Scheme 28.2) [12], The ligands induced remarkably high enantioselectivities, comparable to those obtained using the best bidentate phosphines, in the rhodium-catalyzed enantioselective alkene hydrogenation. All three being based on a BINOL backbone, and devoid of chirality on phosphorus, these monophosphonites [13], monophosphites [14] and monophosphoramidites [15] are very easy to prepare and are equipped with a variable alkyl, alkoxy, or amine functionality, respectively. 

Phosphite compounds, which have been discussed in the context of their application in asymmetric hydrogenation reactions (see Section 6.1.2.6), can also be used to effect the copper salt-mediated asymmetric conjugate addition of diethylzinc to enones.74 As shown in Scheme 8-33, in the presence of diphosphite 92 and copper salt [Cu(OTf)2], the asymmetric conjugate addition proceeds smoothly, giving the corresponding addition product with high conversion and ee. In contrast, the monophosphite 93 gave substantially lower ee. 

The H-phosphonate approach entails phosphitylation of a hydroxylic component with an activated unprotected derivative of phosphonic acid to furnish H-phosphonate (a stable tetra-coordinate form of the unprotected monophosphite) followed by oxidation on phosphorus. The H-phosphonate approach is not generally used for the synthesis of monophosphates due to the difficulty of oxidation of the H-phosphonate monoesters and the necessity either to render the phosphorus atom of H-phosphonate into the three-coordinate form by silylation (to form disilylphosphite) or to introduce a protecting group (to form H-phosphonate diester) prior to oxidation. Nevertheless, in rare cases, the H-phosphonate procedure turns out to be a method of choice if, for instance, the simultaneous oxidation of functional groups other than H-phosphonate is required.51... 

Another special case are bidentate ligands. With an appropriate distance of the two phosphorus atoms, they bind the metal like pincers (chelate structure), shifting the equilibria in Fig. 14 directly to column C. Those ligand systems are known to be appreciably slower than the corresponding bulky monophosphites, but highly regioselective (e.g., ligand 4 in Fig. 9). 

This process is also applicable to hydroformylation of 1-butene to produce pentanal with high linearity of approx. 98%, if bisphosphite ligands are used. In contrast to phosphanes monophosphite/rhodium catalysts often... 

Breakthroughs that took place around the year 2000 have shown, in contrast to the common view, that indeed chiral monodentate phosphorus ligands can also lead to high enantioselectivities in a number of asymmetric hydrogenations. In the years following, monophosphines, monophos-phonites, monophosphoramidites, and monophosphites have been successfully used in the enantioselective hydrogenation of a-dehydroamino acids and itaconic acid derivatives [25],... 

Table 6.2 Hydroformylation using rhodium bulky monophosphite catalysts. ...

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. 

Since the hydroformylation reaction for most substrates shows a first order dependence on the concentration of rhodium hydride, the reaction becomes slower when considerable amounts of rhodium are tied up in dimers. This will occur at low pressures of hydrogen and high rhodium concentrations. Dimer formation has mainly been reported for phosphine ligands [17, 42, 45], but similar dimeric rhodium complexes from monophosphites [47] and diphosphites [33, 39] have been reported. The orange side product obtained from HRh(15)(CO)2 was characterized as the carbonyl bridged, dimeric rhodium species Rh2(15)2(CO)2 [39]. 

Industrial efforts have been focused on manufacturing of t>r/ 7/-aldehydes ( linear aldehydes) from olefins. Here, we briefly summarize its history on the development of phosphorus ligands, which are classified as monophosphine, monophosphite, bis-phosphite, and bis-phosphine, all useful for the normal-sc cct vc hydroformylation. 

Reetz, M.T. and Mehler, G. (2000) Highly enantioselective Rh-catalyzed hydrogenation reactions based on chiral monophosphite ligands. Angew. Chem., Int. Ed., 39, 3889-3890. 

A monophosphite complex has been prepared by the reaction of either chromium(III) perchlorate 28 or chromate829 with the ligand. The related monohypophosphite complex [Cr(H20)4H2P02]2+ has also been reported.830... 

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