Base catalysis rearrangement

This type of ring interconversion is represented by the general expression shown in Scheme 15. Analogous rearrangements occur in benzo-fused systems. The known conversions are limited to D = O in the azole system, i.e. cleavage of the weak N—O bond occurs readily. Under the reaction conditions, Z needs to be a good nucleophile in its own right or by experimental enhancement (base catalysis, solvent, etc.) and Z is usually O, S, N or C. 

The ot-ketol rearrangement can also be brought about by base catalysis, but only if the alcohol is tertiary, since if or = hydrogen, enolization of the substrate is more favored than rearrangement. 

A thermally stable sulfonium ylide is also obtained from the CuS04-catalyzed reaction between dimethyl diazomalonate and thioxanthene or its 9-alkyl derivatives 339 > rearrangement to the thioxanthen-9-ylmalonate occurs only with base catalysis. 

In summary, zirconocene and half-sandwich zirconium-based catalysis has been developed both in surface and solution systems. In general, the activation of Zr-POSS and silica-supported zirconium system with MAO proved inappropriate (see silsesquioxane displacement and leaching, respectively, vide supra), while the acti-vahon with BArl proved more efficient, albeit structural rearrangement complicates the activation chemistry and simple Zr(IV) cationic alkyl species are seldom the outcome of the activation. 

The-Claisen rearrangement to the ortho position is a first-order reaction,67 68 and the process does not require catalysis by acids and bases. The rearrangement is intramolecular, since rearrangement of mixtures of ethers such as allyl /3-naphthyl ether and cinnamyl phenyl ether,60 or cinnamyl 4-methylphenyl ether and allyl 4-aminophenyl ether,68 yields none of the cross products which would result from an intermolecular reaction. The process is best represented by the cyclic mechanism, in which the following processes take place, with the electronic shifts during reaction indicated by the arrows.23 39 69... 

Barium thiocyanate, 118 Base catalysis, 34 in acylations, 47, 48 of chelate formation, 95 of clevage, 128 of cydization, 127 in hydrolysis, 115 Basicity, of 2-aminothiazole, 90 of 2-imino-4-thiazolines, 124 Beckmann rearrangement, of oxime, 16 Benzaldehyde, with alkyl-acetamidothi-azole, 46... 

The reduction of methyl 2-siloxycyclopropanecarboxylates can also be started at the ester function when lithium aluminum hydride in ether is the reagent. The resulting alcohols undergo the wellknown cyclopropylcarbinyl/homoallyl rearrangement upon treatment with acid to provide P/y-unsaturated carbonyl compounds 117. These are synthesized isomerically pure and in good yields in a number of cases, if the two-phase-system 2N hydrochloric acid/pentane is employed 78). Otherwise the very easy isomerization to the conjugated a,p-unsaturated compounds 118 occurs to some extend, which can intentionally be completed by base catalysis. 

In acid-base catalysis,proton addition to or abstraction from reactant molecules (with Bronsted acids or bases), or formation of coordination bonds (with Lewis acids), and subsequent bond breaking and rearrangement are the key reaction processes. Most cases involve ionic reaction intermediates bound to the surface by electrostatic interactions. 

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