Lewis-Brpnsted acid , asymmetric

In 1997, Yamamoto and co-workers described an elegant enantioselective protonation of the allyl tin derivative (Scheme 31.2). This methodology was based on the Lewis-acid-assisted Brpnsted acid strategy using a stoichiometric amount of Lewis-Brpnsted acid (LBA) B. This asymmetric protonation proceeds through a 8 2 mechanism. Indeed, the protonation of ( )-3-phenyl-2-butenyltrimethyltin la delivers the (5)-3-phenyl-l-butene 2 (Scheme 31.2). During... 

In 2006, Xu and Xia et al. revealed the catalytic activity of commercially available D-camphorsulfonic acid (CS A) in the enantioselective Michael-type Friedel-Crafts addition of indoles 29 to chalcones 180 attaining moderate enantiomeric excess (75-96%, 0-37% ee) for the corresponding p-indolyl ketones 181 (Scheme 76) [95], This constitutes the first report on the stereoselectivity of o-CSA-mediated transformations. In the course of their studies, the authors discovered a synergistic effect between the ionic liquid BmimBr (l-butyl-3-methyl-l/f-imidazohum bromide) and d-CSA. For a range of indoles 29 and chalcone derivatives 180, the preformed BmimBr-CSA complex (24 mol%) gave improved asymmetric induction compared to d-CSA (5 mol%) alone, along with similar or slightly better yields of P-indolyl ketones 181 (74-96%, 13-58% ee). The authors attribute the beneficial effect of the BmimBr-D-CSA combination to the catalytic Lewis acid activation of Brpnsted acids (LBA). Notably, the direct addition of BmimBr to the reaction mixture of indole, chalcone, d-CSA in acetonitrile did not influence the catalytic efficiency. 

Z)-awh-4-Hydroxy-l-aIkenyl carbamates 363, when subjected to substrate-directed, vanadyl-catalysed epoxidation , lead to diastereomerically pure epoxides of type 364 (equation 99)247,252,269 qqjggg epoxides are highly reactive in the presence of Lewis or Brpnsted acids to form -hydroxylactol ethers 366 in some cases the intermediate lactol carbamates 365 could be isolated . However, most epoxides 364 survive purification by silica gel chromatography . The asymmetric homoaldol reaction, coupled with directed epoxidation, and solvolysis rapidly leads to high stereochemical complexity. Some examples are collected in equation 99. The furanosides 368 and 370, readily available from (/f)-0-benzyl lactaldehyde via the corresponding enol carbamates 367 and 369, respectively, have been employed in a short synthesis of the key intermediates of the Kinoshita rifamycin S synthesis . 1,5-Dienyl carbamates such as 371, obtained from 2-substituted enals, provide a facile access to branched carbohydrate analogues . 

An enantioselective Strecker reaction involving Brpnsted acid catalysis uses a BINOL-phosphoric acid, which affords ees up to 93% in hydrocyanations of aromatic aldimines in toluene at -40 °C.67 The asymmetric induction processes in the stereoselective synthesis of both optically active cis- and trans-l-amino-2-hydroxycyclohexane-l -carboxylic acids via a Strecker reaction have been investigated.68 A 2-pyridylsulfonyl group has been used as a novel stereocontroller in a Strecker-type process ees up to 94% are suggested to arise from the ability of a chiral Lewis acid to coordinate to one of the sulfonyl (g)... 

Achari B, Mandal SB, Dutta PK, Chowdhury C (2004) Synlett 2004 2449 Aggarwal VK, Belfield AJ (2003) Catalytic asymmetric Nazarov reactions promoted by chiral Lewis acid complexes. Org Lett 5 5075-5078 Akiyama T, Itoh J, Fuchibe K (2006c) Adv Synth Catal 348 999 Akiyama T, Itoh J, Yokota K, Fuchibe K (2004) Enantioselective Mannich-type reaction catalyzed by a chiral Brpnsted acid. Angew Chem Int Ed Engl 43 1566-1568... 

S.C. PanandB. List s paper spans the whole field of current organocat-alysts discussing Lewis and Brpnsted basic and acidic catalysts. Starting from the development of proline-mediated enamine catalysis— the Hajos-Parrish-Eder-Sauer-Wiechert reaction is an intramolecular transformation involving enamine catalysis—into an intermolecular process with various electrophilic reaction partners as a means to access cY-functionalized aldehydes, they discuss a straightforward classification of organocatalysts and expands on Brpnsted acid-mediated transformations, and describe the development of asymmetric counteranion-directed catalysis (ACDC). 

