Oxyanion-accelerated C2-C6 cyclization of benzannulated enyne-allenes

The cyclization of oxyanion-substituted, benzannulated enyne-allenes was found to proceed rapidly and efficiently at room temperature, producing substituted indanones and fluorenones through a C2-C6 cyclization pathway. These reactions bear close resemblance to thermal C2-C6 cyclizations of enyne-allenes previously reported by Schmittel and others, though the oxyanion-substituted cases cyclize far more rapidly, and stand in noteworthy contrast to the C2-C7 (Myers) cyclization of (Z)-1,2,4-heptatrien-6-yne, the parent enyne-allene. The rate of reaction was found to be sensitive to the size of the alkyne and allene substituents, though the electronic effects of substitution are not known. The acceleration imparted by the oxyanion substituent is consistent with the electronic stabilization of a proposed diradicaloid transition state for the C2-C6 cyclization, but the available evidence does not permit the distinction between concerted and stepwise mechanisms. Studies are ongoing to further elucidate the mechanism and expand the scope of these transformations.