Catalytic, Interrupted Carbonyl-Olefin Metathesis for the Formation of Functionalized Cyclopentadienes

Cyclopentadienes are scaffolds in organometallic chemistry, synthetic organic chemistry, and catalysis. We herein describe a regioselective Lewis-acid-catalyzed method for the synthesis of highly functionalized cyclopentadienes incorporating electronically and sterically diverse subunits. Our experimental and theoretical investigations support a mechanism that is related to catalytic carbonyl-olefin metathesis reactions wherein Lewis-acid-catalyzed cycloadditions between carbonyl and alkene functionalities afford reactive oxetane intermediates. However, in lieu of a [2+2]-cycloreversion, stepwise oxetane fragmentation to intermediate carbocations results in the formation of functionalized cyclopentadienes via interrupted carbonyl-olefin metathesis. This work provides insights into the design of catalytic carbonyl-olefin metathesis reactions of aliphatic ketone substrates as stepwise oxetane fragmentation was previously only reported as a competing reaction pathway for aryl ketones.