Oxyanion substituents

Although a hydroxyl group strongly activates an aromatic ring toward electrophilic attack an oxyanion substituent is an even more powerful activator Electron delocaliza tion m phenoxide anion leads to increased electron density at the positions ortho and para to oxygen... 

On deprotonation, pyridones and quinolones carry an oxyanion substituent which turns out to be a good director of lithiation. Two equivalents of LDA lead to the compounds... 

The only electrons that count in the open-chain component are those in the continuous linear array of p-orbitals. Substituents, whether at the ends of the conjugated system or anywhere else along its length, do not affect the stereochemistry, although they often affect the rate or the position of equilibrium, as we saw in the effect of the oxyanion substituent in the cyclization 4.26 —> 4.21. 

The oxyanion of the enolate (or a silyloxy group) appears to speed up this reaction relative to an unadorned Claisen system, but an even more dramatic acceleration is seen when Cope systems have an oxyanion substituent on the tetrahedral carbon, as in the reaction 5.59 —> 5.61, which takes place at a... 

Only one other substituent is capable of stabilizing trityl cations more effectively than amino or dimethylamino groups namely, the oxyanion substituent O . The stabilization of the trityl cation form of the dianion of the indicator dye phenolphthalein depends on the presence of two such substituents ... 

The ability of charged substituents to accelerate the 3,3-sigmatropic reiarrangement of allyl vinyl ethers (the Claisen rearrangement) has also been documented. The effect of oxyanion substituents on the rate and course of aliphatic Claisen rearrangements has been the subject of particular attention. " In 1972, Ireland and Mueller reported that the lithium enolate derivatives of allyl esters undergo rapid and effi-... 

The ability of an oxyanion substituent to facilitate 1,5-hydrogen sigmatropy was demonstrated by Paquette et al. in 1980. For example, rearrangement of the potassium salt of 2,4-cycloheptadienol occurs more than 1(P times faster than the rearrangement of the corresponding alcohol (equation 12). Accelerated 1,5-shifts of alkyl, vinyl, aryl and cyclopropyl groups in cyclopentadiene derivatives e.g. equation 13) have also been reported. ... 

An oxyanion substituent on the diene is more powerfully electron donating than a methoxy substituent. It seems likely, therefore, that it is the model which is inadequate, not the idea of explaining the high nucleophilicity of the a position along these lines. Thus the reactivity at C-4 shown by silyl enol ethers is the expected behaviour—it is the reactivity at C-2 in the enolate anions that is hard to explain. 

With an alkyl group as the substituent, the product anions are not substantially different, nor are the coefficients on C-2 and C-3. In practice, the benzyne 4.189 gives attack equally at C-2 and C-3, except when the nucleophile is larger.416 With an oxyanion substituent, and even more so with an amide anion 4.190, attack at C-2 becomes quite substantial, perhaps avoiding the formation of a C—Li bond adjacent to the anion.417... 

---