Lewis acid-assisted Bronsted acid

Figure 5-5. Bronsted acid-assisted lewis acids.

Bronsted Acid-Assisted Chiral Lewis Acid Catalysts 285... 

Bronsted acid-assisted chiral Lewis acid... 

Asymmetric Mannich reactions provide useful routes for the synthesis of optically active p-amino ketones or esters, which are versatile chiral building blocks for the preparation of many nitrogen-containing biologically important compounds [1-6]. While several diastereoselective Mannich reactions with chiral auxiliaries have been reported, very little is known about enantioselective versions. In 1991, Corey et al. reported the first example of the enantioselective synthesis of p-amino acid esters using chiral boron enolates [7]. Yamamoto et al. disclosed enantioselective reactions of imines with ketene silyl acetals using a Bronsted acid-assisted chiral Lewis acid [8]. In all cases, however, stoichiometric amounts of chiral sources were needed. Asymmetric Mannich reactions using small amounts of chiral sources were not reported before 1997. This chapter presents an overview of catalytic asymmetric Mannich reactions. 

Yamamoto and co-workers have introduced a conceptually interesting series of catalysts that incorporate an acidic proton into the active catalyst. Termed Bronsted acid-assisted chiral Lewis acid (BLA), catalyst 14 selectively catalyzes a number of diene-aldehyde cycloadditions reactions (Scheme 16) [67]. While extremely selective for the substrates shown, no aldehydes lacking an a-substitu-ent were reported to be effective in this reaction. This feature was addressed in... 

Bronsted acid assisted chiral Lewis acid catalysts (BLA) are designed for the asymmetric Diels-Alder reaction [11]. Both BLA (17) [11] and (18) [12] were employed in... 

Yamamoto and Ishihara have used catalysts such as (8.21) for the Diels-Alder reaction, which they describe as Bronsted acid assisted Lewis acids . These catalysts provide enhanced stereocontrol and rate with the a-substituted enals reported. In... 

Ishihara and Yamamoto et al. have found that chiral boron ate complexes prepared from chiral tetraols and BHj THF or B(OR)j give new catalysts [Bronsted acid-assist-ed chiral Lewis acids (BLA)] in enantioselective synthesis, which achieve selectivity... 

Mixtures of polyphosphoric acid (PPA) or K-10 clay and Bi(OTf)3 xH20 have been reported to be efficient systems for the rearrangement of indanone oximes (Equation 36) [73]. The apparent synergy between PPA and Bi(OTf)3 xH20 might be rationalized as Bronsted acid assisted Lewis acid (BLA) catalysis. 

One possible way to take advantage of such abilities may be to apply a combined acids system [2] to the catalyst design. The concept of combined acids, which can be classified into Bronsted acid-assisted Lewis acid (BLA), Lewis acid-assisted Lewis acid (LLA), Lewis acid-assisted Br0nsted acid (LBA), and Bronsted acid-assisted Bronsted acid (BBA), can be a particularly useful tool for the design of asymmetric catalysis, because combining such acids will bring out their inherent reactivity by associative interaction, and also provide more organized structure, which will allow an effective asymmetric environment to be secured. 

Since the first report of chiral oxazaborolidine-based Bronsted acid-assisted chiral Lewis acid (BLA) for enantioselective Diels-Alder reactions by Corey and coworkers... 

Bronsted acid-assisted Lewis acid catalyst (BLA)... 

Enantioselective protonation of silyl enol ethers using a SnCl4-BINOL system has been developed (Scheme 83). 45 This Lewis-acid-assisted chiral Bronsted acid (LBA) is a highly effective chiral proton donor. In further studies, combined use of a catalytic amount of SnCl4, a BINOL derivative, and a stoichiometric amount of an achiral proton source is found to be effective for the reaction.346 347... 

