Phosphine-Mediated Reductive Insertion of α-Keto Esters and Isatins into Phthalic Anhydride Derivatives

Herein, we report an unprecedented P(NMe₂)₃-mediated reductive insertion of 1,2-dicarbonyl compounds including α-keto esters and isatins into phthalic anhydride-derived alkenes and phthalic anhydrides, which furnishes the corresponding isochroman-1-ones and isochroman-1,4-diones, respectively, in moderate to excellent yields with high chemo- and regioselectivity. Furthermore, the asymmetric version of the ring expansion reaction could be realized by using a chiral auxiliary strategy. Mechanistically, the nucleophilic attack of the Kukhtin-Ramirez adduct, generated from P(NMe₂)₃ and 1,2-dicarbonyl compound, to the anhydride derivative, followed by a cascade ring-opening and ring-closure process, affords the ring expansion product. The reaction represents a novel metal-free carbon insertion ring expansion of aliphatic rings and also the first [1 + 5] annulation involving the Kukhtin-Ramirez adducts.