The regiochemistry of intramolecular photochemical [2+2] cycloaddition reactions: Biradical conformation control.

A basic MM2 force-field is shown to give minimum energy conformations for radicals which are generally comparable to those obtained previously using a modified force-field. The conformations of biradicals determined by the method are used to assess whether this factor plays a role in determining the regiochemistry of photochemical [2+2] cycloaddition reactions. This concept of biradical conformation control is shown to account for behaviour which is difficult to rationalise using the 'rule of five'.