Stabilization, anions, catalysis

If you want to do a conjugate addition of a carbonyl compound without having a second anion-stabilizing group, you need some stable and relatively unreactive enol equivalent. In Chapters 27 and 28 you saw how enamines are useful in alkylation reactions. These neutral species are also perfect for conjugate addition as they are soft nucleophiles but are more reactive than ends and can be prepared quantitatively in advance. The reactivity of enamines is such that heating the reactants together, sometimes neat, is all that is required. Protic or Lewis acid catalysis can also be used to catalyse the reaction at lower temperature. 

Vinylsilanes react with chloral in the presence of Lewis acids (Scheme 33), but this type of reaction is little used, probably because the products are allylic alcohols, which are apt to undergo ionization in the presence of Lewis acids to give allyl cations, and hence further reaction. Reactions employing nucleophilic catalysis, although free of this problem, are also limited, only anion-stabilized systems undergoing reaction (Scheme 34). On the other hand, there is less of a problem with 3-elimination of a halide ion, as there would be with most metals 3 to a halogen. ... 

Few studies of amide hydrolysis have been carried out under conditions of micellar catalysis. Gani and Viout find that there is weak acceletation of the hydrolysis of activated amides (cturying electron-withdrawing alkyl groups or anion-stabilizing N-substitutents) by cetyltrimethylammonium bromide in the pH range 10—14 but the pH dependence can be quite different from the... 

For the abovementioned reasons this review will concentrate on the silylium ions and related silicon cations with silicon in the oxidation state (+1V). In particular in view of silylium-like reactivity of species such as the solvent- or anion-stabilized silylium ions of the types 11-13,17 and intramolecular stabilized species, as, for example, 21, 22 will be also included in this overview. Emphasis is given here to structural and Si NMR spectroscopic properties of the silylium ions, synthetic aspects and the use of silylium ions in organic synthesis and catalysis will be also addressed. 

The reaction can be performed with base catalysis as well as acid catalysis. The former is more common here the enolizable carbonyl compound 1 is depro-tonated at the a-carbon by base (e.g. alkali hydroxide) to give the enolate anion 5, which is stabilized by resonance ... 

The author anticipates that the further development of transition metal catalysis in ionic liquids will, to a significant extent, be driven by the availability of new ionic liquids with different anion systems. In particular, cheap, halogen-free systems combining weak coordination to electrophilic metal centers and low viscosity with high stability to hydrolysis are highly desirable. 

The two main conditions (besides the stability of the a radical towards the solvent to observe such an electron catalysis are a sufficient high rate of addition of the nucleophile and the thermodynamic inequality E°ll >E°12 implying a fast displacement of the latter equilibrium to the direction of the formation of the anion radical of 71. 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

The intermediate N-acylpyridinium salt is highly stabilized by the electron donating ability of the dimethylamino group. The increased stability of the N-acylpyridinium ion has been postulated to lead to increased separation of the ion pair resulting in an easier attack by the nucleophile with general base catalysis provided by the loosely bound carboxylate anion. Dialkylamino-pyridines have been shown to be excellent catalysts for acylation (of amines, alcohols, phenols, enolates), tritylation, silylation, lactonization, phosphonylation, and carbomylation and as transfer agents of cyano, arylsulfonyl, and arylsulfinyl groups (lj-3 ). 

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