Catalysis Lewis bases

Lewis-Base Catalysis via Intermediate Formation of a Chiral Zwitterionic Enolate... 

Other reactions not described here are formal [3 -i- 2] cycloadditions of a,p-unsaturated acyl-fluorides with allylsilanes [116], or the desymmetrization of meso epoxides [117]. For many of the reactions shown above, the planar chiral Fe-sandwich complexes are the first catalysts allowing for broad substrate scope in combination with high enantioselectivities and yields. Clearly, these milestones in asymmetric Lewis-base catalysis are stimulating the still ongoing design of improved catalysts. 

First Test System Thermomorphic Lewis Base Catalysis. 71... 

An important area of organocatalysis that was also initiated in the early 1990s has been the realm of Lewis base catalysis. ° On the basis of the noninmitive activation principle of silyl bonding rehybridization, Denmark and Iseki introduced chiral and DMF variants as effective catalysts for enantioselec-... 

One section in this review will deal with silyl cations, another with hypervalent silicon compounds. The concept of hypervalent sihcon compounds belongs, strictly speaking, to the class of Lewis base catalysis. However, since a Lewis base forms in situ with a silicon containing reagent or SiCl an intermediate, which functions as a Lewis acid to activate substrates during the reaction, we would also present a few examples in this review. Since silicon is a semimetal we leave it up to the reader to decide whether silicon catalysts should be considered as organocatalysts. 

In conclusion, the distinguishing characteristic of this type of phosphoramide-based neutral Lewis base catalysis is the potential of the reaction to proceed through an associative ( closed ) transition structure. Thus pronounced diastereo-selection results and control of the absolute configuration are possible. Currently, however, it seems difficult to explain the sense of induction based on transition state models [59, 60],... 

A key step is the formation of a stable hydronium ion upon formation of dimethylether. The concept of Bronsted acid-Lewis base catalysis also allows us to understand the formation of ethylene from methanol, as formed in zeolite-catalysed reactions. A possible mechanism is sketched in Fig. 4.68. 

Abstract. Within the context of Lewis base catalysis /V-heterocyclic carbenes represent an extremely versatile class of organocatalyst that allows for a great variety of different transformations. Starting from the early investigations on benzoin, and later Stetter reactions, the mechanistic diversity of /V-hctcrocyclic carbenes, depending on their properties, has led to the development of several unprecedented catalytic reactions. This article will provide an overview of the versatile reactivity of A-heterocyclic carbenes. 

Referring to a mechanistic classification of organocatalysts (Seayad and List 2005), currently the two most prominent classes are Brpnsted acid catalysts and Lewis base catalysts. Within the latter class chiral secondary amines (enamine, iminium, dienamine activation for a short review please refer to List 2006) play an important role and can be considered as—by now—already widely extended mimetics of type I aldolases, whereas acylation catalysts, for example, refer to hydrolases or peptidases (Spivey and McDaid 2007). Thiamine-dependent enzymes, a versatile class of C-C bond forming and destructing biocatalysts (Pohl et al. 2002) with their common catalytically active coenzyme thiamine (vitamin Bi), are understood to be the biomimetic roots ofcar-bene catalysis, a further class of nucleophilic, Lewis base catalysis with increasing importance in the last 5 years. 

Another synthetic pathway recently used is the cyclization of linear triazenes, like (224), which in aprotic media with Lewis base catalysis yield 5-amino-1-aryl-1,2,3-triazoles (225). However, in protic media Dimroth rearrangement affords the isomeric 5-(arylamino)-l,2,3-triazoles (226) (70JOC2215, 8UOC856). The 2-tetrazene (227) gives methyl l-benzyl-1,2,3-triazolecarboxylate (228) by photochemically induced homolysis of a single N—N bond <81TL227). 

This concept of Lewis base catalysis has been widely developed by Denmark and coworkers in the asymmetric aldol additions of trichlorosilyl enolates on aldehydes. These reactions were shown to be highly susceptible to acceleration by catalytic quantities of chiral phosphoramides [69-77]. In particular, a phos-phoramide derived from (S,S)-stilbenediamine was remarkably effective not only in accelerating the reaction but also in modulating the diastereoselectivity and in providing the aldol addition products in good to excellent enantioselec-tivity. For example, trichlorosilyl enolate 61 reacts with benzaldehyde in very high enantio- and diastereoselectivity with 10 mol% of phosphoramide 62 in favor of the anti diastereomer (antifsyn 60/1). The catalyzed aldol reaction depends on the bulkiness and loading of the catalyst. On the other hand, the hindered phosphoramide (S,S)-63 afforded the syn aldol product in excellent diastereoselectivity (anti syn 1/97) but with modest enantioselectivity. 

Cyclohexadione was recently shown to react with a,fS-unsaturated aldehydes to form bicyclo[3.2.1]octanes 46 under Lewis base catalysis [22]. The best results were... 

BR0NSTED AND LEWIS BASE CATALYSIS—BIFUNCTIONAL CATALYSIS 613... 

Denmark SE, Beutner GL (2008) Lewis base catalysis in organic synthesis. Angew Chem Int Ed 47 1560... 

Denmark SE, Burk MT (2010) Lewis base catalysis of bromo- and iodolactonization and cycloetherification. Proc Natl Acad Sci USA 107 20655—20660... 

This is a particular case of general Lewis base catalysis of a substitution reaction. It may be troublesome for other studies because of the ease with which some phosphines are oxidized to phosphine oxides. This type of preassociation and catalysis is favored by the low-polarity solvents used in this area and may be unobserved in more polar and hydroxylic solvents. 

Figure 18 Merging Lewis base catalysis and hydrogen bonding catalysis by Seidel.

The Morita-Baylis-Hillman (MBH) reaction can be broadly defined as a coupling reaction between an alkene activated by an electron-vfithdrawing group and an electrophile that occurs under Lewis base catalysis, l,4-diazabicyclo[2.2.2]octane (DABCO) normally being used as the base (Scheme 3.1) [3]. 

Competing ideas drawn from the literature on the catalyzed epoxy-phenolic curing mechanism are reviewed here and will be evaluated in subsequent chapters with regards to the results of experiments carried out in this study the most common catalysts employed are Lewis bases. Many reaction mechanisms have been suggested for the Lewis-base catalysis of the epo -phenolic reaction. " The basic reactions these authors attempted to describe are shown in Rxns 1 and 2. These models differ significantly in their description of which elementaiy molecular reactions occur to achieve the net reaction. In some of these models, the Lewis-base acts as a true catalyst in others, it first forms a complex with one or two of the reactants before the catalytic nature of the complex is activated. 

To test the viability of the method for such applications, 12 hydrolytic enzymes were investigated with two automated multiple enzyme analysis systems [46]. Hydrolases in continuous-flow systems were successfully appHed to determine glycogen without interference from free glucose [47] and for the determination of total serum cholesterol [48]. Microreactor technology combined with a hydrolase-based enzymatic method was applied for high-throughput optimization of the HCN addition to aldehydes by enantioselective Lewis acid/Lewis base catalysis [49]. 

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