Enol radicals

Allylic radical are relatively stable, and the pentadienyl radical is particularly stable. In such molecules, (E), E), (E),(Z), and (Z),(Z) stereoisomers can form. It has been calculated that (Z),(Z)-pentadienyl radical is 5.6 kcal mol less stable than the ( ),( )-pentadienyl radical. ° It is noted that vinyl radical have (E) and (Z) forms and the inversion barrier from one to the other increases as the electronegativity of substituents increase. Enolate radicals are also known. ... 

The possibility of predicting solid state reactivity from calculated thermochemical data was first addressed with ketodiesters 65a-e, which were substituted with methyl groups to vary the extent of the RSE in the radicals 65-BRl - 65-BR3 involved along the photodecarbonylation pathway (Scheme 7.19). " All ketones reacted in solution to give complex product mixtures from radical combination (66a-e) and disproportionation processes. Calculations revealed RSEs of 8.9 kcal/mol, 15.1 kcal/mol, and 19.8 kcal/mol for radicals 65-BRl (primary enol radical), 65-BR2 (secondary enol radical), and 65-BR3 (tertiary enol radical), respectively. In the... 

Fig-1 Influence of the pH value on the ratio of the electron transfer, forming enol ether 8, and the water trapping, yielding products 9+10, of the enol radical cation 7 in a DNA double strand... 

Schmittel, M. Umpolung of Ketones via Enol Radical Cations. 169, 183-230 (1994). 

Schmittel, M. Umpolung of Ketones via Enol Radical Cations. 169, 183-230 (1994). Schroder, A., Mekelburger, H.-B., and Vogtle, F. Belt-, Ball-, and Tube-shaped Molecules. 172, 179-201 (1994). 

As in the reductive ring-opening, titanocene—oxygen bonds have to be protonated. Here, a titanium enolate, which is generated after reductive trapping of an enol radical, has to be protonated, in addition to a simple titanocene alkoxide. As before, 2,4,6-collidine hydrochloride constitutes a suitable acid to achieve catalytic turnover, but here zinc dust turned out to be the reductant of choice [31c], The features of the stoichiometric reaction are preserved under our conditions. Acrylates and acrylonitriles are excellent radical acceptors in these reactions. Methyl vinyl ketone did not yield the desired addition product. Under the standard reaction conditions, a-substituted acceptors are readily tolerated, but (3-substitution gives the products only in low yields. 

The key features of the catalytic cycle are trapping of the radical generated after cycliza-tion by an a,P-unsaturated carbonyl compound, reduction of the enol radical to give an enolate, and subsequent protonation of the titanocene alkoxide and enolate. The diaster-eoselectivity observed is essentially the same as that achieved in the simple cyclization reaction. An important point is that the tandem reactions can be carried out with alkynes as radical acceptors. The trapping of the formed vinyl radical with unsaturated carbonyl compounds occurs with very high stereoselectivity, as shown in Scheme 12.21. 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

The main features are a le reduction of the hydrogen-bonded enone leading to an enol radical that tautomerizes to a more easily reducible keto radical or dimerizes. 

Example More extensive substitution at the oxirane system brings additional dissociation pathways for the molecular ions. Nevertheless, one of the main reaction paths of molecular ions of glycidols gives rise to enol radical ions by loss of a aldehyde (R = H) or ketone molecule. [218] The reaction mechanism can be rationalized by the assumption of a distonic intermediate (Scheme 6.78) ... 

The one-electron chemistry of enols has been intensively studied by Schmit-tel [108]. He has shown that the thermodynamic stability order of the ketone tautomer and the enol tautomer in the solution phase is inverted upon one-electron oxidation [109, 110]. Therefore enols are much more easily oxidized than the corresponding ketone tautomer. Supposing that the enolization is faster than the electron transfer, it ought to be possible to oxidize the enol present in small amounts beside the ketone in the equilibrium mixture. The following cyclization reactions are as useful approach to the chemistry of enol radical cations and can be considered as the a-umpolung of ketones. 

