Electron-transfer catalyzed cycloaddition reactions of unactivated cyclic olefins in weakly coordinating anion electrolyte

Cis-Cyclooctene, C₈H₁₄, undergoes an efficient electron-transfer (ET) catalyzed cycloaddition reaction in dichloromethane containing [NBu₄][B(C₆F₅)₄] as supporting electrolyte. The reactions were initiated by in-situ anodic generation of catalytic amounts of either [N(2,4-C₆H₃Br₂)₃]⁺ (1⁺, "magic green") or [ReCp(CO)₃]⁺ (2⁺, Cp = (η⁵-C₅H₅)). The olefin radical chain reaction induced by ET mediation was complete in less than 10 min, some 100-1000 times faster than the optimum photochemically induced preparations of cyclobutane derivatives from cyclic olefins. Yields of up to 87% of mixtures of C₁₆H₂₈ isomers were obtained at an electro-synthetic level. Use of the weakly coordinating [B(C₆F₅)₄]^- anion, rather than a traditional small anion such as [PF₆]^- or [ClO₄]^-, was required to carry out the cyclization reactions. The more easily oxidized trans-cyclooctene, when anodized directly at an electrode, gave a similar isomeric mixture of cyclobutanes. Cycloheptene and cyclohexene underwent similar [2 + 2] coupling to give cyclobutane products. Cyclopentene gave a [2 + 2 + 2] cyclized trimer. The mechanism appears to involve a key radical-substrate propagation step in which an olefin radical cation reacts with a neutral olefin to give a cyclobutyl radical cation, which then accepts an electron, most likely from another olefin, to form the final neutral compound. Although the E_1/2 potentials of the ET mediators are 660-900 mV lower than the estimated oxidation potentials of the olefins, the endergonic nature of the ET initiation step works to the advantage of the chain process by favoring the radical-substrate propagation step.