Chiral ligands, uses

The directive effect of allylic hydroxy groups can be used in conjunction with chiral catalysts to achieve enantioselective cyclopropanation. The chiral ligand used is a boronate ester derived from the (VjA jA N -tetramethyl amide of tartaric acid.186 Similar results are obtained using the potassium alkoxide, again indicating the Lewis base character of the directive effect. 

Chiral ligands used in addition reactions of diorganozincs with aldehydes 383... 

Addition of Organozinc Reagents to Aldehydes, Ketones, and a-Ketoesters 2.06.16.2.1 Chiral ligands used in addition reactions of diorganozincs with aldehydes... 

Figure 85 Examples of chiral ligands used for the alkylation and arylation of imines by diorganozinc reagents.

Fig. 7.4. Various chiral ligands used in the copper

Figure 1.16 lists other chiral ligands useful for asymmetric hydrogenation of ot-and/or p-keto esters.A Ru complex with BPE, a fully alkylated diphosphine. 

Figure 1.16. Chiral ligands useful for hydrogenation of keto esters.

Figure 4.1. Chiral ligands used in asymmetric hydroformylation.

The directive effect of allylic hydroxyl groups can be used in conjunction with chiral catalysts to achieve enantioselective cyclopropanation. The chiral ligand used is a boronate... 

The reaction is quite sensitive to the chiral ligand used. Diphosphines with an axially disymmetric biaryl moiety in the backbone give the best results. The effectiveness of p-Tol-BINAP as ligand, for example, is similar to that of BINAP. The related atropisomeric ligand BIPHEMP can also be used [9]. Among chiral aliphatic diphosphines tested, CyDIOP, which only differs from DIOP in the type of P-substituents, also gives satisfactory results. 

FIGURE 12.1 Examples of chiral ligands used in asymmetric hydrogenation catalysts. 

Figure 2.1 Chiral ligands used in catalytic asymmetric [3 + 2] cycloadditions.

As a consequence of the development of the N-methylmorpholine N-oxide (NMO) and later the potassium ferricyanide cooxidant systems the amounts of osmium tetroxide and chiral ligand used in the reaction could be considerably reduced. However, the method remains problematic for large-scale applications. The cooxidants for Os(VI) are expensive and large amounts of waste are produced (Table 5). Lately, several groups have addressed this problem and new reoxidation processes for osmium(VI) species have been developed. 

Asymmetric reaction is one of the most exciting features of catalyzed hydroboration since optically active phosphine ligands are the chiral auxiliaries most extensively studied for metal-catalyzed reactions (Scheme 13).134 The chiral ligands used for asymmetric hydroboration of alkenes include BINAP,136 1 03-106,167-170 QUINAP,171-173 107-109,172,174-176 and BDPP.177,178... 

Fig. 3.23 Selected examples of chiral ligands used in homogeneous hydrogenation catalysts.

The chiral ligand used is based on a phthalazine (PHAL) modified by two dihydroquinidine (DHQD) substituents. Other asymmetric dihydroxylation reactions for the synthesis of pharmaceuticals have been developed at Chirex and Pharmacia/Upjohn. 

Asymmetric dihydroxylation Sharpless developed a catalytic system (AD-mix- 3 or AD-mix-a) that incorporates a chiral ligand into the oxidizing mixture which can be used for the asymmetric dihydroxylation of alkenes. The chiral ligands used in Sharpless asymmetric dihydroxylation are quinoline alkaloids, usually dihydroquinidine (DHQD) or dihydroquinine (DHQ) linked by a variety of heterocyclic rings such as 1,4-phthalhydrazine (PHAL) or pyridazine (PYR) (see section 1.6, reference 32 of Chapter 1). 

Chiral Ligand of L1A1H4 for the Enantioselective Reduction of Alkyl Phenyl Ketones. Optically active alcohols are important synthetic intermediates. There are two major chemical methods for synthesizing optically active alcohols from carbonyl compounds. One is asymmetric (enantioselective) reduction of ketones. The other is asymmetric (enantioselective) alkylation of aldehydes. Extensive attempts have been reported to modify Lithium Aluminum Hydride with chiral ligands in order to achieve enantioselective reduction of ketones. However, most of the chiral ligands used for the modification of LiAlHq are unidentate or bidentate, such as alcohol, phenol, amino alcohol, or amine derivatives. 

Figure 3-1 Some chiral ligands used in asymmetric Heck reactions (see Table 3-8).

Availability and cost of ligands. In the majority of cases the ligands of the organometallic catalysts are chiral diphosphines which need special synthetic know-how and can be rather expensive. Typical prices are US 100-500/g for laboratory quantities and US 5000 to >US 20000/kg on a larger scale. Chiral ligands used for early transition metals are usually cheaper. 

Table 2.14 gives an overview of industrial processes using asymmetric catalysis, including examples both of asymmetric hydrogenation and other types of reactions, as well as of the use of biocatalysts for the reaction. Selected chiral ligands used in asymmetric catalytic reactions are also shown in the table. 

The second report of catalytic enantioselective organolithium additions to the imine group came in 1991 from the laboratories of Itsuno and co-workers [23aj. These researchers studied the addition of -BuLi to AT-(trimethylsilyl)benzalde-hyde imine 5a in the presence of chiral modifiers such as alcohols, diols, and amino alcohols (<1 g of imine, ca. 0.13 M). The chiral ligands used were easily prepared according to literature procedures. The enantiomerically enriched primary amine 6 was obtained in 27-90% yield after appropriate workup, depending on the nature of the chiral ligand and the reaction solvent (Scheme 4). 

Figure 1. Representative chiral ligands used for the enantiosclcctive alkylation of 1.3-diphcnyl-2-propenyl acetate with dimethyl mulonate. Figures in parentheses refer to the relevant reference(s).

I.5.8.I.2. Chiral Ligands Used in Asymmetric Homogeneous Catalysis... 

Use of chirally modified salicylimines is reported in early examples of cobalt complex catalysed asymmetric hydroformylation of styrene. A prominent example is (Aj-A -a-methyl-benzylsalicylaldimine [(+ )-A -SalH, 1] prepared from (5)-l-phenylethylamine and salicyl-aldehyde21-131-134. ( + )-(S)-3-5ec-Butylpyridine (2)164 is one of the few further examples of nonphosphorus-containing chiral ligands used in asymmetric hydroformylation23. 

Scheme 8.15. An early example of a catalytic dihydroxylation reaction [71]. The chiral ligand used here is the p-chlorobenzoate ester of DHQD (Figure 8.4a).

Other enantioselective catalysts for the D-A reactions have been developed. The chiral ligands used include the TADDOLs (a, a, a, a-tetraaryl-l,3-dioxolane-4,5-dimethanols) ° and BINOL derivatives. These are discussed in Section 6.4 of Part B. 

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