Add catalysis

When exclusively considering Lewis-add catalysis, the literature on ligand effects can be divided into studies describing quantitatively the effect of ligands on rates and equilibria of the individual steps in the catalytic cycle on one hand, and studies focused on the enantioselectivity of the reaction on the other. Interestingly, in the majority of the former investigations, aqueous media are employed. 

Literature claims of Lewis-acid catalysis of Diels-Alder reactions in water At the time of the printing of this thesis eight reports describe Lewis-acid catalysis of Diels-Alder reactions in water. This small number indicates that Lewis-add catalysis in aqueous media suffers not only from unpopularity, but also from an intrinsic disadvantage. Three of these reports originate... 

Inspired by the work of Burk and Feaster ) we attempted to use (2-pyridyl)hydrazine (4.36) as a coordinating auxiliary (Scheme 4.10). Hydrazines generally react effidently with ketones and aldehydes. Hence, if satisfactory activation of the dienophile can be achieved through coordination of a Lewis acid to the (2-pyridyl)hydrazone moiety in water. Lewis-add catalysis of a large class of ketone- and aldehyde-activated dienophiles is antidpated Subsequent conversion of the hydrazone group into an amine functionality has been reported previously by Burk and Feaster ... 

The first attempt to synthesize and characterize Kegj -type heteropoly acid supported on various mesoporous silicas and its application to add catalysis in the formation of acetic anhydride via dehydration of acetic acid were described in this study. A variety of characterization techniques such as Na adsorption, TEM and XRD were applied... 

This is known as specific add catalysis, specific in that H30 is the only acidic species that catalyses the reaction the reaction rate is found to be unaffected by the addition of other potential proton donors (acids) such as NH4 , provided that [H3Offi], i.e. pH, is not changed, indirectly, by their addition. The mechanism of the above acetal hydrolysis is believed to be,... 

Lewis add catalysis has been and continues to be of great interest in organic synthesis.111 While various kinds of Lewis add-promoted reactions have been developed and many have been applied in industry, these reactions must generally be carried out under stridly anhydrous conditions. The presence of even a small amount of water stops the reaction because most conventional Lewis adds read immediately with water, rather than with the substrates, and decompose. This destrudive reaction has restrided the use of Lewis acids in organic synthesis. From a viewpoint of today s environmental consciousness, however, it is desirable to use water instead of organic solvents as a reaction solvent.1231... 

Since our first paper181 on Lewis add catalysis in aqueous media appeared, many investigations and results in this area have been reported. Water-stable Lewis adds are now becoming common and useful catalysts in organic synthesis. These catalysts have been applied to various types of Lewis acid-catalyzed reactions. 

S. Kobayashi, T. Wakabayashi, S. Nagayama, H. Oya-mada, Lewis Add Catalysis in MiceUar Systems. Sc(OTf)3-Catalyzed Aqueous Aldol Reactions of Silyl Enol Ethers with Aldehydes in the Presence of a Surfactant Tetrahedron Lett. 1997,38, 4559-4562... 

The decomposition of the reduced complex is facilitated, most probably, by an electrophilic substitution of I+ by H+. This is a relatively fast step, as no add catalysis or H-D isotope effect could be demonstrated. The presence of Cu(I) in a favorable position close to the iodine atom facilitates the reduction of the ligand. 

Solvnlysis and other renciiims invnlv ing add catalysis by the mend ion reaction lype... 

Nucleophilic King Opening. Opening of the elhyleneimine ring with add catalysis can generally be accomplished by the formation of an intermediate aziridiniunt salt, with subsequent nucleophilic substitution on the carbon atom which loses the amino group, In the follow ing. R represents a Lewis acid, usually H + A- — the nucleophile. 

The achiral triene chain of (c//-frans-)-3-demethyl-famesic ester as well as its (6-cir-)-isomer cyclize in the presence of acids to give the decalol derivative with four chiral centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Eschenmoser, 1959 W.S. Johnson, 1968,1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric add catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such adds activate oxygen functions selectively (K.B. Sharpless, 1970). 

Bronsted acid/base catalysis is the most common enzymatic mechanism, since nearly all enzymatic reactions involve a proton transfer. This means that nearly all enzymes have acidic and/or basic groups in their active site. In add catalysis, the substrate is protonated by one of the amino add residues at the active site (typically aspartic acid, glutamic acid, histidine, cysteine, lysine, or tyrosine). This residue itself must therefore be protonated at the readion pH (typically between pH 5 and 9), with a pKa just above this value. Conversely, in base catalysis, the pJCa of the deprotonating residue must be just below the physiological pH. Some enzymes can even carry out bifunctional catalysis, by protonating and deprotonating two different sites on the same substrate molecule simultaneously. 

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