In contrast to some related reviews, which use reaction class or electrophiles as organizational elements, this chapter is divided into three main sections according to catalyst class (i) Bronsted acid catalysis by phosphoric acid and phosphoramide derivatives, (ii) N—H hydrogen bond catalysis by organic base and ammonium systems, and (iii) combined acid catalysis including Bronsted-acid-assisted Bronsted acid, Lewis-acid-assisted Bronsted acid, and Lewis-acid-assisted Br0nsted acid systems (Figure 5.1). 

Lewis-Acid-Assisted Bronsted Acid Catalysis [124]... 

Yamamoto et al. reported full research details on catalytic enantioselective protonation under acidic conditions in which prochiral trialkylsilyl enol ethers and ketene bis(trialkyl)silyl acetals were protonated by a catalytic amount of Lewis acid assisted Bronsted acid (LBA15 or 16) and a stoichiometric amount of 2,6-dimethylphenol as an achiral proton source [20]. 

Most chiral organoboron Lewis acids reported to date are based on an organoborane that is attached to a chiral organic moiety such as a diol, aminoalcohol, or other readily available chiral substrates.Organoboron derivatives recently used as catalysts in enantioselective Diels-Alder reactions include the family of chiral acyloxyboranes (CAB) with (196) and (197) as representative examples and various cyclic boronic esters such as (198) and (199). An interesting system that combines the favorable Lewis acid properties of fluorinated arylboranes with a chiral Bronsted acid has been developed by Ishihara and Yamamoto. The Bronsted acid-assisted chiral Lewis acids (BLA) (200) was found to be highly effective in enantioselective cycloadditions of Q ,jS-enals with various dienes. The presence of the Bronsted acid functionality leads to significant acceleration of the reaction. 

We will illustrate this also for water adsorption. The low dielectric constant of the zeolite leads to the unexpected result that contact of a single water molecule with a proton does not lead to its protonation. Bonding of a water molecule can be considered as Bronsted acid-Lewis base assisted adsorption. Only when two water molecules adsorb around a proton, protonation occurs. The hydronium-ion now is stabilized by the second polar water molecule (compare Figs. 4.55a and 4.55b). 

Silyl enol ethers, known as chemically stable and easy handled enolates, can be protonated by a strong Bronsted acid. Our group demonstrated that a Lewis acid-assisted Bronsted acid (LBA 17), generated from optically pure binaphthol and tin tetrachloride, was a chiral proton source of choice for asymmetric protonation of silyl enol ethers possessing an aromatic group at the a-position [33, 34]. Binaphthol itself is not a strong Bronsted acid, however, LBA 17 can proto-nate less reactive silyl enol ethers since the acidity of the phenolic protons of 17 is enhanced by complexation with tin tetrachloride. The catalytic asymmetric protonation of silyl enol ethers was accomplished for the first time by LBA 18. Treatment of ketene bis(trimethylsilyl)acetal 60 with 0.08 equiv of LBA 18 and a stoichiometric amount of 2,6-dimethylphenol as an achiral proton source afforded (S)-2-phenylpropanoic acid (61) with 94% ee (Scheme 10) [35]. LBA 19 derived from binaphthol monoisopropyl ether has been successfully applied to the enantioselective protonation of meso 1,2-enediol bis(trimethylsilyl) ethers under stoichiometric conditions [36]. 

All of the above methods introduce the aryl group during the enantiodetermining step. An alternative strategy would be to already have the aryl group in place and to generate the tertiary stereocentre via an asymmetric protonation of an enolate complex. This was first reahsed by the pioneering work of Yamamoto in this area with the use of Lewis acid assisted chiral Bronsted acid (LBA) catalysts in the enantioselective synthesis of a-aryl cyclohexanones ((2), Scheme 4.34). Initially developed with the use of stoichiometric quantities of a BlNOL-SnCLi catalyst for the asymmetric protonation of silyl enol ethers, [63] the extensive development of this reaction has resulted in a catalytic variant with an achiral proton donor [64] and expansion of the scope to include tertiary a-aryl carboxylic acids. [65] Further improvement was made with the development of a metal free IV-triflyl thiophos-phoramide BINOL derived proton source (126) [66] and more recently a Lewis base-tolerant chiral LBA [67]. 

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