Enolate ions electron distribution

The key intermediate in this process, the conjugate base of the carbonyl compound, is referred to as an enolate ion, since it is the conjugate base of an enol. The term enolate is more descriptive of the electron distribution in this intermediate in that oxygen bears a greater share of the negative charge than does the a-carbon atom. 

The reactivity of 18 may be understood in terms of the electron distribution in the donor-substituted pyrylium ion, which apparently does not correspond to a cyclically delocalized 6jt-system (18a), but rather to that of a localized trimethine cyanine (18b/c). (6) Due to the charge distribution in the pyrylium system, CH3 groups in the positions 2, 4, and 6 display marked C-H-addity, since they are deprotonated by base. In the resulting enol ethers 20 or 22, the methylene groups can take part in electrophilic reactions of the aldol or Claisen type (side-chain reactivity, compare p. 358) ... 

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

In the presence electron-rich alkenes such as 2,3-dimethylbut-2-ene, irradiation of CA gives the allylethers 59 and 60, whereas with BQ, a substantial amount of the spiro-oxetane is also formed.The product distribution of the allyl ethers is rationalized by steric effects on the H+ abstraction and on the recombination of radicals as well as spin densities. The crucial role of solvent polarity in CA photochemistry is well illustrated by the results of a study into the reaction between the quinone and cyclohexanone enol trimethylsilyl ether 61 using time-resolved (ps) spectroscopy. The influence of the solvent occurs following the formation of the radical ion pair (CA - 61+-). The CA- species is short lived in nonpolar solvents and cyclohex-2-en-l-one and 62 are the reaction products, whereas in acetonitrile, the lifetime is much longer, which allows diffuse separation of the radical ion pair and transference of the TMS to the solvent. The resulting ketyl radical couples to CA - yielding 63. 

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