Ethyl radical dimerization

A reinterpretation has appeared of the behavior of 5-ethyl-5-hydro-phenazinyl in solution on lowering the temperature.262 Originally, it was concluded that the radical dimerized to a diamagnetic form.263-265 Now, however, it is believed that diamagnetic crystallites are formed. Characteristic spectroscopic absorptions which appear as paramagnetism is lost are also ascribed to the solid phase.262,266 Electronic fluorescence spectra of 5-ethyl-5-hydrophenazinyl have been reported as have pK values for its ground and first excited states.267 An interpretation of the ESR spectrum of this radical has also been published.268... 

It seems that styrene and ethylbromide compete for the sites on the lithium surface. The adsorption of styrene possessing easily polarizable 7i electrons is more favorable than that of ethylbromide. Hence, Wurtz coupling is hindered, while electron-transfer to the adsorbed styrene yields radical-anions and their mobility on the surface allows for their dimerization. Eventually, the dimeric dianions are desorbed and since their reaction with ethylbromide is faster than propagation, the ethyl capped dimers are the main products. 

Scheme 9.100. A representation of the Kolbe electrolysis of the sodium salt of propanoic acid. The voltage for the electrolysis must be greater than that required for the reduction of water to hydroxide anion (OH") and hydrogen (H2) which occurs at the cathode. It is generally accepted that one electron is lost by the carboxylate anion at the anode to generate a carboxyl radical. The arrows shown on the carboxyl radical (with half-heads) are drawn to account for the apparent movement of one electron (in contrast to the usual cartoons showing two electron arrows) at a time to produce carbon dioxide (CO2) and the ethyl radical (CH3CH2 ).The latter dimerizes to butane or loses a hydrogen to a second radical, producing ethane and ethene.

Interestingly, treatment of a-nitrostilbene in water-ethyl acetate mixture by the cation-radical of A,A -dioctyl-4,4 -bipyridinium (the octylviologen cation radical) leads to the formation of products derived from the nitro group reduction. No dimerization is observed (Tomioka et al. 1986). Water is responsible for the fixation of transferred electron within the nitro group. Further reactions result in the formation of the corresponding oxime and ketone as shown in Scheme 2.10. 

The kinetic and activation parameters for the decomposition of dimethylphenylsilyl hydrotrioxide involve large negative activation entropies, a significant substituent effect on the decomposition in ethyl acetate, dependence of the decomposition rate on the solvent polarity (acetone-rfe > methyl acetate > dimethyl ether) and no measurable effect of the radical inhibitor on the rate of decomposition. These features indicate the importance of polar decomposition pathways. Some of the mechanistic possibilities involving solvated dimeric 71 and/or polymeric hydrogen-bonded forms of the hydrotrioxide are shown in Scheme 18. 

The derivation of a similarly atypical rate expression is required for the simulation of the electrochemical behavior encountered in electrohydrodimerization studies. In these studies, the variation of the bulk concentration of the olefin (e.g., ethyl cinnamate, diethyl furmarate) reveals that there is a concentration dependence to the reaction order associated with the dimerization of the electrogenerated radical ion [33]. This variation in apparent reaction order with concentration can only be attributed to a two-step mechanism [25] involving two independent rate or equilibrium processes. A mechanism that meets this criterion and appears to fit the electrochemical data is the preequilibrium mechanism [36] in which the electrogenerated radical ions first engage in an equilibrium dimerization before the rate-determining ring closure of the dimer takes place. Symbolically, this mechanism may be written ... 

Reduction of ketones other than 48 to dimers has been examined. Ethyl pyridyl ketone (51) gave a poor yield of the corresponding pinacol (52).79 Pinacols can also be made from crossed coupling of ketones with 3-(48).80 Finally, a study of the radical anions formed from the isomeric benzoylpyr-idines (53) was done, and the rates of rotation of the pyridyl ring were determined.81 The mechanism of reduction of the oxime and thiosemicar-bazone derivatives of 53 was determined by voltammetry techniques.82 The monoimine derivative of the pyridil 54 was reduced to the a-amino ketone 55 (Scheme 18).83 Unsaturated pyridyl ketones and heteroaryl pyridyl ketones have also been studied by voltammetry.84,85... 

The dimeric compound was also prepared by an independent synthesis492). Similar investigations had been carried out before with ethyl derivatives686). Kinetic measurements are available for the radical H-splitting by t-butylper-oxide486). 

The C8 aldehyde ester may be produced by cleavage of the 9-hydroperoxide of ethyl llnoleate followed by terminal hydroperoxidation. Further oxidation would produce the corresponding dicarboxylic acid which upon decarboxylation would give rise to ethyl heptanoate. The 8-alkoxy radical may also decompose to give the C7 alkyl radical, which would yield ethyl heptanoate or form a terminal hydroperoxide, and so on. Polymerization, both intra- and intermolecular, is also a major reaction in high temperature oxidation. Combination of alkyl, alkoxy, and peroxy radicals yields a variety of dimeric and polymeric compounds with C-O-C or C-O-O-C crosslinks. 

The dimerization of butadiene, aryl olefins and ethyl vinyl ether is best rationalized by postulating a radical cation 89 (Eq. (174) ) as first intermediate. As the fi -carbon of 89 has the highest free valence, the highest positive charge density and the lowest atom localization energy radical or electrophilic reactions of 89... 

Reduction of 3 -iodopropionitrile at the potential of the first polarographic (1 e) wave yields Hg (CH2 CH2 CN)2, Sn2 (CH2 CH2CN)6, Pb(CH2 CH2 CN)4 and T1(CH2CH2CN)2I at Hg, Sn, Pb, or T1 cathodes 481 When ethyl or butyl iodide is reduced at platinum or copper cathodes in DMF dimers R2 and disproportionation products, RH, RH(-H2), are formed 482,483,48S Formation of these are attributed to radicals originating both from tire cathodic cleavage of the R-I and the R2-N(CH3)2 bond 482 The ammonium salt is formed by alkylation of DMF. 

These dimerizations are analogous to those of the radical anions R2C 0 which are intermediates in the reduction of ketones to pinacols. Indeed, in the presence of magnesium amalgam, pyridine condenses with acetophenone to give alcohol 396 by oxidation of the intermediate dihydropyridine. In a similar reaction type, pyridine with zinc and acetic anhydride or ethyl chloroformate yields (397 R = Me or OEt, respectively). 

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