Enantioselectivity catalytic asymmetric reactions

Shibasaki et al. also developed catalytic reactions of copper, some of which can be applied to catalytic asymmetric reactions. Catalytic aldol reactions of silicon enolates to ketones proceed using catalytic amounts of CuF (2.5 mol%) and a stoichiometric amount of (EtO)3SiF (120 mol%) (Scheme 104).500 Enantioselective alkenylation catalyzed by a complex derived from CuF and a chiral diphosphine ligand 237 is shown in Scheme 105.501 Catalytic cyanomethyla-tion by using TMSCH2CN was also reported, as shown in Scheme 106.502... 

Asymmetric catalysis is a vital and rapidly growing branch of modern organic chemistry. Within this context, Ti- and Zr-based chiral catalysts have played a pivotal role in the emergence of a myriad of efficient and enantioselective protocols for asymmetric synthesis. In this chapter, a critical overview of enantioselective reactions promoted by chiral Zr-based catalysts is provided. Since an account of this type is most valuable when it provides a context for advances made in a particular area of research, when appropriate, a brief discussion of related catalytic asymmetric reactions promoted by non-Zr-based catalysts is presented as well. 

In the course of our investigations to develop new chiral catalysts and catalytic asymmetric reactions in water, we focused on several elements whose salts are stable and behave as Lewis acids in water. In addition to the findings of the stability and activity of Lewis adds in water related to hydration constants and exchange rate constants for substitution of inner-sphere water ligands of elements (cations) (see above), it was expected that undesired achiral side reactions would be suppressed in aqueous media and that desired enanti-oselective reactions would be accelerated in the presence of water. Moreover, besides metal chelations, other factors such as hydrogen bonds, specific solvation, and hydrophobic interactions are anticipated to increase enantioselectivities in such media. 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

This article provides a brief overview of several recent total syntheses of natural and unnatural products that have benefited from the use of catalytic asymmetric processes. The article is divided by the type of bond formation that the catalytic enan-tioselective reaction accomplishes (e.g C-C or C-0 bond formation). Emphasis is made on instances where a catalytic asymmetric reaction is utilized at a critical step (or steps) within a total synthesis however, cases where catalytic enantioselective transformations are used to prepare the requisite chiral non-racemic starting materials are also discussed. At the close of the article, two recent total syntheses are examined, where asymmetric catalytic reactions along with a number of other catalyzed processes are the significant driving force behind the successful completion of these efforts (Catalysis-Based Total Syntheses). 

The field of asymmetric catalysis in synthesis is far from mature however. There are a considerable number of important chemical transformations that do not yet have a catalytic enantioselective variant, and few of the existing catalytic asymmetric reactions - perhaps none of them - are truly general. Some argue that there are no general methods, asymmetric or not several asymmetric variations of the same general transformation may therefore be needed. Furthermore, many of the applications that were discussed here were employed by researchers that also developed the asymmetric methods we have not reached a stage yet, when scientists not involved in the discovery and development of catalytic asymmetric reactions regularly utilize such protocols. It is hoped that this article... 

The virtue of performing the PKR in an enantioselective manner has been extensively elaborated during the last decade. As a result, different powerful procedures were developed, spanning both auxiliary-based approaches and catalytic asymmetric reactions. For instance, the use of chiral N-oxides was reported by Kerr et al., who examined the effect of the chiral brucine N-oxide in the intermolecular PKR of propargylic alcohols and norbornadiene [59]. Under optimized conditions, ee values up to 78% at - 60 °C have been obtained (Eq. 10). Chiral sparteine N-oxides are also able to induce chirality, but the observed enantioselectivity was comparatively lower [60]. 

The first part of this chapter describes recent advances in the use of novel, chiral, alkali metal free-lanthanoid-BINOL derivative complexes for a variety of efficient, catalytic, asymmetric reactions. For example, using a catalytic amount of chiral Ln-BINOL derivative complexes, asymmetric Michael reactions and asymmetric epoxidations of enones proceed in a highly enantioselective manner. 

Conceptually new multifunctional asymmetric two-center catalysts, such as the Ln-BINOL derivative, LnMB, AMB, and GaMB have been developed. These catalysts function both as Brpnsted bases and as Lewis acids, making possible various catalytic, asymmetric reactions in a manner analogous to enzyme catalysis. Several such catalytic asymmetric reactions are now being investigated for potential industrial applications. Recently, the catalytic enantioselective opening of meso epoxides with thiols in the presence of a heterobimetallic complex has... 

