Enolate compounds basic principles

Although the reason for acid or basic catalysis is specially clear in reactions involving the addition or removal of water, the effect is not by any means confined to such reactions. It is very prominent in prototropic reactions such as the enolization of acetone. Here the first step appears to be the acceptance by the acetone of a proton from the acid to give an addition compound which readily isomerizes by a redistribution of charge. The result is a molecule from which any proton acceptor present, including water, will readily remove H+ to leave the enol. The general principle is still the same. 

The pH dependence of the hydrolysis of all compounds studied is, in principle, consistent with the mechanism of Scheme 6 that applies to the alkoxyphenylcarbene complexes, and so are the products of the reactions of 68, 66 and 8. However, the products obtained in the hydrolysis of 144 and the fact that in basic solution the hydrolysis of all the compounds is subject to a substantial kinetic solvent isotope effect are inconsistent with Scheme 6, at least at pH >8.5. The mechanism that accounts best for all experimental observations at pH >8.5, including the isotope effect, is shown in Scheme 17 for the example of 68. It involves rapid deprotonation of 68 followed either by slow protonation of 135 with water ( 2 )) or a buffer acid (fe [BH]) and subsequent rapid conversion of 161 to 162, or slow concerted water (fe2c) or buffer acid catalyzed (fe [BH]) conversion of 135 to 162 (more on these two alternatives below). Complexation between (CO)sCr and the enol ether activates the latter toward basic hydrolysis which rapidly leads to the vinyl alcohol and tautomerization to the aldehyde. Control experiments demonstrated that the kind of complexation indicated by 162 indeed promotes rapid hydrolysis of the enol ether. ° In the reactions of 144 complexation of the enol ether ()8-methoxystyrene) appears to be weak, presumably because of steric crowding, and hence the reaction... 

For Michael additions of CH acidic compounds (e.g. diethyl malonate with a,3-unsaturated ketones) the following recommendations are given "When possible, relatively weak basic catalysts such as piperidine... should be selected. If stronger bases are required, it is normally appropriate to use only 0.1 to 0.3 equivalent of the base." The analogy of these conditions to those specified by our rule A is obvious (concentration control). On the other hand, preformed carbanions (organometallies) are usually employed when the addend is more basic than the enolate produced by attack at the unsaturated carbonyl compound. Though the nature of the metal ion plays a crucial rule in many "carbanionic" addition reactions, a first understanding of the principles involved can be... 

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