Reactions with ketyls

The inhibitory function of the sulfur compounds then would appear to be to prevent the subsequent reaction of the initially formed ketyl radicals by catalyzing hydrogen transfer reactions. With this in mind, one can write the following mechanism for the inhibition of the photoreduction of benzo-phenone by sulfur compounds ... 

The transformation of2-734 involves an initial generation of an organosamarium species 2-735 with subsequent nucleophilic addition to the lactone carbonyl. Presumably, a tetrahedral intermediate 2-736 is formed that collapses to yield the ketone 2-737. This reacts with Sml2 to give a ketyl radical 2-738, which undergoes an intramolecular S-exo radical cyclization reaction with the alkene moiety. The resultant... 

The following mechanism can be assumed. Initially, the ketyl intermediate 3-159 is formed, which leads to 3-160 by a radical epoxide opening. Reaction with the second molecule of Sml2 gives samarium-chelated hydroxy-ketyl 3-161, which cyclizes to afford the products 3-157 and 3-158. 

Reaction of Peroxides with Ketyl Radicals References... 

The experimental data on the reactions of ketyl radicals with hydrogen and benzoyl peroxides were analyzed within the framework of IPM [68]. The elementary step was treated as a reaction with the dissociation of the O—H bond of the ketyl radical and formation of the same bond in acid (from acyl peroxide), alcohol (from alkyl peroxide), and water (from hydrogen peroxide). The hydroperoxyl radical also possesses the reducing activity and reacts with hydrogen peroxide by the reaction... 

Rate Constants of Reducing Reactions of Ketyl Radicals with Peroxides... 

The comparison of the values of Ee0 for reactions of ketyl radicals with peroxides with those for reactions R1R2C 0H + R2C(0) show that the reaction of the ketyl radical with peroxide occurs with sufficiently higher Ee0 (see Table 6.30). Obviously, this is the result of strong additional repulsion in the nonlinear polyatomic transition state of the ketyl radical reaction with peroxide... 

Ketyl radicals are more active reducing agents than alkyl radicals. It is seen from the comparison of the rate constants and the activation energies for reactions of ketyl and alkyl radicals with hydrogen peroxide [68],... 

Rate Constants of Ketyl Radical Reactions with Dioxygen... 

The blue color is attributed to the solvated electron. The blue solutions react with ketones to form highly colored substances resembling the metal ketyls formed by reaction with the alkali metals. The blue solutions decompose into trialkylamine and hydrocarbon on standing at the boiling point of liquid ammonia. 

In cyclization reactions of ketyls with hydrazone, Skrydstrup and coworkers used different ligands to control the face selectivity in these coupling reactions [154]. Only low enantioselectivities (< 15%) and moderate yields (< 65%) were obtained for the trans-cyclized products (data not shown). 

The reaction of the Scheme 8.23 results in the formation of a substitnted phenolate ion and a radical. The radical is further reduced by the next AQ (Dimmel et al. 1985). The keto groups in lignin can be involved in the analogous reaction with AQ. This leads to formation of the ketyl fnnctions which originate analogous fragmentation processes. 

Reactions with Radicai ions of Like Charge. In the final section, we briefly mention reactions between radical ions of like charge. One of the longstanding problems of radical ion chemistry involves the actual stmcture of ketyls. Following extensive conductivity and magnetic susceptibility studies in the 1930s detailed ESR and optical studies have demonstrated the existence and interconversion of at least four distinct species a free ketyl anion a monomer ion pair a... 

Photoreduction was quenched by high concentrations of biacetyl, slightly retarded by iodonaphthalene, but not affected by azulene or anthracene.113 These observations led to the unsatisfying conclusion that reduction proceeded via a triplet state which could be only selectively quenched. However, later work114 using flash photolysis showed that the benzophenone ketyl radical was generated upon irradiation of solutions of benzophenone and acridine, and that its predominant mode of disappearance was by reaction with... 

Rate Constants of Ketyl Radical Reactions with Dioxygen Radical Solvent T (K) (L mol-1 s-1) Ref. 

The model had been substantiated by measuring the potential dependent electrosorption isotherms of all species involved that show that the proto-nated alcaloids are those species that are by far most strongly adsorbed, whereas the acetyl pyridines are least strongly adsorbed, especially at lower pH relative to the electrosynthesis, which is performed at pH 4 to 4.5. The optically inductive reduction of 2- and 4-acetyl pyridine to the optically active carbinols demands the formation of a dense but not too densely packed surface layer of adsorbed protonated alcaloid, which still allows for insertion of the oxo-compound or the ketyl radical, respectively. Performing the reaction with too high alcaloid concentrations leads to compaction on the adsorbate layer, the ketyl radical is squeezed out, and optimal induction is no longer observed. 

A similar cyclization of a ketyl anion (produced via reaction with SmE) onto an allene was reported by Gillmann in 1993 (equation 29)85 The regiochemistry and high diasterose-lectivity associated with this process was explained on the basis of a chelation-controlled cyclization (e.g. TS 48). The same reduction carried out with n-Bu3SnH resulted in only a 37% yield and a 5 1 ratio of diastereomers. 

The electrocarboxylation of aldehydes and ketones leads to the corresponding a-hydroxycarboxylic acids that can easily be converted into carboxylic acids via a hydrogenation reaction [7]. It has been reported that the electrocarboxylation of aromatic ketones occurs through the reaction of C02 onto the activated carbon atom of the carbonyl group of the ketyl radical anion generated upon electron transfer to the ketone [7]. Otherwise, the aforementioned intermediate is likely to be a resonance hybrid (see Equation 12.23), and its electrophilic reaction with C02 may take place both at the carbon or the oxygen atom [42, 43]. 

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