Cycloaddition Reactions of Unactivated Olefins Catalyzed by an Organorhenium Electron-Transfer Mediator

An electrochemical process is described for the rapid and efficient conversion of unsubstituted cyclic olefins into cycloaddition products. When a potential of 1.25 V vs ferrocene was applied to a solution of cis-cyclooctene (COE) in dichloromethane/[NBu4][B(C6F5)4] containing a catalytic amount (4 mol %) of ReCp(CO)3, 1, the olefin was converted to a diastereomeric mixture of the C8-dimer tricyclohexadecane, with the major isomer having a cis-anti-cis geometry. Smaller cyclic olefins (C5 to C7) also formed cycloaddition products. The cyclization reactions require the electrogeneration of 1 + as an electron-transfer mediator which triggers the one-electron oxidation of the olefin in spite of the fact that the potentials of the latter lie at considerably higher values. The very low coulomb count required (ca. 0.1 F per olefin) for the conversion of olefin to product is consistent with a radical-chain mechanism. The electrochemical conversion of COE to cyclized products is complete in a few minutes, in contrast to the week-long photochemical processes previously needed for preparation of the same cycloaddition products.