Abstract
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.
1 Introduction
2 Important Features of β-C Elimination in Small Cycloalkanes
2.1 Reversibility of Ring-Opening β-C Elimination
2.2 Syn-Coplanarity of Ring-Opening β-C
Elimination
2.3 Oxidative Addition during Ring-Opening β-C Elimination
2.4 Comparison with Other Transition-Metal-Catalyzed Ring-Opening
Reactions
3 Case Studies
3.1 β-C Elimination Involving a Carbon-Metal
Bond
3.1.1 Methylenecyclopropanes
3.1.2 Alkylidenecyclopropanes
3.1.3 Vinylcyclopropanes
3.1.4 Alkynylcyclopropanes
3.1.5 Allenylcycloalkanes
3.1.6 Phenyl-Substituted Cyclopropanes
3.1.7 Regioselective Ring Opening of Cyclobutane Derivatives to
Cyclooctenones
3.2 β-C Elimination Involving a Nitrogen-Metal
Bond
3.3 β-C Elimination Involving an Oxygen-Metal
Bond
4 Conclusion
Key words
regioselectiviy - ring opening - ring expansion - transition-metal catalysis - C-C bond activation