The solvent as hydrogen-atom donor in organic electrochemical reactions. Reduction of aromatic halides

Abstract

The first step following the initial formation o f the anion radical i n the electrochemical reduction o f aromatic halides i s the cleavage o f the C-X bond leading to the neutral Are radical. The latter species undergoes three concurrent reactions: Hatom abstraction f r o m the solvent and electron transfer at the electrode and/or f r o m the initial anion radical. The quantitative analysis o f this threefold competition allows one to predict the effect o f the intrinsic (rate constants) and operational (concentration, stirring rate, cell geometry) parameters. I t i s the basis on which the results o f deuterium incorporation by deuterated water or solvent can be used as a tool to investigate the reaction mechanism and to determine the characteristic rate constants. This is illustrated by the study o f the chloro, bromo, and iodo derivatives of three aromatic residues (9-anthryL I-naphthyl, and 4-cyanophenyl) in MezSO and acetonitrile. It i s shown that the proposed reduction mechanism i s indeed followed in each case and the degree o f competition between the three concurrent steps can be evaluated. Using the values obtained independently (by cyclic voltammetry or redox catalysis) for the cleavage rate of the anion radicals o f the chlorocompounds, the correspondi n g rates for the bromo and iodo derivatives are determined as well as the rates o f H -a t o m transfer t o the three aromatic radicals and the magnitude of the corresponding deuterium isotope effects.

Electroreductive Radical Cyclization of Ethyl 2‐Bromo‐3‐allyloxy‐ and‐3‐(propargyloxy)propanoates Catalyzed by (Tetramethylcyclam)nickel(I) Electrogenerated at Carbon Cathodes in Dimethylformamide

“…Considerable evidence exists that DMF can act as a hydrogen atom donor. [37][38][39][40] An inevitable consequence of electrolytic reductions in DMF is that small quantities of electrogenerated bases are produced. [41] Under such conditions, 3 can undergo deprotonation, due to the acidity of the proton adjacent to the carbonyl moiety, to give the corresponding resonance-stabilized carbanion 11; protonation of this carbanion results in the formation of 4 [reaction (4)].…”

Structural Effects in Radical Clocks and Mechanisms of Grignard Reagent Formation: Special Effect of a Phenyl Substituent in a Radical Clock when the Crossroads of Selectivity is at a Metal/Solution Interface

“…[33,52] Ashby's contribution concerning the role of radicals in the formation of alkylMgX clearly showed such participation. [78] Saveant's report on the reactivity patterns of aryl halides suggests that the reduction of the σ aryl radicals formed by the cleavage of radical anions is fast enough to overcome their dimerisation and hydrogen abstraction reactivity.…”

“…[44,51] In a previous report we showed that the linear/cyclised selectivity is best explained by use of the equations proposed by Saveant's group to interpret the selectivity observed in the yields of rearranged products in the vicinity of a cathode. [46,47,52] In these equations, the diffusion coefficients associated with the reactive species play a definite role. Several reports have suggested that the diffusion coefficients of isomers may slightly depend on the shapes of these isomers (here Z vs. E isomers).…”

“…[73] The electrochemical treatment of the cathodic reactivity of 9-chloroanthracene was a simplified case of intramolecular redox catalysis: here the cascade consisted of only two electron transfers. [52] Such cascades of intramolecular electron transfers driven by an irreversible last reaction provide simplified models for the electron transport chains involved in photosynthetic reaction centres. Scheme 4. For the fates of aryl radicals, we follow Bickelhaupt's group's proposition.…”

Photo-induced iodination of aryl halides under very mild conditions

“…Our recent report about the photo-induced aromatic Finkelstein reaction is mechanistically based on the photo-induced single electron transfer from NaI to aryl bromides or aryl chlorides 17 . In aryl iodides are important precursors in synthetic chemistry that form carbon-carbon and carbon-heteroatom bonds.…”