Substituent Steric Strain: Enthalpic Estimates for the Consequences of Interactions between Bulky Substituents

The thermodynamic consequences of substituent steric interactions are assessed by calculated enthalpies for cyclobutane dimerization of ethenes with substituents of varying sizes and show that bulky substituents such as 2,4,6-tri-tertbutylphenyl (Mes*) render the ethene thermodynamically stable with respect to the corresponding cyclobutane. The conclusions have general implications and allow for a better understanding of the chemistry of compounds containing multiply bonded and/or low-coordinate environments for the main group elements.