Medium-sized cyclophanes. Part XVI. Substitution versus transannular reaction of [2.2]metacyclophanes with benzoyl peroxide and cupric chloride. Importance of a cation radical intermediate in the transannular dehydrogenation

Abstract

Two types of reactions occur when [2.2]metacyclophane (1) was treated with benzoyl peroxide (BPO) and cupric chloride in acetonitrile; one is nuclear substitution giving the 4-benzoate and the 8-cyanomethyl derivatives, each formed by a radical attack via a cyclohexadienyl intermediate, and the other is transannular dehydrogenation to give pyrene and hydropyrenes. The latter reaction occurs with BPO alone or with other metal halides. Transannular dehydrogenation is favoured for alkyl derivatives of (1). No nuclear substitution occurs with the 5,13-dimethyl derivative (12a). Relative rates of dehydrogenation induced by cupric chloride are 1, 160, 120, and 40 for (1), (12a), the diethyl derivative (12b), and the tetramethyl derivative (19), respectively. These rates are related to ionization potentials and the stability of an intermediate. The importance of a cation radical intermediate formed by an one-electron transfer process is advanced.