Lewis reaction with water

Despite the interest in B(C6Fs)3 as a Lewis acid catalyst for organic reactions or a co-catalyst for olefinic polymerizations, little is known of its reaction with water. There is a report of the compound [NHEt3][HOB(C6F5)3] [158] and of the platinum complex, [Pt[HOB(C6Fs)3 Me(Bu2bpy)](Bu2bpy = 4,4 -di-ferf-butyl-2,2 -bipyridine),... 

Aqueous Aldol Reaction with Water-stable Lewis Acids... 

Initiators include strong protonic acids and the electrophilic species generated by reaction of a Lewis acid with water, an alcohol, ester or alkyl halide (Scheme 6). In this case, initiation occurs by electrophilic addition of the vinyl monomer to the proton or carbenium ion, which may be accompanied by reversible or irreversible collapse of the ion pair (Eq. 21). 

The step of cationic initiation can be subdivided into two separate reactions. The first one consists of formation of ionic species and the second one of reactions of these ionic species with the olefins, a cationization process. This reaction, termed priming by Kennedy and Marechal, is a process of ion formation in a non-nucleophilic media through (1) dissociation of protonic acids to form protons and counterions, (2) reactions of Lewis acids with Bronsted acids, (3) dissociation of dimeric Lewis acids, (4) complexation of Lewis acids with water or with alkyl halides or with ethers, and so on. These reactions may take place through a series of complicated steps. The second reaction, the cationization of the olefins, may also include several intermediate steps that will eventually lead to propagating species. 

STEP 2 Reaction of a nucleophile and an electrophile to form a new covalent bond. Reaction of the carbocation intermediate (a Lewis acid) with water (a Lewis base) completes the valence shell of carbon and gives an oxonium ion ... 

Orthoboric acid, also called boracic or boric acid, is usually made by precipitation from a solution of borax on treatment with sulfuric acid. The acid is a solid at room temperature and is only sparingly soluble in water. Dilute aqueous solutions are often used as mild antiseptics. Orthoboric acid is a very weak Lewis acid that, on reaction with water, produces hydronium ions as follows ... 

Details of the manufacture of polytetrahydrofuran have not been disclosed. Because of the stability of the 5-membered ring in tetrahydrofuran, combinations of Lewis acids with water are not very effective initiators. More active cationic initiators are preformed carbonium salts (e.g., (C6Hs)3C SbCl6") and oxonium salts (e.g., (C2H5 )30 BF4"). The mechanism of reaction is thought to be fundamentally similar to that given above (Section 8.4.5) for the polymerization... 

Lewis acid sites are the strongest sites detected on MTS materials, and they undoubtedly impact catalytic activity, either directly through electrophihc activation of the substrates or via the generation of strong Bronsted sites by reaction with water. Because of this, it is clear - as already said above - that the thermal history of MTS-based catalysts will determine to a great extent their catalytic behavior. 

CH3)3C-Br (CH3)3C+ + Br-Carbocation is a Lewis acid and the reaction with water is fast ... 

There are three main types of rare earth -diketonates the tris complexes, the Lewis base adducts of the tris complexes, and tetrakis complexes. The tris complexes have three fi-diketonate Ugands for each rare earth ion, i.e. R( -diketonate)3. The Lewis base addircts are formed by reaction with water or other organic Lewis bases. The tetrakis complexes have forrr j8-diketonate hgands aroimd rare earth ion and have the general formula [R( -diketonate)4] . 

This chapter introduces the experimental work described in the following chapters. Some mechanistic aspects of the Diels-Alder reaction and Lewis-acid catalysis thereof are discussed. This chapter presents a critical survey of the literature on solvent ejfects on Diels-Alder reactions, with particular emphasis on the intriguing properties of water in connection with their effect on rate and selectivity. Similarly, the ejfects of water on Lewis acid - Lewis base interactions are discussed. Finally the aims of this thesis are outlined. 

A combination of the promoting effects of Lewis acids and water is a logical next step. However, to say the least, water has not been a very popular medium for Lewis-acid catalysed Diels-Alder reactions, which is not surprising since water molecules interact strongly with Lewis-acidic and the Lewis-basic atoms of the reacting system. In 1994, when the research described in this thesis was initiated, only one example of Lewis-acid catalysis of a Diels-Alder reaction in water was published Lubineau and co-workers employed lanthanide triflates as a catalyst for the Diels-Alder reaction of glyoxylate to a relatively unreactive diene . No comparison was made between the process in water and in organic solvents. 

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