Chiral Bronsted acid catalysis

Hodous BL, Fu GC (2002) Enantioselective addition of amines to ketenes catalyzed by a planar-chiral derivative of PPY possible intervention of chiral Bronsted-acid catalysis. J Am Chem Soc 124 10006-10007... 

Recent progress in chiral Bronsted-acid catalysis. (T. Akiyama, 2006) [Igj. 

Following the pioneering reports on chiral Bronsted acid catalysis, a range of chiral phosphoric acid-catalyzed reactions have been reported [76]. 

Akiyama T, Itoh J, Fuchibe K (2006) Recent progess in chiral Bronsted acid catalysis. Adv Synth Catal 9 999... 

Rueping and co-workers have reported one of the most impressive examples of a Nazarov cyclization under asynunetric catalysis. They have shown that chiral Bronsted-acid catalysis outperforms the chiral Lewis acid catalysts used to date and have demonstrated the efficient cyclization of a variety of substrates 79 with moderate to excellent diastereoselectivity and good to excellent enantioselectivity. Only low catalyst loadings (2 mol %) are required of chiral acid 81 to catalyze the reaction efficiently. Interestingly, the reaction primarily generates the cw-cyclopentanones 80a, as opposed to the Lewis acid-catalyzed reactions that provide the trans-product (see 73). 

Kanomata, K. Toda, Y. Shibata, Y. Yamanaka, M. Tsuzuki, S. Gridnev, I. D. Terada, M. Secondary Stereocontrolling Interactions in Chiral Bronsted Acid Catalysis Study of a Petasis-Ferrier-Type Rearrangement Catalyzed by Chiral Phosphoric Acids. Chem. Sci. 2014,5,3515-3523. 

In the same year, Zhou and List reported a novel one-pot tandem reaction which, for the first time, combined chiral Bronsted acid catalysis with enamine and iminium catalysis. Later, on the basis of control experiments and ESI-MS/MS analysis, a reasonable mechanism was proposed (Scheme 2.2). The initial step of this tandem reaction was mediated by achiral /j-ethoxyaniline (PEP-NH2) and chiral phosphoric acid (R)-TRIP either reagent alone was inefficient in promoting this aldol condensation to afford the first iminium intermediate. The following step was a conjugate reduction which was also Bronsted acid and amine co-catalysed, and no further conversion took place in the absence of either catalyst. The final step was an acid-catalysed reductive amination. This novel sequence allowed the highly enantioselective synthesis of pharmaceutically active chiral ds-3-substituted cyclohejyl or heterocyclohexyl amines in high diastereo- and... 

I n this chapter, we wish to discuss the recent advancements in chiral Bronsted acid catalysis. We will focus chiefly on (i) thiourea catalysts [55], (ii) guanidium salts (protonated amine catalysts), (iii) diol catalysts, and (iv) phosphoric acids [56]. The first three catalysts are classified as neutral or weak Bronsted acids, which may be called hydrogen bond catalysts [57]. In contrast, phosphoric acids, being stronger acids, are Bronsted acid catalysts in a narrow sense (Figure 2.9). 

In 2002, Fu et al. reported planar-chiral 4-(pyrrolidino)pyridine derivative (27) catalyzed enantioselective addition of amines with ketene to give acylpyroles with high ees (Scheme 2.68). They proposed the intervention of chiral Bronsted acid catalysis [129]. They employed (27) as a catalyst for the enantioselective synthesis of esters from ketenes [130]. 

The last few years have witnessed spectacular advancement in chiral Bronsted acid catalysis. Thiourea and phosphoric acids, in particular, have emerged as novel chiral catalysts of choice in a number of synthetic reactions. A number of chiral alcohols and nitrogen containing materials have been obtained with excellent optical purity in a simple protocol. The scope of the catalysts will be expanded by further study. They are considered to be artificial enzymes. One of the advantages of chiral Bronsted acid over chiral Lewis acid is the stability toward moisture and... 

Because imine has a basic nitrogen atom, a Bronsted add turned out to be effective as the activator of imines. The seminal work of Akiyama and Terada in 2004 paved the way for chiral Bronsted acid catalysis in nucleophUic addition to imines. 

Rueping et al. reported achiral Br0nsted acid assisted chiral Bronsted acid catalysis in the direct Marmich reaction of acyclic ketones. The reaction of N-aryl imines with acetophenone was conducted using a chiral phosphoric acid in the presence of acetic acid as the co-catalyst and the resulting products were obtained in moderate yields [11]. 

More recently, several approaches to catalytic, enantioselective Nazarov cyclizations have been developed [28]. Trauner reported that substrate 266 underwent electrocyclic rearrangement, furnishing 268 in 94 % ee in the presence of the chiral Sc-PYBOX catalyst 267 (Equation 30) [134]. The enantio-discriminating step in this cyclization is believed to be protonation of the intermediate enolate that ensues from the Nazarov cyclization. Rueping has documented enantioselective cyclizations of related dienones in the presence of the chiral phosphoric acid derivative 270 (Equation 31) [135]. Chiral Bronsted acid catalysis thus effected the cyclization of 269 to give products 271 (87% ee) and 272 (95% ee). 

New Developments in Enantioselective Bronsted Acid Catalysis Chiral Ion Pair Catalysis and Beyond... 

Some other very important events in the historic development of asymmetric organocatalysis appeared between 1980 and the late 1990s, such as the development of the enantioselective alkylation of enolates using cinchona-alkaloid-based quaternary ammonium salts under phase-transfer conditions or the use of chiral Bronsted acids by Inoue or Jacobsen for the asymmetric hydro-cyanation of aldehydes and imines respectively. These initial reports acted as the launching point for a very rich chemistry that was extensively developed in the following years, such as the enantioselective catalysis by H-bonding activation or the asymmetric phase-transfer catalysis. The same would apply to the development of enantioselective versions of the Morita-Baylis-Hillman reaction,to the use of polyamino acids for the epoxidation of enones, also known as the Julia epoxidation or to the chemistry by Denmark in the phosphor-amide-catalyzed aldol reaction. ... 

By applying a new mode of cooperative catalysis involving the combination of a chiral Bronsted acid and a -symmetric biaryl saturated imidazolium precatalyst, Lee and Scheldt disclosed a highly enantioselective NHC-cata-lyzed [3 + 2] annulation reaction between a,p-alkynals and a-keto esters to generate the desired y-crotonolactones in high yields and excellent levels of enantioselectivity (up to 92% yield, 92% ee). The authors proposed that NHC-bound allenolate underwent addition to the a-keto ester activated by the chiral Bronsted acid derived co-catalyst (Scheme 7.43). 

Using menthol as a chiral Bronsted acid source, upon the Lewis acid activation with the Au(I) center, cooperative catalysis was achieved to prepare substituted pyrrohdine heterocycles with excellent enantiomeric excesses (Scheme 15.83) [300],... 

Numerous investigations of highly enantioselective reactions catalyzed by chiral phosphoric acids (86) continue to be reported. The potential of this type of Bronsted acid in asymmetric catalysis has been demonstrated. The first asymmetric direct hetero Diels-Alder reaction catalyzed by a chiral Bronsted acid has been described. Thus chiral phosphoric acid (86) exhibited superior enantioselectivity, affording fairly good yields and enantioselectivities for reactions of aromatic aldimines with cyclohexenone (Scheme 21). ... 

Tang, W. Johnston, S. Li, C. Iggo, J. A. Bacsa, J. Xiao, J. Cooperative catalysis Combining an achiral metal catalyst with a chiral bronsted acid enables highly enantioselective hydrogenation of imines. Chem. 2013,19,14187-14193. 

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