Manganese-Catalyzed Deoxygenative [3+2] Annulations of Ketones and Aldehydes via C–H Activation

Conventional reactive sites of ketones with aldehydes lie on the carbonyl andα-carbon positions,which lead to a wide range of classic reactions such as pinacol-coupling and aldol-type condensations.Herein,an unprecedented reactive site of aromatic ketones toward aldehydes has been revealed by using earth-abundant manganese catalysis,which enabled the first deoxygenative[3+2]annulations of ketones and aldehydes through C–H activation affording isobenzofuran derivatives.Mechanistic studies give hints on the dual role of triphenylborane additive in the reaction,that is,promoting C–H activation as a transmetalation reagent and activating aldehydes as a Lewis acid.