The absolute stereo-preference in the Diels-Alder reaction can be easily understood in terms of the most favorable transition-state assembly 5, in which an attractive donor-acceptor interaction favors coordination of the dienophile at the face of boron which is cis to the 2-hydroxyphenyl substituent. At this time, the conformation of a,y3-enal has a strong s-trans preference. We believe that the coordination of a proton of the 2-hydroxyphenyl group with an oxygen of the adjacent B-O bond in complex 5 plays an important role in asymmetric induction this hydrogen-bonding interaction via a Brpnsted acid would cause the Lewis acidity of boron and the jr-basicity of the phenoxy moiety to increase, and the transition-state assembly 5 would be stabilized. The jr-basic phenoxy moiety and the jr-acidic dienophile could then assume a parallel orientation at the ideal separation (3 A) for donor-acceptor interaction. In this conformation, the hydroxyphenyl group blocks the si face of the dienophile, leaving the re face open to approach by diene. 

For benzoic acid acceleration in Yb(OTf)3-catalyzed allylation of aldehydes in acetonitrile, (a) As-pinall, H.C. Greeves, N. Mclver, E. G. Tetrahedron Lett. 1998, 39, 9283. For acetic acid acceleration in Yb(fod)3-catalyzed ene reaction of aldehydes with alkyl vinyl ethers, ene reaction of aldehydes with alkyl vinyl ethers, (b) Deaton, M. V. Ciufolini, M.A. Tetrahedron Lett. 1993, 34, 2409. Yamamoto et al. reported Brpnsted acid-assisted chiral Lewis acids and Lewis acid-assisted Brpnsted acids which were used for catalytic asymmetric Diels-Alder reactions and protonations and stoichiometric asymmetric aza Diels-Alder reactions, aldol-type reactions of imines, and an aldol reaction, (c) Ishihara, K. Yamamoto, H. J. Am. Chem. Soc. 1994, 116, 1561. (d) Ishihara, K. Kurihara, H. Yamamoto, H. J. Am. Chem. Soc. 1996, 118, 3049. (e) Ishihara, K. Nakamura, S. Kaneeda, M. Yamamoto, H. J. Am. Chem. Soc. 1996, 118, 12854. (f) Ishihara, K. Miyata, M. Hattori, K. Tada, T. Yamamoto, H. J. Am. Chem. Sc c. 1994, 116, 10520. (g) Yamamoto, H. J. Am. Chem. Soc 1994, 116, 10520. (h) ishihara, K. Kurihara, H. Matsumoto, M. Yamamoto Ishihara, K. Kurihara, H. Matsumoto, M. Yamamoto, H. J. Am. Chem. Soc 1998, 120, 6920. 

Their 3,3 -substituents are utilized not only for their steric bulk, but also for the coordination to metals. Yamamoto and coworkers employed a boron complex of 3,3 -bis(2-hydroxyphenyl) BINOL in the asymmetric Diels-Alder reaction of cyclopentadiene and acrylaldehyde (equation 70) . The ligand possesses two additional hydroxy groups and forms a helical structure on coordination. The catalyst is considered to function as a chiral Brpnsted acid and a Lewis acid. The complex was also used in the Diels-Alder reactions and aldol reactions of imines. Although addition of diethylzinc to aldehydes gives low ee using BINOL itself or its 3,3 -diphenyl derivative, the selectivity can be increased when coordinating groups are introduced at the 3,3 -positions. Katsuki and... 

A number of chiral acid catalysts for the asymmetric AFC reaction of indoles and pyrroles have been reported. However, the less reactive aromatic substrates such as phenols have only been explored with limited success. In this context, Yamamoto s combined acid strategy has been used to design chiral acids with stronger acidity. In a recent work reported by Luo and co-workers, a binary-acid catalytic system consisting of chiral phosphoric acid 13 (Brpnsted acid) and MgFa (Lewis acid) forming multi acidic centers induced... 

The use of chiral Brpnsted acid catalysis as a mode of asymmetric activation burgeoned dramatically in the early part of the twenty first century [35]. The role of hydrogen in this process is, in essence, similar to that of Lewis acid catalysts - i.e. activation of the C=X bond (X=0, NR, CR ) by decreasing the LUMO energy and ultimately leading to promotion of nucleophilic addition to the C=X bond (Fig. 1.5). 

