Lewis acid catalysis asymmetric

Nishiyama H., Motoyama Y. Other Transition Metal Reagents Chiral Transition-Metal Lewis Acid Catalysis for Asymmetric Organic Synthesis in Lewis Acid Reagents 1999 225, Ed Yamamoto H., Pb. Oxford Univ. Press, Oxford Keywords asymmetric Diels-Alder reactions, chiral transition metal Lewis-acid catalysis, asymmetric synthesis... 

KuU T, Peters R. Contact ion pair directed Lewis acid catalysis asymmetric synthesis of trans-configured p-lactones. Angew. Chem. Int. Ed 2008 47 5461. 

The most frequently encountered, and most useful, cycloaddition reactions of silyl enol ethers are Diels-Alder reactions involving silyloxybutadicncs (Chapter 18). Danishefsky (30) has reviewed his pioneering work in this area, and has extended his studies to include heterodienophiles, particularly aldehydes. Lewis acid catalysis is required in such cases, and substantial asymmetric induction can be achieved using either a chiral lanthanide catalyst or an a-chiral aldehyde. 

Kiindig EP, Saudan CM, Bemardinelli G (1999) A stable and recoverable chiral Ru Lewis acid synthesis, asymmetric Diels-Alder catalysis and stmcture of the Lewis acid methacro-lein complex. Angew Chem Int Ed 38 1220-1223... 

Lewis acids as water-stable catalysts have been developed. Metal salts, such as rare earth metal triflates, can be used in aldol reactions of aldehydes with silyl enolates in aqueous media. These salts can be recovered after the reactions and reused. Furthermore, surfactant-aided Lewis acid catalysis, which can be used for aldol reactions in water without using any organic solvents, has been also developed. These reaction systems have been applied successfully to catalytic asymmetric aldol reactions in aqueous media. In addition, the surfactant-aided Lewis acid catalysis for Mannich-type reactions in water has been disclosed. These investigations are expected to contribute to the decrease of the use of harmful organic solvents in chemical processes, leading to environmentally friendly green chemistry. 

These first examples of the catalytic asymmetric aldol reaction not only provided first results that could be utilized for such transformations but also highlighted the problems that had to be overcome in further elaborations of this general method. It was shown that truly catalytic systems were required to perform an enantioselective and diastereoselective vinylogous aldol reaction, and it became obvious that y-substituted dienolates that serve as propionate-acetate equivalents provide an additional challenge for diastereoselective additions. To date, the latter problem has only been solved for diastereoselective additions under Lewis acid catalysis (vide infra) (Scheme 4, Table 3). 

In the neutral BIPHEP-Pt complex, the axial chirality of BIPHEP moiety is controlled by chiral diol BINOL as shown in Scheme 8.29. However, the diastereo-meric purity is not high enough (95 5). Therefore, recrystallization is essential to obtain the single BIPHEP-Pt diastereomer and subsequent enantiomer. It has thus been required that complete chirality control of both neutral and cationic BIPHEP-Pt complexes without recrystallization and its application to asymmetric Lewis acid catalysis (Scheme 8.32)." Interestingly, both enantiopure (5)- and (7 )-BIPHEP-Pt complexes can be obtained quantitatively through the... 

The Catalysis Concept of Iminium Activation In 2000, the MacMillan laboratory disclosed a new strategy for asymmetric synthesis based on the capacity of chiral amines to function as enantioselective catalysts for a range of transformations that traditionally use Lewis acids. This catalytic concept was founded on the mechanistic postulate that the reversible formation of iminium ions from a,p-unsaturated aldehydes and amines [Eq. (11.10)] might emulate the equilibrium dynamics and 7i-orbital electronics that are inherent to Lewis acid catalysis [i.e., lowest unoccupied molecular orbital (LUMO)-lowering activation] [Eq. (11.9)] ... 

The [3+2] cycloaddition strategy provides an effective method to access valuable intermediates for the construction of biologically important alkaloids, amino acids, amino carbohydrates and P-lactams [58-62]. The reaction involves the concerted pericyclic addition of a dipole and a dipolarophile and considerable efforts have been made to render these reactions asymmetric nsing Lewis acid catalysis and chiral anxiliaries [63]. 

Due to increasing demands for optically active compounds, many catalytic asymmetric reactions have been investigated in this decade. However, asymmetric catalysis in water or water/organic solvent systems is difficult because many chiral catalysts are not stable in the presence of water [19]. In particular, chiral Lewis acid catalysis in aqueous media is extremely difficult because most chiral Lewis acids decompose rapidly in the presence of water [20, 21]. To address this issue, catalytic asymmetric reactions using water-compatible Lewis acids with chiral ligands have been developed [22-29]. 

