Intramolecular Anodic Olefin Coupling Reactions: A Useful Method for Carbon-Carbon Bond Formation

The utility of intramolecular anodic olefin coupling reactions for effecting carbon-carbon bond formation has been examined. All of the successful cyclizations studied utilized either an alkyl or silyl enol ether as one of the participating olefins. The enol ethers could be coupled to simple alkyl olefins, styrenes, and allylsilanes in isolated yields ranging from 57 to 84%. The reactions were found to be effective for generating both five- and six-membered rings. The best conditions for cyclization utilized a reticulated vitreous carbon anode, constant-current conditions in an undivided cell, and a lithium perchlorate in either 50% methanol/tetrahydrofuran or 20% methanol/dichloromethane electrolyte solution. The use of an allylsilane as one of the participating olefins allowed for the regiospecific formation of olefinic products. In addition to the olefinic products, these reactions produced a small amount of a cyclized ether product in which the silyl group had not been eliminated. Deuterium-labeling studies showed that at least half of this ether byproduct arose from intramolecular migration of the methoxy group that was initially part of the starting enol ether to the carbon β to the silyl group. Intramolecular migration reactions of this type were found to participate in a number of the reported cyclization reactions.