Chiral catalysts transition metals

Perhaps the most successful industrial process for the synthesis of menthol is employed by the Takasago Corporation in Japan.4 The elegant Takasago Process uses a most effective catalytic asymmetric reaction - the (S)-BINAP-Rh(i)-catalyzed asymmetric isomerization of an allylic amine to an enamine - and furnishes approximately 30% of the annual world supply of menthol. The asymmetric isomerization of an allylic amine is one of a large and growing number of catalytic asymmetric processes. Collectively, these catalytic asymmetric reactions have dramatically increased the power and scope of organic synthesis. Indeed, the discovery that certain chiral transition metal catalysts can dictate the stereo-... 

In contrast to the maturity of asymmetric synthesis utilizing chiral transition metal catalysts, asymmetric phase transfer catalysis is still behind it and covers organic reactions to lesser extent. Thus, it is further necessary in wide range to explore efficient asymmetric phase transfer catalysis keeping its superiority of easy operation, mild reaction conditions, and environmental binignancy. 

This chapter has discussed the transition metal-catalyzed synthesis of allenes. Because allenes have attracted considerable attention as useful synthons for synthetic organic chemistry, effective synthetic methods for their preparation are desirable. Some recent reports have demonstrated the potential usefulness of optically active axially chiral allenes as chiral synthons however, methods for supplying the enantiomerically enriched allenes are still limited. Apparently, transition metal-catalyzed reactions can provide solutions to these problems. From the economics point of view, the enantioselective synthesis of axially chiral allenes from achiral precursors using catalytic amounts of chiral transition metal catalysts is especially attractive. Considering these facts, further novel metal-catalyzed reactions for the preparation of allenes will certainly be developed in the future. 

In this chapter, recent advances in asymmetric hydrosilylations promoted by chiral transition-metal catalysts will be reviewed, which attained spectacular increase in enantioselectivity in the 1990s [1], After our previous review in the original Catalytic Asymmetric Synthesis, which covered literature through the end of 1992 [2], various chiral Pn, Nn, and P-N type ligands have been developed extensively with great successes. In addition to common rhodium and palladium catalysts, other new chiral transition-metal catalysts, including Ti and Ru complexes, have emerged. This chapter also discusses catalytic hydrometallation reactions other than hydrosily-lation such as hydroboration and hydroalumination. 

The first chiral transition-metal catalyst designed for an enantioselective transformation was applied to the reaction between a diazo ester and an alkene to form cyclopropanes [1]. In that application Nozaki and coworkers used a Schiff base-Cu(II) complex (1), whose chiral ligand was derived from oc-phenethylamine, to catalyze the cyclopropanation of styrene with ethyl diazoacetate (Eq. 5.1) [2],... 

The development of chiral transition-metal catalysts for these transformations has taken place during the evolution of mechanistic understanding for metal carbene generation. Concepts applied to the design of chiral catalysts have been influenced by these mechanistic developments, and further refinements can be expected. 

Some chiral transition metal catalysts bind with face-selectivity toward the substrate, which enables the enantioselective conversion of prochiral starting materials ... 

In principle, steric control may be exercised by a chiral transition metal catalyst in which the metal center is the asymmetry origin rather than the phosphine or its substituents. Gladysz and coworkers have separated the enantiomers of rhenium complexes using HPLC. 

What two steps are needed to convert A to L-dopa, an uncommon amino aoid that is effective in treating Parkinson s disease These two steps are the key reactions in the first oommercial asymmetrio synthesis using a chiral transition metal catalyst. This process was developed at Monsanto in 1974. 

A wide variety of synthetic processes have been rendered asymmetric through the use of a chiral catalyst. In addition to the types of reaction described above, chiral transition metal catalysts have been used to influence the stereochemical course of isomerization, cyclization, and coupling reactions. As an example, an approach towards the natural product (-)-epibatidine (158) was recently reported by Namyslo and Kaufmann (166). Epibatidine is a potent analgesic and a nicotinic receptor agonist. The synthesis involves an asymmetric Heck-type hydroarylation between the bicyclic alkene (155) and pyridyl iodide (156). A number of bidentate chiral li-... 

Acyclic arsines are superior to phosphines in their ability to retain chirality, with the R3AS inversion barrier, being ca 40kcalmol" compared to that of RjP, ca 30kcalmoP . Where the thermal degradation of P-chiral transition metal catalyst is significant, the substitution of arsenic for phosphorus may lead to an acceptable stability. 

