Molybdenum-catalyzed carbonyl–carbonyl olefination reaction for heterocycle syntheses

The carbonyl–carbonyl olefination reaction promoted by (over)stoichiometric amounts of titanium and metal reductants, which is known as the McMurry reaction, represents an attractive strategy for the formation of carbon–carbon double bonds. However, the one-step direct coupling of two carbonyl groups in a catalytic fashion remains challenging and less explored. Herein, we report a Mo-catalyzed intramolecular carbonyl–carbonyl olefination reaction for the syntheses of heterocycles from carbonyl or 1,2-dicarbonyl compounds. By utilizing the commercially available molybdenum catalyst, various substituted indoles, benzofurans, and benzothiophene were obtained in up to 98% yield. The synthetic utility of the method is further demonstrated by gram-scale syntheses and derivatization of several bioactive molecules. Preliminary mechanistic studies suggest that the Mo-carbene species is presumably involved in the reaction. Moreover, this strategy could be further applied to the syntheses of valuable 2-pyrone, coumarins and their derivatives through the direct intermolecular deoxygenative cross-coupling of readily available salicylic aldehydes or 1,3-dicarbonyl compounds with α-ketoesters.