PYRPHOS

Rhodium-catalyzed enantioselective hydrogenation of acctamido -cinnamic in water was also achieved using pyrphos bound to poly-acrylic acid as ligand.337 Roucoux described some Rh° nanoparticles which function as reusable hydrogenation catalyst for arene derivatives in a biphasic water-liquid system.338... 

Figure 4.31. PPI and PAMAM dendrimers functionalized with Pyrphos.[41,42]...

The electronic properties around the phosphorus atom can be varied by manipulation of the groups on that atom. MOD-DIOP (12c) was developed by Achiwa and used to reduce itaconic acids [68-72, 75, 103]. Some variations built on BCPM (18e), itself a variant of BPPM, such as the MOD-BCPM (18f) and MCCPM (18g) [88-93, 95-98, 104]. Other variants are PYRPHOS (21a also called DeguPHOS) [105, 106], DPCP (22) [107], NorPhos (23) [108], BDPP (24a) (also called SkewPHOS) [109-111], and PPCP (25) [112]. 

Several chiral ligands, such as PYRPHOS [78b], have been shown to be very efficient ligands for the hydrogenation of a-dehydroamino acid derivatives in terms of both high enantioselectivity and reactivity. 

A PYRPHOS ligand was found to be effective for the hydrogenation of a / -aryl- or alkyl-substituted monoamido itaconate [107]. A MeO-BIPHEP-Ru catalyst was successfully applied for the enantioselective hydrogenation of an intermediate for the drug candoxatril in a mixed solvent (THF/H20) (Scheme 26.12) [108]. 

Rh complexes with ChiraPhos, PyrPhos, or ferrocenyl phosphines lacking amino alkyl side chains (such as BPPFA) are much less active toward tetrasubstituted olefins. Table 6-1 shows that in asymmetric hydrogenations catalyzed by 5a-d, the coordinated Rh complex exerts high selectivity on various substrates. It is postulated that the terminal amino group in the ligand forms an ammonium carboxylate with the olefinic substrates and attracts the substrate to the coordination site of the catalyst to facilitate the hydrogenation. 

The two N-containing phosphines, N3P 206 [179] and PAA-pyrphos 207 [180] served as useful components of hydroformylation and hydrogenation catalysts. 

In a subsequent paper (87), the same authors investigated the palladium-catalyzed allylic aminations with pyrphos-functionlized PPI and PAMAM dendrimers as multidentate ligands. Zero to four generation PPI-(pyrphosPdCl2)x and PAMAM-(pyrphosPdCl2)x neutral dendrimers (97) showed a strong positive dendritic effect on the selectivity of the allylic amination of 1,3-diphenyl-1-acetoxypropene with morpholine (at 45°C in DMSO). 

High catalytic activities have been achieved by the PYRPHOS- [18], PPCP [20], BICHEP- [21], Et-DuPHOS-Rh [19] complexes among others, allowing the reaction with a substrate-to-catalyst molar ratios (S/C) as high as 50,000. With a [2.2]PHANEPHOS-Rh complex, the reaction proceeds even at -45°C [27], Supercritical carbon dioxide, a unique reaction medium, can be used in the DuPHOS and BPE-Rh-catalyzed hydrogenation [43], A highly lipophilic counteranion such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) or trifluoromethanesulfonate is used to enhance the solubility of the cationic Rh complexes. Under the most suitable reaction conditions of 102 atm of carbon dioxide, 1 atm of hydrogen, and 22°C, a-amino acid derivatives are produced with up to 99.7% ee. 

PYRPHOS and BDPP forming a chiral five- and six-membered chelate ring are useful for Rh-catalyzed enantioselective reduction of (Z)-oe-(acetamido)cinnamic acid with HCOOH/HCOONa [161]. 

Pyrphos-Based Catalysts Bearing Dendritic Wedges. 82... 

Scheme 1 Synthesis of Pyrphos-functionalized PPI and PAMAM dendrimers by peptide coupling methods...

In an effort to extend the use of the Pyrphos-derived dendrimers to asymmetric Pd-catalyzed coupling reactions, strongly positive selectivity effects were observed upon going to very large multi-site chiral dendrimer catalysts. This enhancement of the catalyst selectivity was observed in palladium-catalyzed allylic substitutions, such as that displayed in Scheme 3, which are known to be particularly sensitive to small changes in the chemical environment of the active catalyst sites [17]. 

The mononuclear catalyst [(Boc-Pyrphos)PdCl2], which is very unselective for this transformation [9% enantiomeric excess (ee)], provided the point of reference for the subsequent studies with the dendrimer catalysts. This system and the metalladendrimers PPI(PyrphosPdCl2)4 -PPI(PyrphosPdCl2)64... 

Scheme 3 Asymmetric allylic amination of l,3-diphenyl-l-propene-3-acetate catalyzed by Pyrphos-palladium complexes...

Fig. 17 A Pyrphos derivative bearing an N-bound poly(aryl ether) dendron...

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