Enol ethers electron distribution

For both types of substituent, the effects are more marked on the more distant ((3) proton. If these shifts reflect the true electron distribution, we can deduce that nucleophiles will attack the electron-deficient site in the nitroalkene, while electrophiles will be attacked by the electron-rich sites in silyl enol ethers and enamines. These are all important reagents and do indeed react as we predict, as you will see in later chapters. Look at the difference—there are nearly 3 p.p.m. between the nitro compound and the enamine ... 

The oxidation of enol acetates in acetic acid containing tetraethylammonium p-toluenesulfonate gives four types of compounds (equation 23) conjugated enones (A), a-acetoxycarbonyl compounds (B), geminal diacetoxy compounds (C) and triacetoxy compounds (D). Similar to enol ethers, the Erst reactive intermediates are cation radicals generated from enol acetates by one-electron oxidation. The yields and the distribution of products A, B, C and D depend on the structure of the starting enol acetates and the reaction conditions. ... 

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). 

The Matlin group has shown that irradiation of cyclooctadienone 36 in the presence of vinyl ethers produces 7-norbornanones 41 and oxa-triquinanes 42 (Table 81.5). The reaction is envisioned as proceeding via the step-wise cycloaddition to oxyaUyl 37. The electron-rich vinyl ether stabilizes the incipient cationic center in 43, which then undergoes internal O- and C-alkylation of the enolate moiety to give 41 and 42. The product distribution and yields did not show any significant variation with the polarity of the solvent (hexanes, benzene, CHjClj or neat vinyl ether). It is interesting to note that no enone-alkene [2-1-2]-adduct was observed, even when ethyl vinyl ether was used as solvent. This is in contrast to the behavior of pyran-4-one, where irradiation in neat furan produced [2-1-2]-adducts. The photo-excited state lifetime of 36 is limited by rapid cis,trans-isomerization to 40, which does not permit intermolecular capture to be competitive. 

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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