Spirocyclization enhances the Diels–Alder reactivities of geminally substituted cyclopentadienes and 4H-pyrazoles

Abstract

The Diels–Alder reactivity of 5-membered dienes is tunable through spirocyclization at the saturated center. As the size of the spirocycle decreases, the Diels–Alder reactivity increases with the cyclobutane spirocycle, spiro[3.4]octa-5,7-diene, being the most reactive. Density functional theory calculations suggest that spiro[3.4]octa-5,7-diene dimerizes 220,000-fold faster than 5,5-dimethylcyclopentadiene and undergoes a Diels–Alder reaction with ethylene 1200-fold faster than 5,5-dimethylcyclopentadiene. These findings show that spirocyclization is an effective way to enhance the Diels–Alder reactivity of geminally substituted 5-membered dienes.