Lewis acid cationic chiral

This review will concentrate on metal-free Lewis acids, which incorporate a Lewis acidic cation or a hypervalent center. Lewis acids are considered to be species with a vacant orbital [6,7]. Nevertheless, there are two successful classes of organocatalysts, which may be referred to as Lewis acids and are presented in other chapter. The first type is the proton of a Brpnsted acid catalyst, which is the simplest Lewis acid. The enantioselectivities obtained are due to the formation of a chiral ion pair. The other type are hydrogen bond activating organocatalysts, which can be considered to be Lewis acids or pseudo-Lewis acids. 

To overcome the problem that many Lewis acidic cations hydrolyze very easily in water, Kobayashi and co-workers (187) screened various metal cations and found Pb + as a stable Lewis acid in water. And then the authors studied the combination of lead salts and chiral ligands for the catalytic Mukaiyama aldol reaction. In their efforts, chiral BINOL-derived crown was promising chiral ligand under aqueous conditions and up to 87% ee of aldol adduct was obtained in syn-selectivity (Scheme 47). 

Corey et al. synthesized a chiral bis(oxazoline)Fe(III) catalyst 30, the ligand of which was prepared from chiral phenylglycine. The catalyst was formed by the reaction of the ligand with Fel3 in the presence of I2.12 greatly enhances the Lewis acidity of the catalyst owing to the formation of a cationic species [39] (Scheme 1.49). 

Cationic oxazaborinane 59 (Figure 3.10) is a chiral super-Lewis-acidic catalyst recently described by Corey and coworkers [61]. The catalyst is in equilibrium with 59a and the oxazaborinane system 59 59a is unstable and undergoes... 

Hydroxy-L-prolin is converted into a 2-methoxypyrrolidine. This can be used as a valuable chiral building block to prepare optically active 2-substituted pyrrolidines (2-allyl, 2-cyano, 2-phosphono) with different nucleophiles and employing TiQ as Lewis acid (Eq. 21) [286]. Using these latent A -acylimmonium cations (Eq. 22) [287] (Table 9, No. 31), 2-(pyrimidin-l-yl)-2-amino acids [288], and 5-fluorouracil derivatives [289] have been prepared. For the synthesis of p-lactams a 4-acetoxyazetidinone, prepared by non-Kolbe electrolysis of the corresponding 4-carboxy derivative (Eq. 23) [290], proved to be a valuable intermediate. 0-Benzoylated a-hydroxyacetic acids are decarboxylated in methanol to mixed acylals [291]. By reaction of the intermediate cation, with the carboxylic acid used as precursor, esters are obtained in acetonitrile (Eq. 24) [292] and surprisingly also in methanol as solvent (Table 9, No. 32). Hydroxy compounds are formed by decarboxylation in water or in dimethyl sulfoxide (Table 9, Nos. 34, 35). 

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. 

These examples serve to illustrate several general points about use of chiral catalysts for D-A reactions. A cationic metal center is present in nearly all of the catalysts developed to date and has several functions. It is the anchor for the chiral ligands and also serves as a Lewis acid with respect to the dienophile. The chiral ligands establish the facial selectivity of the complexed dienophile. There are several indications of the importance of the anions to catalytic activity. Anions, in general,... 

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. 

This chapter will begin with a discussion of the role of chiral copper(I) and (II) complexes in group-transfer processes with an emphasis on alkene cyclo-propanation and aziridination. This discussion will be followed by a survey of enantioselective variants of the Kharasch-Sosnovsky reaction, an allylic oxidation process. Section II will review the extensive efforts that have been directed toward the development of enantioselective, Cu(I) catalyzed conjugate addition reactions and related processes. The discussion will finish with a survey of the recent advances that have been achieved by the use of cationic, chiral Cu(II) complexes as chiral Lewis acids for the catalysis of cycloaddition, aldol, Michael, and ene reactions. 

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

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 cationic chiral Lewis acids 10, generated from the corresponding oxazaboroli-dines by protonation by trifluoromethanesulfonic acid, are excellent catalysts for the enantioselective reaction of 2-substituted acroleins, a-unsaturated a,p-enones, a-unsaturated acrylic acid esters, and a-unsaturated acrylic acids with a variety... 

Axially chiral phosphoric acid 3 was chosen as a potential catalyst due to its unique characteristics (Fig. 2). (1) The phosphorus atom and its optically active ligand form a seven-membered ring which prevents free rotation around the P-0 bond and therefore fixes the conformation of Brpnsted acid 3. This structural feature cannot be found in analogous carboxylic or sulfonic acids. (2) Phosphate 3 with the appropriate acid ity should activate potential substrates via protonation and hence increase their electrophilicity. Subsequent attack of a nucleophile and related processes could result in the formation of enantioenriched products via steren-chemical communication between the cationic protonated substrate and the chiral phosphate anion. (3) Since the phosphoryl oxygen atom of Brpnsted acid 3 provides an additional Lewis basic site, chiral BINOL phosphate 3 might act as bifunctional catalyst. 

Williams group observed low enantioselectivities for the Michael addition of a prochiral nucleophile, ethyl 2-cyanopropionate 623, to methyl vinyl ketone 624 catalyzed by chiral platinum complexes (Scheme 8.196)." The NMR analysis indicated that these cationic Pt complexes act as Lewis acids toward nitriles. The X-ray crystal structure as well NMR analysis showed that the solvent ligand that is readily displaced by an organic substrate is situated cis to the nitrogen donor in the Pt complex and, therefore, is in a chiral pocket created by the oxazoline ring. 

Evans has reported that the cationic C2-symmetric chiral Lewis acid Cu(ll)bis(oxazoline) complex promotes the hetero-Diels-Alder reaction of 0 ,/3-unsaturated acyl phosphonates with enol ethers to give the cycloadducts with excellent ee (Scheme 52). As well as simple dihydropyrans, various fused bis-dihydropyrans are also reported <1998JA4895, 2000JA1635>. 

However, in the presence of Lewis acids such as AlBr3, without the presence of the CpFe(CO)2+ Lewis acid the [3 + 2[-cycloaddition process was more effective, giving the product in 45% yield. The application of a chiral cationic CpFe(diphosphine) complex as the catalyst (Scheme 9.37) for the asymmetric [3 + 2]-cycloaddition of nitrones to acrolein derivatives was described by Kiindig and coworkers [95],... 

The mechanism A very detailed mechanistic study of this phosphoramide-catalyzed asymmetric aldol reaction was conducted by the Denmark group (see also Section 6.2.1.2) [59, 60], Mechanistically, the chiral phosphoramide base seems to coordinate temporarily with the silicon atom of the trichlorosilyl enolates, in contrast with previously used chiral Lewis acids, e.g. oxazaborolidines, which interact with the aldehyde. It has been suggested that the hexacoordinate silicate species of type I is involved in stereoselection (Scheme 6.15). Thus, this cationic, diphosphoramide silyl enolate complex reacts through a chair-like transition structure. 

---