Halogenation effects in intramolecular furan Diels–Alder reactions: broad scope synthetic and computational studies
For the first time a comprehensive synthetic and computational study of the effect of halogen substitution on both furan and dienophile for the intramolecular Furan Diels–Alder (IMDAF) reaction has been undertaken. Contrary to our initial expectations, halogen substitution on the dienophile was found to have a significant effect, making the reactions slower and less thermodynamically favourable. However, careful choice of the site of furan halogenation could be used to overcome dienophile halogen substitution, leading to highly functionalised cycloadducts. These reactions are thought to be controlled by the interplay of three factors: positive charge stabilisation in the transition state and product, steric effects and a dipolar interaction term identified by high level calculations. Frontier orbital effects do not appear to make a major contribution in determining the viability of these reactions, which is consistent with our analysis of calculated transition state structural data.