Yamamoto and co-workers (135,135-137) recently reported a new method for stereocontrol in nitrile oxide cycloadditions. Metal ion-catalyzed diastereoselective asymmetric reactions using chiral electron-deficient dipolarophiles have remained unreported except for reactions using a-methylene-p-hydroxy esters, which were described in Section 11.2.2.6. Although synthetically very useful and, hence, attractive as an entry to the asymmetric synthesis of 2-isoxazohnes, the application of Lewis acid catalysis to nitrile oxide cycloadditions with 4-chiral 3-(2-aIkenoyl)-2-oxazolidinones has been unsuccessful, even when > 1 equiv of Lewis acids are employed. However, as shown in the Scheme 11.37, diastereoselectivities in favor of the ffc-cycloadducts are improved (diastereomer ratio = 96 4) when the reactions are performed in dichloromethane in the presence of 1 equiv of MgBr2 at higher than normal concentrations (0.25 vs 0.083 M) (140). The Lewis acid... 

Asymmetric Diels-Alder reactions. The acrylate of (+ )-l is markedly superior to ( —)-menthyl acrylate in effecting asymmetric induction in Diels-Alder reaction with cyclopentadiene (Lewis acid catalysis, equation I). A chiral intermediate in Corey s prostaglandin synthesis was obtained by the reaction of (-1- )-8-phenylmenthyl acrylate with a cyclopentadiene derivative.1... 

Lewis acid catalysis, apparently dispensible due to the very high reactivity of nitroso dienophiles, has not yet been investigated although such a study has been suggested by Streith and Defoin [8]. Thus, examples of asymmetric catalysis lack completely in this area of hetero Diels-Alder chemistry. Nevertheless, cycloadditions involving nitroso dienophiles have reached an advanced level concerning stereoselectivity and therefore much attention has been paid towards the preparation and application of chiral, enantiopure dienophiles and dienes for these reactions. 

Mikami et al. [18] demonstrated that under Lewis acid catalysis silyl enol ethers, bearing at least one hydrogen atom in the allylic position, form carbonyl-ene products. They succeeded in using the titanium catalyst 32 for the asymmetric catalysis of this reaction (Scheme 11b). If the aldehyde contains an activating substituent, as in the case of the glyoxolate 62, an excess of the enecompound is not necessary. For example, the reaction of 62 with the silyl enol ether 27 to the carbonyl-ene adduct 63 still proceedes with good stereoselectivity, but yields drop to a moderate value. 

Activation of C=N double bonds by copper Lewis acids for nucleophilic addition has also been reported (Sch. 37) [73]. The a-imino ester 157 undergoes alkylation at the imine carbon with a variety of nucleophiles when catalyzed by copper Lewis acids. The presence of the electron-withdrawing ester group increases the reactivity of the imine and also assists in the formation of a stable five-membered chelate with the Lewis acid. Evidence for Cu(I) Lewis-acid catalysis and a tetrahedral chelate was obtained by FTIR spectroscopy, from the crystal structure of the catalyst, and from several control experiments. The authors rule out the intermediacy of a copper enol-ate in these transformations. The asymmetric alkylation of A,0-acetals with enol silanes mediated by a copper Lewis acid proceeding with high selectivity has been reported [74],... 

Chiral (helical) Lewis acids for asymmetric Diels-Alder catalysis are prepared from titanium tetraisopropoxide 5 and a chiral binaphthol ligand 4 [13]. The titanium reagent 6 plays an important role as chiral template for the fixation of a,fi-unsaturated aldehydes and thereby for the enantioface recognition of substrates. The asymmetric Diels-Alder reaction, e. g., of cyclopentadiene 7 and acrolein 8, is effected in the presence of catalytic (P)-6 (10 mol%), producing the endo adduct 9 (R =R2=H) in 88% ee (Scheme 1). 

The best results before organic catalysis were with amides 125 and Lewis acid catalysts based on Al, Ti, and Cu(II) with C2-symmetric ligands. Corey s aluminium complex 127 derived from the diamine whose resolution was described in chapter 22 works well with substituted cyclo-pentadienes 124 and the product 126 was used in prostaglandin synthesis.28 There are three aspects of stereoselectivity in this reaction which diastereotopic face of 124 is attacked (that anti to the CH2OBn group), is the product exo or endo (endo) and which endo product is formed, 126 or its enantiomer Only for the last question is asymmetric catalysis necessary, though Lewis acid catalysis of any kind enhances endo/exo selectivity. 

Transformations such as Diels-Alder reactions, ene-reactions and condensations of enoxysilanes with carbonyl compounds often require Lewis acid catalysis. For this reason, many organometallic reagents bearing chiral residues have recently been used as Lewis acids in asymmetric synthesis. Mary of these chiral Lewis acids feature structures similar to those described in Chapter 2. 

Lanthanide triflates are stable Lewis acids in water and are successfully used in several carbon-carbon bond-forming reactions in aqueous solutions. The reactions proceed smoothly in the presence of a catalytic amount of the triflate under mild conditions. Moreover, the catalysts can be recovered after the reactions are completed and can be re-used. Lewis acid catalysis in micellar systems will lead to clean and environmentally friendly processes, and it will become a more important topic in the future. Finally, catalytic asymmetric aldol reactions in aqueous media have been attained using Ln(OTf)3-chiral crown ether complex as a catalyst. 

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