Organic Bronsted acid catalyzed

The catalytic, enantioselective, vinylogous Mannich reaction has recently emerged as a very powerful tool in organic synthesis for the assembly of highly functionalized and optically enriched 6 amino carbonyl compounds. Two distinctly different strategies have been developed. The first approach calls for the reaction of preformed silyl dienolates as latent metal dienolates that react in a chiral Lewis acid or Bronsted acid catalyzed Mukaiyama type reaction with imines. Alternatively, unmodified CH acidic substrates such as a,a dicyanoalkenes or 7 butenolides were used in vinylo gous Mannich reactions that upon deprotonation with a basic residue in the catalytic system generate chiral dienolates in situ. 

The Mukaiyama aldol reaction is beyond doubt a brilliant triumph of modem synthetic organic chemistry however, the reaction products are contaminated with pre-activated silyl enol ethers derived from the carbonyl compounds with stoichiometric amounts of silylation agent and base. In addition, silylated wastes are inherently formed. Circumventing the pre-activation process improves atom efH-ciency in this case, the carbonyl nucleophiles react directly with the carbonyl electrophiles in the presence of catalyst. The first Bronsted acid-catalyzed direct aldol reactions have been achieved using chiral Hg-BINOL-derived phosphoric acid 96 (Scheme 28.12) [66], The aldol products (127) have syn-configurations and, thus, this reaction is complementary to (S)-proHne catalysis in Brpnsted acids, which in general yields the anti configuration [11]. 

Instead of Bronsted acids, lanthanide triflates can be used to catalyze the reaction of indole with benzaldehyde (Eq. 7.7). The use of an ethanol/water system was found to be the best in terms of both yield and product isolation. The use of organic solvent such as chloroform resulted in oxidized byproducts.17... 

The catalytic asymmetric aldol reaction has been applied to the LASC system, which uses copper bis(-dodecyl sulfate) (4b) instead of CufOTf. 1261 An example is shown in Eq. 6. In this case, a Bronsted add, such as lauric add, is necessary to obtain a good yield and enantioseledivity. This example is the first one involving Lewis acid-catalyzed asymmetric aldol reactions in water without using organic solvents. Although the yield and the selectivity are still not yet optimized, it should be noted that this appredable enantioselectivity has been attained at ambient temperature in water. 

The surfaces of clay minerals can catalyze the polymerization of organic compounds through a free radical-cationic initiation process. This type of reaction is believed to be initiated by the abstraction of an electron by Lewis acid sites on mineral surfaces however, Bronsted acidity has also been shown to be important in certain cases (see Chapter 22). 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

This review aims at reporting on the synthesis of aliphatic polyesters by ROP of lactones. It is worth noting that lactones include cyclic mono- and diesters. Typical cyclic diesters are lactide and glycolide, whose polymerizations provide aliphatic polyesters widely used in the frame of biomedical applications. Nevertheless, this review will focus on the polymerization of cyclic monoesters. It will be shown that the ROP of lactones can take place by various mechanisms. The polymerization can be initiated by anions, organometallic species, cations, and nucleophiles. It can also be catalyzed by Bronsted acids, Lewis acids, enzymes, organic nucleophiles, and bases. The number of processes reported for the ROP of lactones is so huge that it is almost impossible to describe aU of them. In this review, we will focus on the more... 

The abiotic hydrolysis of triazines in soil environments is catalyzed by acidic sites on the surfaces of both organic and inorganic soil constituents. The surfaces of soil constituents have both Lewis acid sites (which accept electron pairs) and Bronsted acid sites (which donate protons). However, triazines are not competitive with water and OH groups for complexation with Lewis acid sites, so in soil environments hydrolysis is catalyzed primarily by Bronsted acid sites. Four types of Bronsted acid sites are found on soil surfaces (Mortland, 1970) ... 

Such observations lead to the designation of reactions as acid- or base-catalyzed. When the catalysis is limited to the species H+ (or OH ), the reaction is spoken of as being subject to specific ion (or OH ion) catalysis. Many reactions of both organic and inorganic chemistry fit such a designation. However, very early work on such systems soon showed that the catalysis was not limited to or OH but did extend to other species which could be subsumed under the category of what are now called Bronsted acids and bases. 

Sc(() l f) ( is an effective catalyst of the Mukaiyama aldol reaction in both aqueous and non-aqueous media (vide supra). Kobayashi et al. have reported that aqueous aldehydes as well as conventional aliphatic and aromatic aldehydes are directly and efficiently converted into aldols by the scandium catalyst [69]. In the presence of a surfactant, for example sodium dodecylsulfate (SDS) or Triton X-100, the Sc(OTf)3-catalyzed aldol reactions of SEE, KSA, and ketene silyl thioacetals can be performed successfully in water wifhout using any organic solvent (Sclieme 10.23) [72]. They also designed and prepared a new type of Lewis acid catalyst, scandium trisdodecylsulfate (STDS), for use instead of bofh Sc(OTf) and SDS [73]. The Lewis acid-surfactant combined catalyst (LASC) forms stable dispersion systems wifh organic substrates in water and accelerates fhe aldol reactions much more effectively in water fhan in organic solvents. Addition of a Bronsted acid such as HCl to fhe STDS-catalyzed system dramatically increases the reaction rate [74]. 

Dialkyl-(4-hydroxypheny)lsulfonium salts also undergo reversible photodissociation to yield Bronsted acid, useful in principle for catalyzing cationic polymerizations [86]. The organic product may be written as an ylid or, more appropriately, in quinoidal form, VIII. 

The most widely employed photoactive compound that triggers the chain of chemical reactions is the photoacid generator (PAG), a molecule that upon absorption of a photon undergoes photolysis, producing Bronsted acid and other photoproducts. The acid catalyzes a chemical reaction and it is regenerated at the end of the reaction (Scheme 12.3) [ 1,4,6,11 ]. The PAG may be an ionic organic salt or a nonionic organic compound, vide infra. 

Diarylborinic acids and arylboronic acids bearing electron-withdrawing substituents are useful as air-stable Lewis acid catalysts for various organic transformations. In particular, the potential of 3,5-(CF3)2C6H3B(OH)2 as a Lewis acid catalyst has strikingly increased since Yamamoto and Ishihara s initial study [32b]. Continued exploratory research on the application of arylboron compounds as air-stable and reusable catalysts is expected to provide powerful and practical methods for various Bronsted-Lewis acid-catalyzed organic transformations. 

While hundreds of Lewis acid catalysts have been developed for organic reactions, Brousted acid catalysts have been paid less attention until recently. Conventional Lewis acids, such as titanium chloride and aluminium chloride, are known to be incompatible with aqueous media. On the other hand, Bronsted acids are stable toward water and oxygen. Thus, they are potential candidates as activators of electrophilic substrates in water. It was found that among various Bronsted acids, HBF4 efficiently catalyzed Mannich -type reactions of silyl enol ethers with aromatic aldehydes derived from activated imines to afford the corresponding p-amino ketones (Scheme 3.1). ... 

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