Chiral phosphate catalysts

Based on acid-base combination chemistry, Ishihara developed a catalytic enantioselective cyanosilylation of ketones by using chiral (J )-Ph2-BINOL-derived lithium phosphate, which was prepared in situ from phosphoric acid 

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Muller and co-workers [52] have studied the intermolecular rhodium (11) catalyzed C-H insertion of a nitrene derived from [M-(p-nitrobenzenesulfonyl)imi-nojphenyliodinane into cycloalkanes, cycloalkenes, and cyclic ethers. In one example involving indane as the substrate with Pirrimg s chiral phosphate catalyst, the product showed an enantioenrichment of 31% ee, Eq. (34). 

It is noteworthy that Toste and coworkers described the new concept of an anionic phase-transfer catalyst, whereby a chiral phosphate catalyst was used in the asymmetric fluorination reactions. They used selectfluor as a versatile cationic fluorinating agent, which would normally be insoluble in nonpolar organic solvents. 

Axially chiral phosphoric acid 3 was chosen as a potential catalyst due to its unique characteristics (Fig. 2). (1) The phosphorus atom and its optically active ligand form a seven-membered ring which prevents free rotation around the P-0 bond and therefore fixes the conformation of Brpnsted acid 3. This structural feature cannot be found in analogous carboxylic or sulfonic acids. (2) Phosphate 3 with the appropriate acid ity should activate potential substrates via protonation and hence increase their electrophilicity. Subsequent attack of a nucleophile and related processes could result in the formation of enantioenriched products via steren-chemical communication between the cationic protonated substrate and the chiral phosphate anion. (3) Since the phosphoryl oxygen atom of Brpnsted acid 3 provides an additional Lewis basic site, chiral BINOL phosphate 3 might act as bifunctional catalyst. 

Muller has explored enantioselective C-H insertion using optically active rhodium complexes, NsN=IPh as the oxidant, and indane 7 as a test substrate (Scheme 17.8) [35]. Chiral rhodium catalysts have been described by several groups and enjoy extensive application for asymmetric reactions with diazoalkanes ]46—48]. In C-H amination experiments, Pirrung s binaphthyl phosphate-derived rhodium system was found to afford the highest enantiomeric excess (31%) of the product sulfonamide 8 (20equiv indane 7, 71% yield). 

A new catalyst salt (20) that consists of an achiral ammonium ion and a chiral phosphate anion and which catalyses highly enantioselective transfer hydrogenations of ,/J-unsaturated aldehydes to the corresponding saturated derivatives has been developed. The underlying principle, namely asymmetric counteranion-directed catalysis, is claimed to be a new strategy for highly enantioselective synthesis.357... 

Hydrogenation of Ketimines Chiral Phosphates Are Efficient Metal-Free Catalysts.213... 

In 2009, Xiao employed a half sandwich Cp lr(l 11) complex with different chiral monosulfonated OPEN ligands (Figure 7.2), earlier developed for imine reduction by the same group, in combination with a BINOL based chiral phosphate counteranion (TRIP anion), to reductively aminate 24 acetophenone derivatives (1.2 equiv) with p anisidine (limiting reagent) under the conditions of 1.0 mol% of the indicated Ir catalyst (Table 7.1), 5 bar (73 psi) H2, toluene, 35 °C, and 15 24h [14]. 

To a suspension of 150 mg of 4 A molecular sieves in 3 mL of toluene, 10 mL of a toluene solution of about 0.07 mmol of the chiral titanium catalyst (prepared by the procedure above) are added and the mixture is cooled to OX. 0.7 mmol of the fumaric acid derivative 1 in 4mL of toluene is added to the mixture, and then 5 mL of hexane and 1 mL of isoprene (3, R1 = CH3) are added. The mixture is stirred overnight at 0 X. pH 7 phosphate buffer is added, and the mixture is worked up as above. 

Besides the metal complexes based on Ni and Pd that were previously proved suitable for the asymmetric hydrovinylation reactions, Ru and Co complexes have also been employed in this type of transformation. The one sole example of Ru-catalyzed enantioselective hydrovinylation was reported by the List group in 2012. In their initial studies, they envisioned that Ni complexes with chiral counteranions might enable the desired reaction. However, it was found that these species either were unreactive or did not furnish enantioselective control in the hydrovinylation reactions. In this context, Jiang and List focused their attention on Ru catalysis (Table 9.6). Systematic evaluation on the effects of Ru complexes ligated by different phosphine ligands as well as various chiral phosphate anions introduced by the corresponding silver salts demonstrated that the combination of Ru complex Ru-3 and additive Ag-1 was the optimal catalyst, leading to acceptable results (entry 8). 

Aldimines (R -CH=N-R ) have been phosphinated with diphenylphosphine oxide to give a-amino phosphine oxides (42) in up to 99% ee, using a chiral magnesium phosphate catalyst. ... 

A chiral TRIP anion combined with a palladacycle provides an active catalyst for enantioselective rearrangement of allylic imidates to the corresponding amide products (Scheme 98). Although the catalyst contains a chiral palladacycle, the stereoselectivity is induced by the chiral phosphate anion (Scheme 98). 

An optimised chiral binol phosphate catalyst (87) for the hydrocyanation of imines provides a convenient strategy for the enantioselective synthesis of a-amino acids and diamines (Scheme 22). The same catalyst (87) has been used for cascade... 

The same catalyst (93) was applied with success to the first organocatalytic multi component asymmetric Biginelli reaction providing medicinally relevant chiral 3,4-dihydropyrimidini-2-(l//)-ones (94) (Scheme 29)7 Chiral acid-catalyzed inverse electron-demanding aza Diels-Alder reaction of aldimines (95) with electron-rich alkenes (Scheme 30). A new catalyst salt (96) that consist of an achiral ammonium ion and chiral phosphate anion has been developed that catalyzes highly... 

Direct diastereo- and enantioselective hydrohydroxyalkylations of butadiene, an abundant petrochemical feedstock, required the use of a mthenium catalyst modified by a chiral phosphate counterion derived from Hg-BINOL. The anion is attached to the metal center through the acid-base reaction of H2Ru(CO)(PPh3)3 with the indicated chiral phosphoric acid. With the chiral counterion as the sole chiral inducing element, primary benzylic alcohols hydrohydroxyalkylate butadiene with good levels of anri-diastereo- and enantioselectivity (Scheme 3) [37]. 

Conjugate addition of O-alkylhydroxylamines to a,P-unsaturated ketones was effectively catalyzed by an isolable complex of scandium with a chiral phosphate ligand (Sc[(R)-BNP]3) [124]. The corresponding P-amino ketones were generated in almost quantitative yields with high enantioselectivities (Scheme 12.48). These products could be quantitatively converted into the 2-acyl aziridines upon treatment with a base catalyst without losing their enantiopurities. 

Antilla et al. also investigated chiral calcium BINOL phosphate catalysts [94] for reviews of asymmetric phospination reactitMis, see [95, 96]. This type of calcium catalyst was apphed to asymmetric phosphinatimi reactions (Table 24). Not only a chiral magnesium phosphate but also a chiral calcium phosphate was... 

The transformation of ort/zo-alkynylaryl ketones (275) to isochromene derivatives (276) through a cyclisation/enantioselective-reduction sequence has been realised in the presence of a chiral silver phosphate catalyst (277). The reaction afforded the li -isochromene derivatives (276) in high yield with fairly good to high enantioselectivity (Scheme 75). ... 

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