Transition-Metal-Catalyzed Rearrangements of Small Cycloalkanes: Regioselectivity Trends in β-Carbon Elimination Reactions

In this review, the regioselectivity of transition-metal-catalyzed ring cleavage of cyclopropanes and cyclobutanes by β-carbon elimination is analyzed using experimental results and rationalization models from the literature. Most models proposed are generally consistent with the following trends: Cleavage of the less substituted bond of the small cycloalkane is favored by a minimization of steric hindrance, whereas cleavage of the more substituted bond becomes possible if steric hindrance is limited and if the substituent is electron-withdrawing or if it can coordinate the metal. Importantly, steric congestion can easily offset electronic activation. Moreover, regioselectivity models appear to be more generally applicable to cyclopropanes than to cyclobutanes.