Electrophilic reactions nucleophilic catalysis

Heterolytic scission of the -S-S- bond in which only electrophilic assistance is involved is the exception rather than the rule in reactions involving bond fission of this type. Kice et a/.165,166 have demonstrated that a variety of S-S bond cleavages involve concomitant electrophilic and nucleophilic catalysis including (a) the formation of aryl thiolsulfones from aryl thiolsulfinates and aryl sulfinic acids and (b) the hydrolysis (acetic acid—1% water and 60%... 

The reactions of )3- and y-substituted halo and oxy materials apparently may involve predominant electrophilic or nucleophilic catalysis, with solvolytic-type processes also being reported 245). 

In electrophilic covalent catalysis, the enzyme withdraws electrons from the reaction center, thereby activating the substrate. The distinction between electrophilic and nucleophilic catalysis is not always easy or profitable, since electrophilic attack is often preceded by a step in which the catalyst acts as a nucleophile to bind to the reactive center on the substrate. Further, electrophilic catalysis in one direction is likely to be nucleophilic in the other. Often, the definition depends upon whether the step presumed to be decisive is nucleophilic or electrophilic. 

Of the many reagents, both heterogeneous and homogeneous, that can facilitate chemical reactions, the cycloamyloses stand out. Reactions can be catalyzed with many species such as hydronium ions, hydroxide ions, general acids, general bases, nucleophiles, and electrophiles. More effective catalysis can sometimes be achieved by combinations of catalytic species as in multiple catalysis, intramolecular catalysis, and catalysis by com-plexation. Only the latter catalysis can show the real attributes of an efficient catalytic system, namely speed and selectivity. In analogy to molecular sieves, selectivity can be attained by stereospecific complexation and speed can be likewise attained if the stereochemistry within the complex is correct. The cycloamyloses, of any simple chemical compound, come the closest to these goals. 

The hydrolysis of silica alkoxides is a very slow reaction, a sharp acceleration is observed when the pH is shifted to acidic or alkaline side. This is caused by catalysis by protons or hydroxy groups acting as strong electrophilic or nucleophilic agents, respectively [8,18,58],... 

Note how the key interaction boosting the catalytic effect is the protonation of the carbonyl group on the TG. Such catalyst-substrate interaction increases the electrophilicity of the adjacent carbonyl carbon atom, making it more susceptible to nucleophilic attack. Compare this to the base-catalyzed mechanism where the base catalyst takes on a more direct route to activate the reaction, creating first an alkoxide ion that directly acts as a strong nucleophile (Figure 4). Ultimately, it is this crucial difference, i.e., the formation of a more electrophilic species (acid catalysis) v.s. that of a stronger nucleophile (base catalysis), that is responsible for the differences in catalytic activity. 

Nucleophilic catalysis is a specific example of covalent catalysis the substrate is transiently modified by formation of a covalent bond with the catalyst to give a reactive intermediate. There are also many examples of electrophilic catalysis by covalent modification. It will be seen later that in the reactions of pyridoxal phosphate, Schiff base formation, and thiamine pyrophosphate, electrons are stabilized by delocalization. 

The lower effective concentrations found in intramolecular base catalysis are due to the loose transition states of these reactions. In nucleophilic reactions, the nucleophile and the electrophile are fairly rigidly aligned so that there is a large entropy loss. In general-base or -acid catalysis, there is considerable spatial freedom in the transition state. The position of the catalyst is not as closely defined as in nucleophilic catalysis. There is consequently a smaller loss in entropy in general-base catalysis, so that the intramolecular reactions are not favored as much as their nucleophilic counterparts. 

A wide range of donor ketones, including acetone, butanone, 2-pentanone, cyclopentanone, cyclohexanone, hydroxyacetone, and fluoroacetone with an equally wide range of acceptor aromatic and aliphatic aldehydes were shown to serve as substrates for the antibody-catalyzed aldol addition reactions (Chart 2, Table 8B2.6). It is interesting to note that the aldol addition reactions of functionalized ketones such as hydroxyacetone occurs regioselectively at the site of functionaliztion to give a-substitutcd-fi-hydroxy ketones. The nature of the electrophilic and nucleophilic substrates utilized in this process as well as the reaction conditions complement those that are used in transition-metal and enzymatic catalysis. 