A review of formal aza-Diels-Alder reactions of imines with e-rich dienes and enones, in the presence of Lewis acids/Brpnsted acids/organocatalysts, has been presented. Bifunctional A-acyl aminophosphine catalysts (75) are effective asymmetric organocatalysts in the hetero-4- -2-cycloaddition of a-substituted allenoates with tosylaldimines to produce optically active tetrahydropyridines. The Brpnsted acid-catalysed aza-Diels-Alder reactions of cyclopentadiene with iminoacetates possessing two chiral auxiliaries yielded 2-azabicyclo[2.2.1]hept-5-ene cycloadducts with high exo-selectivities. ... 

Both chiral Brpnsted and Lewis acids have been useful in asymmetric Friedel-Crafts reactions. For example, the chiral Brpnsted acid 40 was used in the asymmetric synthesis of chiral fluorenes from an achiral indole and the biarylaldehyde 39 (Scheme 1.11) [32]. Initial steps in the conversion lead to the ion pair 42. Through ion pairing with the electrophilic carbocation, the chiral anion... 

The Lewis acid (LA) is believed to only participate in the catalyzed Friedlander condensation, while the chiral phosphoric acid (B -H) could participate in the first condensation to give 25 and in the asymmetric transfer hydrogenation of A (Scheme 8). The success of this approach relies in the compatibility and synergic effect of both catalysts, the Lewis acid, and the chiral Brpnsted acid. 

Asymmetric Michael addition of a-cyanoacetates to enones, constructing densely adjacent quaternary and tertiary stereocentres, has been attained by combination of a catalytic action of soft Pd complexes as soft Lewis acids and Brpnsted acids, for example, AcOH. Investigation of the kinetics revealed that the C-C-bond-forming step takes place almost instantaneously with the bis-palladium complex right after the substrate coordination. ... 

Keywords Asymmetric synthesis Brpnsted acid Cascade reaction Lewis acid ... 

Several ways how (asymmetric) organocatalysis can be classified have been reported [14]. One general strategy is a classification according to Lewis acids/Lewis bases and Brpnsted acids/Brpnsted bases with the general simphfied activation modes depicted in Scheme 6.16. 

Protons are the simplest and most easily available Lewis acids available to catalyze organic transformations, and the use of a chiral Brpnsted acid combines the potential of proton catalysis with asymmetric induction achieved through the choice of the proper counteranion. It is without doubt that chiral phosphoric acids have been the most successfully used chiral Brpnsted acids so far [62,64] and a variety of different applications have been reported over the last years. Scheme 6.26 gives a summarizing overview about only a few... 

The second part of the chapter deals with several kinds of asymmetric reactions catalyzed by unique heterobimetallic complexes. These reagents are lanthanoid-alkali metal hybrids which form BINOL derivative complexes (LnMB, where Ln = lanthanoid, M = alkali metal, and B = BINOL derivative). These complexes efficiently promote asymmetric aldol-type reactions as well as asymmetric hydrophosphonylations of aldehydes (catalyzed by LnLB, where L = lithium), asymmetric Michael reactions (catalyzed by LnSB, where S = sodium), and asymmetric hydrophosphonylations of imines (catalyzed by LnPB, where P = potassium) to give the corresponding desired products in up to 98% ee. Spectroscopic analysis and computer simulations of these asymmetric reactions have revealed the synergistic cooperation of the two different metals in the complexes. These complexes are believed to function as both Brpnsted bases and as Lewis acids may prove to be applicable to a variety of new asymmetric catalytic reactions.1,2... 

The design for a direct catalytic asymmetric aldol reaction of aldehydes and unmodified ketones with bifunctional catalysts is shown in Figure 36. A Brpnsted basic functionality (OM) in the heterobimetallic asymmetric catalyst (I) could deprotonate the a-proton of a ketone to generate the metal enolate (II), while at the same time a Lewis acidic functionality (LA) could activate an aldehyde to give (III), which would then react with the metal enolate (in a chelation-controlled fashion) in an asymmetric environment to afford a P-keto metal alkoxide (IV). 

Conceptually new multifunctional asymmetric two-center catalysts, such as the Ln-BINOL derivative, LnMB, AMB, and GaMB have been developed. These catalysts function both as Brpnsted bases and as Lewis acids, making possible various catalytic, asymmetric reactions in a manner analogous to enzyme catalysis. Several such catalytic asymmetric reactions are now being investigated for potential industrial applications. Recently, the catalytic enantioselective opening of meso epoxides with thiols in the presence of a heterobimetallic complex has... 

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