Monodentate phosphites

In the studies conducted by Reetz, rhodium catalysts based on mixtures of monodentate phosphites, monodentate phosphonites and combinations of the two were screened in the enantioselective hydrogenation of a- and /9-N-acetyl-de-hydroamino acid esters, enamides and dimethyl itaconate [40], and a number of the more striking positive results are listed in Table 36.3. An enhanced ee-value was found mostly with combinations of two phosphonites, or one phosphonite and one phosphite, in particular when one of the ligands carries a bulky substituent and the other a small one. 

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). 

Switching the roles of the zinc porphyrin template and N-donor adapter provides an alternative mode for the supramolecular construction of biden-tate ligands (Scheme 32). Complex 26 derived from mixing three equivalents of template 24 with two equivalents of monodentate phosphite ligands 23 furnished a rhodium catalyst which displayed good regioselectivity toward... 

The first examples of five-coordinate platinum(II) complexes of the type [Pt(PR3)L]2+ (L = tris(2-(diphenylphosphino)ethyl)phosphine R = Et, OMe, OEt) (104) containing only P-donor atoms have been prepared by the reaction of [PtClL]+ with an appropriate monodentate tertiary phosphine or phosphite ligand.284 Triaryl phosphines and phosphites do not react with the precursor complex, even at elevated temperatures, most probably due to the considerable steric interactions that would occur upon the approach of the P-donor ligand to the platinum(II) center. 

The chiral monodentate phosphites presented in Scheme 28.6 are easily prepared from a diol, phosphoms trichloride, and an alcohol. Usually, the diol is converted into the corresponding phosphoro chloridite, followed by reaction... 

Scheme 28.6 Monodentate phosphite ligands derived from BINOL or related diols.

Rhodium-catalyzed hydrogenation of enamides has been successfully performed using monodentate phosphites 17, with enantioselectivities of up to 95% being obtained [53]. The rate of hydrogenation is low in order to reach full conversion with a SCR of 500, hydrogenation is performed at a pressure of 60 bar for 20 h. The use of ligand 17 am in the rhodium-catalyzed hydrogenation of aromatic enamides resulted in ee-values of up to 95%. 

As described for monodentate phosphonite ligands, monodentate phosphite ligands have also been used in a monodentate ligand combination approach. 

Scheme 28.8 Monodentate phosphite ligands based on carbohydrates.

The use of monodentate phosphoramidites in enantioselective hydrogenation was first reported in 2000, together with reports on the use of phosphites and phospho-nites [15]. Phosphoramidites are prepared in a variety of ways, but the most common route is the treatment of a diol with PC13, followed by addition of an amine [60, 61]. MonoPhos (29a), the first reported phosphoramidite used as a ligand, is prepared from BINOL and HMPT in toluene [62]. Phosphoramidites, especially... 

Reetz and Goossen et al. reported recently the asymmetric hydrogenation of a series of enol esters using monodentate phosphite ligands 17 and 24 based on a combination of BINOL and carbohydrates or simple alcohols the results of these studies are shown in Table 28.6. 

Clearly, the method can also be used for other monodentate ligands such as phosphites. The application of this library approach to copper-catalyzed enantio-selective C-C-bond formation has also been reported [38]. 

It has been established that usually two monodentate ligands (phosphorami-dites, phosphites or phosphonites) are present in their rhodium-based hydrogenation catalysts. This would allow the possibility of testing catalysts based on two different monodentate ligands. Initially, this does not seem very appealing, as the suspected outcome would be the formation of a mixture of the heterocatalyst and the two homocatalysts (Scheme 36.13). 

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