Acids catalysis, addition with

Finally, thioamides can be modified via their S-silylated derivatives (32). With Lewis acid catalysis, addition of a Schiff base yields 3-aminothioamides (33) as a mixture of diastereomers (Scheme 7). ... 

General Bransted acid catalysis begins with the addition of a proton whilst general Bronsted base catalysis begins with the removal of a proton ... 

The external surface of protonic zeolites can be relevant in acid catalysis. Several data suggest that nonshape-selective catalysis can occur at these sites, like in the case of alkylaromatics conversions over H-MFI [225,226]. On the other hand, H-zeolites also catalyze reactions of molecules, which do not enter the cavities due to their bigger size. Therefore, the external surface of zeolites is certainly active in acid catalysis. Additionally, the bulk versus surface Si/Al composition of a zeolite could be different and different preparation procedures can allow to modify this ratio [226]. Corma et al. [84] reported data on the accessibility of protonic sites of different zeolites to 2,6-di-ter-butyl-pyridine (DTBP). This molecule has been considered selective for Brpnsted sites, due to its impossible interaction with Lewis sites for steric hindrance. According to these authors, however, the interpretation of the data is not straightforward, for several reasons such as the presence of different cavities and the big size of the probe itself. Surprisingly, Corma et al. found a complete accessibility of the sites of beta zeolite to DTBP. This contrasts the data of Trombetta et al. [197], who showed that protonic sites exist also on the smaller channels of beta, whose access to DTBP seems very unlikely. In a more recent publication, Farcasiu et al. [227] reported an accessibility of 90% of the protons of H-BEA to DTBP, much higher than the 36% for H-MOR and 31% for H-USY. 

On the basis of the studies described in the preceding chapters, we anticipated that chelation is a requirement for efficient Lewis-acid catalysis. This notion was confirmed by an investigation of the coordination behaviour of dienophiles 4.11 and 4.12 (Scheme 4.4). In contrast to 4.10, these compounds failed to reveal a significant shift in the UV absorption band maxima in the presence of concentrations up to one molar of copper(ir)nitrate in water. Also the rate of the reaction of these dienophiles with cyclopentadiene was not significantly increased upon addition of copper(II)nitrate or y tterbium(III)triflate. 

In a second attempt to extend the scope of Lewis-acid catalysis of Diels-Alder reactions in water, we have used the Mannich reaction to convert a ketone-activated monodentate dienophile into a potentially chelating p-amino ketone. The Mannich reaction seemed ideally suited for the purpose of introducing a second coordination site on a temporary basis. This reaction adds a strongly Lewis-basic amino functionality on a position p to the ketone. Moreover, the Mannich reaction is usually a reversible process, which should allow removal of the auxiliary after the reaction. Furthermore, the reaction is compatible with the use of an aqueous medium. Some Mannich reactions have even been reported to benefit from the use of water ". Finally, Lewis-acid catalysis of Mannich-type reactions in mixtures of organic solvents and water has been reported ". Hence, if both addition of the auxiliary and the subsequent Diels-Alder reaction benefit from Lewis-acid catalysis, the possibility arises of merging these steps into a one-pot procedure. 

Sulphuric acid catalysed nitration in concentrated nitric acid, but the effect was much weaker than that observed in nitration in organic solvents ( 3.2.3). The concentration of sulphuric acid required to double the rate of nitration of i-nitroanthraquinone was about 0-23 mol 1, whereas typically, a concentration of io mol 1 will effect the same change in nitration in mixtures of nitric acid and organic solvents. The acceleration in the rate was not linear in the concentration of catalyst, for the sensitivity to catalysis was small with low concentrations of sulphuric acid, but increased with the progressive addition of more catalyst and eventually approached a linear acceleration. 

A regioselective aldol condensation described by Biichi succeeds for sterical reasons (G. Biichi, 1968). If one treats the diaidehyde given below with acid, both possible enols are probably formed in a reversible reaaion. Only compound A, however, is found as a product, since in B the interaction between the enol and ester groups which are in the same plane hinders the cyclization. BOchi used acid catalysis instead of the usual base catalysis. This is often advisable, when sterical hindrance may be important. It works, because the addition of a proton or a Lewis acid to a carbonyl oxygen acidifies the neighbouring CH-bonds. 

