Tautomerization base-catalyzed

The base-catalyzed tautomerization of 4-methyl-4//-cyclopenta[c]thio-phenes 193 to the corresponding 6-methyl derivative 194 has been investigated quantitatively. Second-order rate constants and activation parameters have been determined (75CS42). 

The lUPAC designation is De -I- Ae. First-order kinetics are predicted and many such examples have been found. Other evidence for the SeI mechanism was obtained in a study of base-catalyzed tautomerization. In the reaction the rate of... 

Recently the synthesis of NHC complexes by a base catalyzed tautomeric rearrangement has been introduced. Af-Phenylimidazole coordinates via the unsubstituted nitrogen atom to Mn to give complex 82 (Fig. 26a) [198]. Subsequent attempts to deprotonate the C2 carbon atom in complex 82 led to the intermediate... 

The tautomerization of an enol to a ketone is catalyzed by either acid or base. In the acid-catalyzed mechanism, H+ is added in the first step (see Figure 11.6). In the base-catalyzed mechanism, H+ is removed in the first step. Show the steps in the mechanism for the base-catalyzed tautomerization. 

Lobry de Bruyn-Alberta van Ekenstein reaction) A base-catalyzed tautomerization that interconverts aldoses and ketoses with an enediol as an intermediate. This enolization also epimer-izes C2 and other carbon atoms, (p. 1115)... 

Schiflf bases are able to undergo base-catalyzed tautomerization according to the following equilibria ... 

Wolff-Kishner Reduction (Section 16.11E) Formation of a hydrazone followed by treatment with base (commonly KOH in diethylene glycol or potassium tert-butoxide in dimethyl sulfoxide) reduces the carbonyl group of an aldehyde or a ketone to a methylene group. The mechanism involves initial hydrazone formation followed by base-catalyzed tautomerization, deprotonation, loss of Nj, and reprotonation of the carbanion intermediate to give the fuUy reduced product. 

The mechanisms for acid-catalyzed and base-catalyzed tautomerization involve the same two steps (protonation at the carbonyl group and deprotonation of the a position). The difference between these mechanisms is the order of events. In acid conditions, the first step is protonation of the carbonyl group, giving a positively charged intermediate. In basic conditions, the first step is deprotonation of the a position, giving a negatively charged intermediate. 

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