Mechanism of the Gibbs Reaction. 3. Indophenol Formation via Radical Electrophilic Aromatic Substitution (SREAr) on Phenols

Article abstract of DOI:10.1021/jo00101a013

Different products are formed, depending on the para substituent (R) when 2,6-dichlorobenzoquinone N-chloroimine (1b) reacts with the anion of the 4-substituted phenol (2).If the group R can leave as a cation (i.e., R is an electrofugal leaving group) such as H, CH2NMe2, CH2OH, etc., then the reaction yields indophenol (3), the normal Gibbs product.If the group R cannot leave as a cation such as CH3, the final product of the reaction will be type 10, 1,1-disubstituted 2,5-cyclohexadienone.If the group R is OH or NH2, then the reaction gives the corresponding benzoquinone 4 or benzoquinone imine 1 and 2,6-dichlorobenzoquinone imine (1d).In all these cases the reaction proceeds at a 1:1 stoichiometry.If, however, the group R can leave as an anion (i.e., R is a nucleofugal leaving group) such as halogen, alkoxy, or OCH2Ph, then the reaction proceeds at a 1:2 stoichiometry.In this case the reaction of a second mole of phenolate with type 26 intermediate yields the indophenol product 3 and the oxidized product of the phenol.If the two ortho positions of the phenolate are substituted then the oxidized product of the phenol will be the corresponding benzoquinone.The mechanism of the reaction has been studied by kinetic and nonkinetic (NMR) methods.It has been concluded that the first step of the mechanism is a single electron transfer (SET) from the phenolate to the benzoquinone N-chloroimine 1b which is the rate-determining process in most of the cases.In some of the nucleofugal cases the final oxidation, involving the second mole of phenolate, is the rate-determining step.For the radical reaction three different alternatives are suggested: a combination of radicals in a solvent cage (direct reaction) and two different chain reactions (chain A and chain B).Quantum chemical calculations revealed that the direct reaction and the chain A mechanisms were energetically more favored than chain B.The reaction shows an extremely large para selectivity although the substitution does follow a radical mechanism.