The thermal dimerization of 2-boryl-1,3-butadienes and the scope of this reaction to prepare six-membered rings difficult to synthesize by other methodologies have been studied. In addition, the nature of this dimerization has been studied theoretically. It has been found that the reaction coordinate associated with the formation of the cycloadduct of lowest energy has significant [4+3] character. This behavior is caused by the favorable carbon-carbon overlap and the large values of the corresponding resonance integrals. However, beyond the transition structure, the [4+2] pathway becomes the preferred one thus leading to the exclusive formation of the [4+2] cycloadduct. Aside from this effect, donating groups at the boryl moiety favor the [4+2] mechanism.