Enolates charge effects

Recent attention has focussed on the solvation of neutral enolates formed through the deprotonation of a-ammonio ° , a-pyridinio and a-imino carbonyl com-pounds . A trimethylammonio (MesN -) group has been shown to stabilize in water-adjacent enolates more dominantly than an unsubstituted ammonio group (HsN -). For example, a-trimethylammonio ester 35 ° is about 1000 times more acidic than the simple a-ammonio ester 36 ° . This is presumably due to the highly solvated ammonio (H3N+-) group having a lower charge effect in water than the less solvated and more... 

The cw effect , observed also in trisubstituted enol ethers , may as well be rationalized by a similar mechanism (Scheme 12). In this case, the electron-donating group that stabilizes the partially charged double bond carbon of the perepoxide in the syn transition state is the alkoxy moiety. Therefore, the favourable interactions of the singlet oxygen with the phenyl or alkoxy substituents direct the orientation of the perepoxide intermediate. 

The possibility of the enolate anion acting as if its charge were effectively concentrated on carbon or on oxygen was discussed previously in connection with aldol addition (Section 17-3B). However, the situation there was quite different from the one here, because aldol addition is easily reversible, whereas alkylation is not. Furthermore, while the aldol reaction involving C-O bond formation is unfavorable (AH° = + 20 kcal mole-1) compared to C-C bond formation (AHn = —4 kcal mole-1), both O- and C-alkylation of the anion have AH° < 0 (see Exercise 17-64). 

---