This process (hetero Diels-Alder reaction leading to a dihydropyran system) may be also conducted in an asymmetric version application of chiral transition-metal catalysts based on BINOL, BDMAP, bisoxazolines, etc. provides adducts in very high optical purity (ee up to 99%) [1,6], In a series of papers Jurczak reported recently a highly enantioselective cycloaddition of 1-methoxy-1,3-butadiene and butyl glyoxylate catalyzed with chiral salen complexes [21],... 

Besides the transition-metal-catalyzed asymmetric addition reactions to prochiral olefins, the substitution reaction of a carbon nucleophile to allylic esters has been investigated using a variety of chiral transition-metal catalysts. Using the aforementioned sugar diphosphites... 

Enantioselective C-H insertion is clearly the domain of chiral dinuclear rhodium catalysts (see Chap. 16.2). Only very few examples of enantioselective copper-catalyzed reactions of this type have been reported. As a possible approach to the mitomycine ring system, Sulikowski has studied the cyclization of diazo esters 28 quite extensively using various chiral transition metal catalysts... 

Development of chiral transition-metal catalysts enables one to perform the catalytic C—H insertion to metal carbenoids, generated from diazo compounds, in an enantioselective manner. Davies et al. reported that the asymmetric intramolecular reaction of the aryldiazoacetates 684 in the presence of Rh2-(S-DOSP)4 gave the C-H insertion products 685 (Scheme 212). 288b The enantioselectivity is strongly dependent on the site of the C-H activation the highest enantioselectivity was obtained for insertion into the methyne C—H bond. 

A facinating application of homogeneous catalysis is asymmetric catalysis. The 2001 Nobel Prize in Chemistry was given for research in the field of chiral transition metal catalysts for stereoselective hydrogenations and oxidations. 

Both intra- and intermolecular aziridination of alkenes can be accomplished with PhI(OAc)2 and an appropriate nitrogen source (eqs 57 and 58). The former reactions have been described using carbamate, sulfonamide, and sulfamate substrates. Typical catalysts utilized for these processes include Ru, Rh, and Cu complexes. By employing chiral transition metal catalysts, asymmetric induction has been realized in both intra- and inter-molecular reactions. 

As mentioned earlier, hydrosilylation can be used as an alternative route of ketone reduction. By performing the addition of hydrosilanes in the presence of suitable chiral transition metal catalysts, prochiral ketones are transformed in the subsequent hydrolysis step into optically active alcohols (5) ... 

A chiral transition metal catalyst generally consists of a metal atom ligated by a chiral organic molecule. This coordination complex can influence the outcome of a reaction by interacting with a substrate. This interaction involves coordination of the substrate to a vacant site on the metal atom. The ligand is covalently bound to the ligand via donor atoms such as P, N, O or S, typically in a bidentate fashion to form the chiral metal complex (Fig. 4.2). The complex can transfer its chiral information to a substrate when it binds to the metal. The nature of the donor atoms and the backbone through which they are linked can have a profound effect on the... 

The reactions discussed in the last sections produce enantioemiched P-stereo-genic compounds by metal-catalysed enantioselective resolutions of chiral but racemic secondary phosphines and derivatives. Another set of methods is based on desymmetrisations of achiral (but prochiral) tertiary phosphine derivatives bearing two enantiotopic groups amenable to functionahsation by chiral transition metal catalysts. 

Arenes suffer dearomatization via cyclopropanation upon reaction with a-diazocarbonyl compounds (Btlchner reaction) [76]. Initially formed norcaradiene products are usually present in equilibrium with cycloheptatrienes formed via electrocyclic cyclopropane ring opening. The reaction is dramatically promoted by transition metal catalysts (usually Cu(I) or Rh(II) complexes) that give metal-stabilized carbenoids upon reaction with diazo compounds. Inter- and intramolecular manifolds are known, and asymmetric variants employing substrate control and chiral transition metal catalysts have been developed [77]. Effective chiral catalysts for intramolecular Buchner reactions include Rh Cmandelate), rhodium carboxamidates, and Cu(I)-bis(oxazolines). While enantioselectivities as high as 95% have been reported, more modest levels of asymmetric induction are typically observed. 

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