PREPARATION OF 2,3-DIHYDROBENZO[b]THIOPHENE BEARING BENZYLIC QUATERNARY CARBON BY PALLADIUM-CATALYZED CASCADE REACTION

A palladium-catalyzed cascade reaction for preparing 2,3-dihydrobenzo[b]thiophenes and the corresponding 1,1-dioxides bearing the C3 benzylic quaternary carbon is described. This cascade reaction involves the oxidative addition of aryl iodide to Pd(0) to form a σ-aryl palladium intermediate, insertion of the internal alkene into the σ-aryl palladium intermediate to form a σ-alkyl palladium intermediate with concomitant formation of a heterocyclic ring bearing a benzylic all-carbon quaternary stereogenic center, which undergoes transmetalation with a TIPS ether of thiol, and finally, reductive elimination to afford the product. The use of N,O-bis(trimethylsilyl)acetamide (BSA) under previously optimized conditions is key to achieving a high yield.