Nonchain processes in nucleophilic substitutions triggered by electron transfer (SRN1). Photochemical and electrochemical induction of the substitution of 1-iodoadamantane by arenethiolate ions

Article abstract of DOI:10.1021/ja00151a012

Photochemical induction of the reaction of 1-iodoadamantane with arenethiolate ions provides a clear-cut example of nonchain S_RN1 substitutions. With 4-methoxy-, 4-methyl-, and 4-cyanobenzene and benzenethiolate ions, excellent yields (over 85%) are obtained in spite of efficient electron transfer quenching of the very easily oxidized anion radical of the substituted product by quaternary ammonium countercations. The production of the corresponding disulfides is used to determine the number of chains. Absorption and fluorescence spectroscopy and laser pulse irradiation experiments indicate that the electron which triggers the substitution process is generated by photoejection from the nucleophile, with an efficiency that decreases in the order 4-methoxybenzene > 4-methyl-benzene > 4-benzene > 4-cyanobenzenethiolate. Electron photoejection does not occur with naphthalenethiolate ions. However substitution can be entrained by addition of benzenethiolate ions. The two nucleophiles were found to have comparable reactivities toward the 1-adamantyl radical. Rather than the formation of its anion radical, the first step of the S_RN1 process is a dissociative electron transfer to 1-iodoadamantane, yielding directly the adamantyl radical. This is the reason that direct electrochemical induction of the substitution was not observed. Induction by electrogenerated aromatic anion radicals could be observed. It was much less efficient than photoinduction because the amount of electron donor species required to trigger the reaction is much larger.