Many of the most interesting and useful reactions of aldehydes and ketones involve trans formation of the initial product of nucleophilic addition to some other substance under the reaction conditions An example is the reaction of aldehydes with alcohols under con ditions of acid catalysis The expected product of nucleophilic addition of the alcohol to the carbonyl group is called a hemiacetal The product actually isolated however cor responds to reaction of one mole of the aldehyde with two moles of alcohol to give gem mal diethers known as acetals... 

Polymerization to Polyether Polyols. The addition polymerization of propylene oxide to form polyether polyols is very important commercially. Polyols are made by addition of epoxides to initiators, ie, compounds that contain an active hydrogen, such as alcohols or amines. The polymerization occurs with either anionic (base) or cationic (acidic) catalysis. The base catalysis is preferred commercially (25,27). 

The acid-catalyzed additions of bromide and chloride ion to thiiranes occurs readily, with halide preferentially but not exclusively attacking the most substituted carbon atom of the thiirane. The reaction of 1-substituted thiiranes with acetyl chloride shows a slight preference for halide attack at the less substituted carbon atom (80MI50601). For further discussion of electrophilic catalysis of halide ion attack see Section 5.06.3.3.2. The reaction of halogens with thiiranes involves electrophilic attack on sulfur (Section 5.06.3.3.6) followed by nucleophilic attack of halide ion on carbon. 

Solutions of unstable enols of simple ketones and aldehydes can also be generated in water by addition of a solution of the enolate to water. The initial protonation takes place on oxygen, generating the enol, which is then ketonized at a rate that depends on the solution pH. The ketonization exhibits both acid and base catalysis. Acid catalysis involves C-protonation with concerted 0-deprotonation. 

Schollenberger added 2% of a polycarbodiimide additive to the same poly(tetra-methylene adipate) urethane with the high level of acid (AN = 3.66). After 9 weeks of 70°C water immersion, the urethane was reported to retain 84% of its original strength. Carbodiimides react quickly with residual acid to form an acyl urea, removing the acid catalysis contributing to the hydrolysis. New carbodiimides have been developed to prevent hydrolysis of polyester thermoplastics. Carbodiimides are also reported to react with residual water, which may contribute to hydrolysis when the urethane is exposed to high temperatures in an extruder [90]. 

In addition to its other properties, interest in the potential use of the vasodilative properties of prostaglandin El, alprostadil ( ), has led to several conceptually different syntheses.For this purpose, the classic Corey process has to be modified by reversing the order of addition of the side chains to allow for convenient removal of the unwanted double bond in the upper side chain. For example, Corey lactone is protected with dihydropyran (acid catalysis), reduced to the lactol with diisobutyaluminum hydride, and then subjected to the usual Wittig reaction to give intermediate This is... 

Similar additions may be performed with the enamine 13. However, with 3-buten-2-one or methyl 2-propenoate Lewis acid catalysis is needed to activate the Michael acceptor chloro-trimethylsilane proved to be best suited for this purpose. A remarkable solvent effect is seen in these reactions. A change from THF to HMPA/toluene (1 1) results in a reversal of the absolute configuration of the product 14, presumably due to a ligand effect of HMPA235. 

A high degree of syn selectivity can be obtained from the addition of enamines to nitroalkenes. In this case, the syn selectivity is largely independent of the geometry of the acceptor, as well as the donor, double bond. Next in terms of selectivity, are the addition of enolates. However, whether one obtains syn or anti selectivity is dependent on both the geometry of the acceptor and the enolate double bond, whereas anti selectivity of a modest and unreliable level is obtained by reaction of enol silyl ethers with nitroalkenes under Lewis acid catalysis. 

It is not the aldehyde or ketone itself that is halogenated, but the corresponding enol or enolate ion. The purpose of the catalyst is to provide a small amount of enol or enolate. The reaction is often done without addition of acid or base, but traces of acid or base are always present, and these are enough to catalyze formation of the enol or enolate. With acid catalysis the mechanism is... 

To add two OH groups in an anti addition, we will employ a two-step process we will first make an epoxide, and then we will open the epoxide with water under conditions of acid-catalysis ... 

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