Bidentate diphosphite

A series of strongly related monophosphites, differing only in the alcohol moiety, were prepared on the basis of a triazole backbone [18]. With a similar approach, bidentate diphosphites also became available (Scheme 2.98). 

In several instances, the geometry of such Rh complexes could be analyzed by X-ray structural analyses [119]. In principle, a bidentate diphosphite ligand can coordinate in a bis-equatorial (ee) or in an equatorial/axial (ea) maimer. Frequently, fluxional behavior was concluded by NMR spectroscopy at room temperature (Scheme 2.103) [59, 120]. To date, the individual contribution of these isomers to the overall catalytic result is not fully clarified [43,121]. 

Scheme 2.103 Different coordination modes with bidentate diphosphites.

Monodentate phosphites performed superior to bidentate diphosphites or PPhg as ligands. A large excess of the ligand retarded the reaction. Noteworthy, the phosphite-modified Ru catalyst remained stable also in a second run. An increase of the H2 partial pressure in comparison to the partial pressure of CO2 led to the hydrogenation of the olefinic substrate. Under all conditions, the aldehyde was formed in minor quantities. Based on this protocol, some other cyclic or acyclic olefins were converted into the corresponding alcohols with yields of 45-88%. 

In a similar manner, Packett at Union Carbide realized a one-pot rhodium-catalyzed version of the methodology by using diols (ethylene glycol, 1,4-butane-diol, or 2,3-butanediol) as acetalization reagents and pyridinium tosylate as co-catalyst [12]. As ligands, bidentate diphosphites and diphosphines were utilized. Under these conditions, the main products were the corresponding diacetals (up to 55%), whereas in the absence of diols mainly isomeric pentenals ( 80%) and valeraldehyde ( 10%) were obtained. 

Bidentate Diphosphinite, Phosphite-Phosphinite, Diphosphite and Phosphine-Phosphoroamidite Ligands... 

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... 

Chiral Bidentate and Multidentate Phosphorus Ligands (Diphosphanes and Polyphosphanes, Diphosphites, Diphosphinites, and Diphosphonites). After testing a number of monodentate phosphanes, a real breakthrough came in the area of enantioselective hydrogenation when various types of chiral diphosphanes were applied. The first two diphosphanes employed were DIPAMP and DIOP (see later) introduced by Knowles and Kagan, respectively. Their achievements stimulated research on a variety of bidentate chiral diphosphanes. 

Platinum-chiral bidentate ligand-tin(ll)chloride catalysts have been used in hydroformylation reactions. MeOBlPHEP, BlNAP-hemioxide, and diphosphites containing 2,4-pentanediyl and 1,3-diphenyl-1,3-propanediyl moieties have been used as a bidentate ligand (288). Pt-complexes with ferrocene-based chiral diphosphines have been screened in enantioselective hydroformylation of styrene (289). 

In RhH(BINAPHOS)(CO)2 the ligand coordinates in an equatorial-apical fashion. Both apical-equatorial isomers of the phosphine-phosphite bidentates have been identified. For BINAPHOS and related systems, the unique dissynunetric enviromuent created by phosphine-phosphite seems to be determinant for enantioselectivities to be high. As for diphosphites, phosphine-phosphite systems require a well-tuned cooperation between the chiral centers in the ligand. 

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. 

Bidentate biphenol-derived diphosphites claimed by Oxeno (today Evonik) can be symmetric like 1 [92] or nonsymmetric, exemplified with 2 [44] and 3 (Figure 2.24) [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 ... 

The first example of catalytic hydroformylation of vinyl acetate was published in 1949 by Adkins and Krsek [26] [CojfCO), CO/H2 = 100-150 atm, 120-125°C] (Scheme 4.25). Rh as a catalytically active metal was tested first by Brown and Wilkinson in 1969 [27]. In general, Rh-catalyzed hydroformylation of vinyl acetate occurs with extremely high iso-regioselectivity with simple monodentate phosphines [28], phosphonites [20], phosphinites [29], and phosphites [30]. Also, with bidentate diphosphine or diphosphite ligands the a-product is predominantly formed [31]. Therefore, this substrate is frequently screened as a model... 

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