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.