The nomenclature used in describing bimolecular electrophilic substitutions involving cyclic transition states reflects, in part, the above-mentioned difficulty. Ingold3 has adopted the nomenclature of Winstein et al.1 and refers to such substitutions as SEi, but to the present author this is not a particularly appropriate choice since it does not indicate the bimolecular nature of the substitution. Dessy et al.8 have used the term SF2 to describe a mechanism, such as that in reaction (5), in which a four-centred transition state is formed, but not only is such a term too restricted, it also provides no indication that the mechanism is one of electrophilic substitution. The view of Reutov4 is that the cyclic, synchronous mechanism is very close to the open mechanism and that both can be described as SE2 mechanisms. Dessy and Paulik9 used the term nucleophilic assisted mechanisms to describe these cyclic, synchronous mechanisms and Reutov4,10 has recently referred to them in terms of internal nucleophilic catalysis , internal nucleophilic assistance , and nucleophilic promotion . Abraham, et al,6 have attempted to reconcile these various descriptions and have denoted such mechanisms as SE2(cyclic). 

The reaction order with respect to the solvent is, however, not determinable and thus there can be no direct kinetic evidence as to the number of solvent molecules in the transition state. In view of this, it is not reasonable to attempt to include nucleophilic catalysis by the solvent in any formal nomenclature of the mechanism, although it is clear that the role of the solvent is of great importance in studies of electrophilic substitutions. 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

The nucleophile initially adds at the multiple bond, forming carbanion A. Further transformations of A occur in line with electronic and steric effects, depending on the reaction conditions and on the use of nucleophilic catalysis. Several routes are possible, leading to different reaction products. Note that the use of nucleophilic catalysis is a general technique in the chemistry of compounds with electrophilic multiple bonds in particular, it is widely employed for dimerization and trimerization of activated olefins, keteneimines, etc. 

Vinyl silanes resemble alkenes in reactivity they combine with reactive electrophiles such as bromine without catalysis but need Lewis acid catalysis for reaction with carbon electrophiles. Reaction usually occurs 189 at the silyl end of the alkene so that the intermediate 190 enjoys the P-silyl stabilisation of the carbocation. The silyl group is removed by a nucleophile, usually a halide ion.45... 

A different set of mechanistic possibilities comes into play when nucleophilic catalysis is used in place of the electrophilic catalysis induced by Lewis acids. The most common form of this reaction is fluoride ion catalysis of allylsilane reactions, where the carbon nucleophile is probably a hypervalent allylsilane (12), with the fluoride ion coordinated to the silicon, making the allylsilane unit nucleophilic enough to react with aldehydes without acid catalysis. ... 

Many chemical reactions involve a catalyst. A very general definition of a catalyst is a substance that makes a reaction path available with a lower energy of activation. Strictly speaking, a catalyst is not consumed by the reaction, but organic chemists frequently speak of acid-catalyzed or base-catalyzed mechanisms that do lead to overall consumption of the acid or base. Better phrases under these circumstances would be acid promoted or base promoted. Catalysts can also be described as electrophilic or nucleophilic, depending on the catalyst s electronic nature. Catalysis by Lewis acids and Lewis bases can be classified as electrophilic and nucleophilic, respectively. In free-radical reactions, the initiator often plays a key role. An initiator is a substance that can easily generate radical intermediates. Radical reactions often occur by chain mechanisms, and the role of the initiator is to provide the free radicals that start the chain reaction. In this section we discuss some fundamental examples of catalysis with emphasis on proton transfer (Brpnsted acid/base) and Lewis acid catalysis. 

The advantage of covalent catalysis where an electrophilic or nucleophilic group on the peptide chain of the enzyme forms a covalent bond with the substrate, is immediately apparent by considering the difference in entropy changes between the equivalent intermolecular (Eqn. 12) and enzyme-catalysed mechanisms (Eqn. 13). In the latter, one of the reactants, B, is covalently bonded to the enzyme and a comparison of this reaction of the intermolecular reaction illustrates the advantage of binding the substrate to the enzyme even if the chemical reactivity of B in the enzyme may be similar to that of B in intermolecular reaction. 

Microwave reactions have been successfully demonstrated for many different organic reactions including metal-mediated catalysis, cyclo-additions, heterocyclic chemistry, rearrangements, electrophilic and nucleophilic substitutions, and reduction. Many reactions work well in water, adding to the techniques green credentials [27]. 

The air-stable and readily available l,3,5,-triaza-7-phosphaadamantane (PTA) was found to be a convenient and efficient nucleophilic trialkylphosphine organocatalyst for the (aza)-MBH reaction. Under the mediation of 15-30mol.% of PTA, various electrophiles such as aldehydes" and imines " " readily undergo the (aza)-MBH reactions with various activated olefins, giving the corresponding adducts in fair to excellent yields. By systematie eomparison with other structurally similar N,P-catalysts, it is concluded that the superiority of PTA in the above nucleophilic catalysis is attributable to its comparable... 

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