Two possible mechanisms are proposed. Primarily the enol radical cation is formed. It either undergoes deprotonation because of its intrinsic acidity, producing an a-carbonyl radical, which is oxidized in a further one-electron oxidation step to an a-carbonyl cation. Cyclization leads to an intermediate cyclo-hexadienyl cation. On the other hand, cyclization of the enol radical cation can be faster than deprotonation, producing a distonic radical cation, which, after proton loss and second one-electron oxidation, leads to the same cyclo-hexadienyl cation intermediate as in the first reaction pathway. After a 1,2-methyl shift and further deprotonation, the benzofuran is obtained. Since the oxidation potentials of the enols are about 0.3-0.5 V higher than those of the corresponding a-carbonyl radicals, the author prefers the first reaction pathway via a-carbonyl cations [112]. Under the same reaction conditions, the oxidation of 2-mesityl-2-phenylethenol 74 does not lead to benzofuran but to oxazole 75 in yields of up to 85 %. The oxazole 75 is generated by nucleophilic attack of acetonitrile on the a-carbonyl cation or the proceeding enol radical cation. 

The greater stability of simple ketones relative to their enol tautomers is reversed on formation of the corresponding radical cations (88a) (88b). In appropriate cases, ionization of the ketone to its cation is followed by spontaneous hydrogen transfer to give the enol radical cation. 1,5-Hydrogen transfer via a six-membered-ring transition state is a common route. Characterization of such mechanisms has been reviewed for a variety of such reactions in cryogenic matrices, where many of the processes that compete in solution are suppressed. ... 

Protonation of the radical-anion occurs on oxygen to give an enol-radical. The latter species is a resonance hybrid. It takes part in a fast irreversible radical-radical dimerization step and since the species has two potential radical sites, three structural isomers of the hydrodimer can be formed. The main product is formed from a transition state with minimum steric hindrence between the radical centres. 

Figure 4.11. Examples of redox-initiated radical reactions. Samarium diiodide reduction of the bromide gives a radical that cyclizes faster than the second reduction reaction. Manganese triacetate oxidation of the P-keto ester gives an enol radical that is not further oxidized by the manganese reagent. The IBX oxidizes anilides to the corresponding radicals. Hexamethylphosphoramide = HMPA and Tetrahydrofuran = THE.

Schmidtke H-H (1994) Vibrational Progressions in Electronic Spectra of Complex Compounds Indicating Stron Vibronic Coupling. 171 69-112 Schmittel M (1994) Umpolung of Ketones via Enol Radical Cations. 169 183-230 Schonherr T (1997) Angular Overtap Model Applied to Transition Metal Complexes and dN-Ions in Oxide Host Lattices. 191 87-152... 

B. Giese, W. Damm, T. Witzel, and H. G. Zeitz, The influence of substituents at prochiral centers on the stereoselectivity of enolate radicals. Tetrahedron Lett. 34 7053 (1993). 

The nature of the previously described photoredox mechanisms for other donors leads us to propose that Chaberek s complex (DE) is an exciplex which leads to electron transfer (eq. 24) to produce D and a semioxidized enolate radical E-, which is the actual initiating species (eq. 25). 

Another example of the application of the spin-center-shift concept is depicted in Scheme 8b. On irradiation of /3-ketoamides 43, hydrogen abstraction occurs from the d-position, followed by smooth elimination of methanesulfonic acid. In contrast with the applications of spin-center-shift mentioned above, the enolate radical moiety of biradicals 44 reacts solely as oxygen radicals 44b to give the oxa-zinones 45 [20],... 

Fig. 17.49. Reductions of a-heterosubstituted ketones to a-unsubstituted ketones (see Figures 15.34 and 17.59 for the preparation of compounds A and B, respectively). Here, a ketyl is formed as a radical anion intermediate (for more details about ketyls see Section 17.4.2). The ketyl obtained from A releases a chloride ion, the ketyl resulting from B releases a hydroxide ion. In each case, an enol radical is formed thereby which picks up an electron. This leads to the formation of a zinc enolate from which the final product is generated by protonation.

---