Importantly, mixtures of E- and Z-olefin substrates could be hydrogenated with comparable enantioselectivities, providing an enantioconvergent process a highly desirable yet rare feature of a catalytic asymmetric reaction. In addition, this transformation effectively differentiates between />,/>-olefin substituents of similar steric demand (e.g., Me/Et, Ar/c-hex), furnishing hydrogenated products with very high enantioselectivity. 

Although this chapter describes details of studies published up until the end of 2005, selected examples of the early 2006 literature are also included. The schemes and tables were chosen to illustrate mainly the catalytic asymmetric reactions. Stoichiometric or non-asymmetric examples, relevant to the main focus, are briefly mentioned in the text, especially when further development is expected but, in general, they are not presented in a graphical form. The key catalytic reactions exhibiting high enantioselectivity and the most successful catalysts are highlighted by being displayed in frames (e.g., Fig. 7.1). We believe that this layout... 

The 1 1 complex b ween bovine serum and an osmate ester is an enantioselective catalyst in the syn hydroxyladon of certain alkenes, although synthetic applications iq>pear to be limited. Asymmetric di-hydroxylation of alkenes is considered in a review on catalytic asymmetric reactions. ... 

The past several years have witnessed enormons advances in the number and variety of reactions that can be catalyzed with excellent enantioselectivities see Enantioselectivity). The area has recently been comprehensively reviewed with volumes edited by the team of Jacobsen, Pfaltz, and Yamamoto as well as Ojima. These important treatises are quite detailed and cannot be summarized here. The goal of this section is to present some of the most important new approaches to asymmetric catalysis. The basic concepts necessary to understanding catalytic asymmetric reactions have been succinctly described by Bosnich in the first edition of Encyclopedia of Inorganic Chemistry and will not be duplicated here. 

Miscellaneous. There are several other reports on the application of this ligand to catalytic asymmetric reactions, although enantioselectivities are modest. Those reports include the Mukaiyama-Michael reaction, allylation of aldehydes, asymmetric Diels-Alder reaction, Mukaiyama-Aldol reaction of ketomalonate, aziridination reaction of a-imino esters, and asymmetric hetero-Diels-Alder reaction. ... 

With regard to the catalytic asymmetric reaction , only a few successful examples, except those reactions using chiral transition metal complexes, have been reported. For example, the cinchona-alkaloid-catalyzed asymmetric 1,4-addition of thiols or 6-keto esters to Michael acceptors quinidine catalyzed the asymmetric addition of ketene to chloral and the highly enantioselective 1,4-addition of ) -keto esters in the presence of chiral crown ethers to Michael acceptors have been most earnestly studied. 

In the last decade a variety of catalytic asymmetric DA reactions wifh alkenes has been developed most, however, involve the use of a cychc diene, particularly cyclopentadiene. There are a few examples of catalytic asymmetric reactions of siloxydienes with alkenes. Corey et al. have reported enantioselective cycloaddition of sil-oxydiene 106 to mefhacrolein as the key step of fhe asymmetric synfhesis of cassiol (Scheme 10.116) [314]. Recently, Rawal et al. have demonstrated that Cr(III)-salen complex 108a catalyzes the cycloaddition of l-amino-3-siloxy-l,3-dienes to a, -unsa-turated aldehydes wifh high enantioselectivity [315]. The highly functionalized cyclohexene products have been used for alkaloid synthesis. Ghosez et al. have introduced asymmetric DA reaction of siloxy-substituted azadienes 109 under catalysis by Cu(II)-box complex 70a [316]. 

Compared with well-established electrophilic it-allylpalladium chemisty, the catalytic asymmetric reaction via umpolung of jt-allylpalladium has received very limited exploration [93]. Zhou and co-workers investigated the Pd-catalyzed asymmetric umpolung allylation reactions of aldehydes [22a, 94], activated ketones [95], and imines [96] by using chiral spiro ligands (5)-18e, (S)-17c, and (5)-17a, respectively. One representative example is that of the Pd/(5)-18e-catalyzed umpolung allylation of aldehydes with allylic alcohols and their derivatives, which provided synthetically useful homoallylic alcohols from readily available allylic alcohols, with high yields and excellent enantioselectivities (Scheme 33). 

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