Chiral precatalyst

Thus, chiral precatalysts includ-RuCl2[(R)-binap](dmf) (oligomeric [NH2(C2H5)2][ RuCl[(R)-binap] 2-... 

Figure 1.25. Examples of chiral precatalyst for asymmetric transfer hydrogenation of ketones.

Achiral precatalysts of type A form racemic products. However, high enan-tioselectivity is obtained by Cl-symmetric chiral precatalysts B. The turnover... 

As expected, the chiral precatalysts also initiate diastereoselective processes ( > 95%) [294c]. 

The chiral precatalyst is a titanium species. It is generated by the in situ treatment of titanium isopropoxide with diethyl or diisopropyl tartarate. The relative amounts of Ti(OPr )4 and the tartarate ester have a major influence on the rate of epoxidation and enentioselectivity. This is because the reaction between Ti(OPr )4 and the tartarate ester leads to the formation of many complexes with different Ti tartarate ratios. All these complexes have different catalytic activities and enantioselectivities. At the optimum Ti tartarate ratio (1 1.2) complex 9.35 is the predominant species in solution. This gives the catalytic system of highest activity and enantioselectivity. The general phenomenon of rate enhancement due to coordination by a specific ligand, with a specific metal-to-ligand stoichiometry, is called ligand-accelerated catalysis. 

Trace amounts of complexes 3 and 20 promote the synthesis of optically pure, multiply functionalized, versatile intermediates such as pyrones or lactones from activated, acid-labile siloxydienes with aldehydes. The reagents typically work under mild conditions and therefore promote the survival of valuable functionality in the dienophile, the diene, and cycloadduct [105-107]. As a consequence this procedure is applied in the total synthesis of various natural products, often requiring an intramolecular Diels-Alder approach [106]. Specific interactivity of the chiral precatalyst Eu(hfc)3 (hfc = 3-(heptafluorpropylhydroxymethylene)-D-camphorate with Danishefsky s diene bearing a chiral auxiliary resulted in cycloaddition products of high diastereofacial excess (95 % eq. (8)) [105]. 

Kinetic studies suggest essentially zero-order dependence on the substrate concentration, but also indicate a competitive inhibitor function of the product heterocycles. The rapidly formed amine-amide adducts of type Cp2Ln(HNR)(Fl2NR) are assumed to be the active catalyst [161]. The turnover-limiting process is the intramolecular olefin insertion into the Ln-N bond which proceeds via a chairlike, seven-membered transition state (see Scheme 8). As expected, the chiral precatalysts also initiate diastereoselective processes (>95 % de). 

Subsequently, the same group reported a synthetically useful protocol to stereospecific synthesis of syn- and a/rti-dihydropyranones containing a stereogenic trifluoromethyl substituent through NHC-catalyzed redox het-ero-Diels-Alder reactions of either E- or Z-p-trifluoromethyl enones with a-aroyloxyaldehydes. Kinetic experiments revealed the formation of 0-acylated enolate species when an achiral precatalyst was used in these reactions, accounting for the difference in reactivity observed compared with a chiral precatalyst. The measurement of a positive KIE suggests that deprotonation of the NHC-aldehyde adduct to form the Breslow intermediate is kinetically significant (Scheme 7.84). 

In 2010, a new monodentate chiral precatalyst 18, which bears a monosubsti-tuted carbon backbone with a single stereocenter, was published by Blechert and CO workers (Scheme 3.6) [25]. An orlfio-substituted phenyl and a mesityl groups were introduced as the Af-substituents. The ort/io-substituted phenyl group next to the stereocenter in the carbon backbone can efficiently transfer the chirality of the backbone to the equatorial coordination sphere (gearing effect). Moreover,... 

While most NHC precatalysts are known to promote the reaction between enals and nitroalkenes via the Stetter route, the novel chiral precatalyst (124) has been found to... 

In 2008 Kita developed a procedure for the asymmetric dearomatization of naph-thols via the formation of ortho-spirolactones using a C2-symmetrical chiral precatalyst 36 (Figure 19.11) and co-oxidant mCPBA with acetic acid [115] (Scheme 19.16). This reaction proceeds via the mCPBA/acetic acid-mediated oxidation of precatalyst 36 to the hypervalent iodine(III) active catalytic species 37. Ishihara has taken this work further with the development of the conformationally more flexible chiral organo-iodine precatalyst 38 [116], offering increased enantioselectivity at... 

By using chiral precatalysts, in many cyclopropanation reactions, significant diastereo- and enantioselectivity have been achieved. Structures of a few typical precatalysts are shown by 7.64-7.66. 

A discovery that extends the scope of asymmetric epoxidation reactions is the hydrolysis of a racemic mixture of epoxides, using the cobalt complex 8.36 as a chiral precatalyst. Under the catalytic conditions, in the presence of air and small amounts of acetic acid, 8.36 is converted to a Co -containing active catalytic intermediate with acetate and water present as additional ligands. 

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