Catalytic asymmetric reactions examples

A lead(II) triflate-crown ether complex functions as a chiral Lewis-acid catalyst for asymmetric aldol reactions in aqueous media (Scheme 86).352 This is the first example of a chiral crown-based Lewis acid that can be successfully used in catalytic asymmetric reactions. 

Transfer of a metal carbene moiety from a metal carbene complex to a heteroatom other than oxygen, sulfur, and nitrogen is possible. One such example is the report by Uemura and co-workers, who disclosed catalytic asymmetric reaction of ethyl diazoacetate with ( )-cinnamyl phenyl selenide 193 (Equation (30))." The reaction afforded 194 as a diastereomeric mixture (58 42). Using Rh2(5[Pg.173]

Shibasaki et al. have reported an asymmetric nitroaldol reaction catalyzed by chiral lanthanum alkoxide 18 to produce an optically active 2-hydroxy-1-nitroalkane with moderate-to-high enantiomeric excesses (Scheme 8B1.10) [27]. Apparently this novel catalyst acts as Lewis base. The proposed reaction mechanism is shown in Scheme 8B1.11, where the first step of the reaction is the ligand exchange between binaphthol and nitromethane. This reaction is probably the first successful example of the catalytic asymmetric reaction promoted by a Lew i s base metal catalyst. Future application of this methodology is quite promising. 

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. 

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 achiral additives likely bind to the catalyst in a bidentate fashion, and in doing so change the chiral enviromnent aronnd the snbstrate-binding site. This example illustrates the potential impact that additives can have on catalytic asymmetric reactions. 

These rare earth-containing heterobimetallic complexes can be utilized for a variety of efficient catalytic asymmetric reactions, as shown in Sch. 13, and quite recently we have succeeded in realizing the first example of a catalytic asymmetric nitro-Mannich-type reaction using the newly developed heterobimetallic complex 71 (Table 17) [72]. 

Therefore, the most crucial step is the catalytic asymmetric reaction with the substrate. The logarithm of the relative rate is varied from, for example, 0.01 to 100 (Fig. 7-12). Let us examine the case that the one activated diastereomeric complex provides the product in 100% ee (R) and that the other diastereomer provides the opposite enantiomeric product in 50% ee (S). Even when two activated diastereomer complexes are formed in 1 1 ratio, more than 98% ee of the product can be established in the case where the relative rate of the two activated diastereomers is 100 (log rel-act = 2). 

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). 

Fig. 4 Selected examples of catalytic asymmetric reactions as part of the preparation of RRR-3.

In this chapter, recent development of rare earth metal-catalyzed organic transformations, mainly highly controlled catalytic asymmetric reactions, in organic solvents and in aqueous media are highlighted. For more or other examples using rare earth metal catalysts, see reviews and references cited therein. ... 

Arguments in favor of catalytic asymmetric reactions are supported by the following examples. 

Fig. 8 Representative examples of catalytic asymmetric reactions promoted by hmnodimiclear bifunctional Schiff base catalysts...

Abstract The concept of bifunctional acid catalysis is very helpful for inventing new catalytic asymmetric reactions. Compared with single functional acid catalysts, cooperative effect of two acid components has the potential to fine tune the reactivity as well as the selectivity of desired reaction pathways. This chapter focuses on some representative examples on the recent developments of bifunctional acid catalysis, including combined acid catalysis and other cooperative acid catalysis. 

Several examples of catalytic asymmetric aza Diels-Alder reactions are shown in Table 5.10 [30]. The reaction always proceeded smoothly to afford the correspond-... 

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