Acid catalysis 1,4-addition

Internal versus External and Extraframework Sites in Zeolite Acid Catalysis the Use of Hindered Basic Probes Catalytically active sites also exist on the external surface and at the pore mouth of zeolite crystals. These sites are considered to be responsible for unwanted non-selective catalysis. On the other hand, H-zeoUtes also catalyze reactions of molecules that do not enter the cavities because of their larger size. So, the external surface of zeolites is certainly active in acid catalysis. Additionally, the bulk and surface Si/Al compositions of a zeolite could be different and different preparation procedures can be chosen to modify this ratio. 

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). ... 

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. 

A similar approach is followed in a recent study of the Lewis-acid catalysis of a Michael addition in acetonitrile. See Fukuzumi, S. Okamoto, T. Yasui, K Suenobu, T. Itoh, S. Otera, J. Chem. Lett. 1997, 667. 

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. 

First of all, given the well recognised promoting effects of Lewis-acids and of aqueous solvents on Diels-Alder reactions, we wanted to know if these two effects could be combined. If this would be possible, dramatic improvements of rate and endo-exo selectivity were envisaged Studies on the Diels-Alder reaction of a dienophile, specifically designed for this purpose are described in Chapter 2. It is demonstrated that Lewis-acid catalysis in an aqueous medium is indeed feasible and, as anticipated, can result in impressive enhancements of both rate and endo-exo selectivity. However, the influences of the Lewis-acid catalyst and the aqueous medium are not fully additive. It seems as if water diminishes the catalytic potential of Lewis acids just as coordination of a Lewis acid diminishes the beneficial effects of water. Still, overall, the rate of the catalysed reaction... 

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... 

Under conditions of acid catalysis the nucleophilic addition step follows protonation of the carbonyl oxygen Protonation increases the carbocat ion character of a carbonyl group and makes it more electrophilic... 

The point was made earlier (Section 5 9) that alcohols require acid catalysis in order to undergo dehydration to alkenes Thus it may seem strange that aldol addition products can be dehydrated in base This is another example of the way in which the enhanced acidity of protons at the a carbon atom affects the reactions of carbonyl com pounds Elimination may take place in a concerted E2 fashion or it may be stepwise and proceed through an enolate ion... 

Since the principal hazard of contamination of acrolein is base-catalyzed polymerization, a "buffer" solution to shortstop such a polymerization is often employed for emergency addition to a reacting tank. A typical composition of this solution is 78% acetic acid, 15% water, and 7% hydroquinone. The acetic acid is the primary active ingredient. Water is added to depress the freezing point and to increase the solubiUty of hydroquinone. Hydroquinone (HQ) prevents free-radical polymerization. Such polymerization is not expected to be a safety hazard, but there is no reason to exclude HQ from the formulation. Sodium acetate may be included as well to stop polymerization by very strong acids. There is, however, a temperature rise when it is added to acrolein due to catalysis of the acetic acid-acrolein addition reaction. 

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). 

Alkyl tertiary alkyl ethers can be prepared by the addition of an alcohol or phenol to a tertiary olefin under acid catalysis (Reycler reaction) sulfuric acid, phosphoric acid, hydrochloric acid, and boron trifluoride have all been used as catalysts ... 

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]. 

General acid catalysis is a catalysis by a Br(4nsted acid (other than the lyonium ion) acting by donating a proton. The addition of thiols to the carbonyl group is general acid catalyzed. ... 

Since the revised Biginelli mechanism was reported in 1997, numerous papers have appeared addressing improvements and variations of this reaction. The improvements include Lewis acid catalysis, protic acid catalysis, non-catalytic conditions, and heterogeneous catalysis. In addition, microwave irradiation (MWI) has been exploited to increase the reaction rates and yields. 

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. 

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