Study On Rh(Ⅲ)-Catalyzed Acetylation Of Sp~2 C-H Bond And[4+2]annulation

1.Rh（Ⅲ）-Catalyzed Regioselective Acetylation of sp² C-H Bond Starting fromParaformaldehydeThe acetyl（-COCH₃）group is a prominent and versatile functional group in organic synthesis,which can be converted to alcohols,imines,oximes,and other functional groups.In addition,it also plays an important role in the synthesis of heterocycles.Conventionally,the acetylation of arenes strongly relies on Friedel-Crafts reactions,which suffer from the limitation of electron-rich arenes,poor regioselectivity and requirement of excessive Lewis acids.To settle these problems,transition metal-catalyzed acetylation of C（sp²）-H bonds starting from acetaldehyde,butanedione,acetate,crotonic acid,acetonitrile,and 2-butyne via C-H,C-C,C-O C=C,C≡N and C≡C bond cleavage has been extensively investigated based on the great progress of transition metal-catalyzed C-H activation over the past decade.Most of reactions require the addition of stoichiometric or excessive amounts of oxidants,causing environmental pollution and waste of resources.Therefore,the development of an environmentally friendly acetyl source has attracted the attention of chemists.Paraformaldehyde as being cheap,stable,low toxicity and simple to operate has been used as a C1 synthon in transition metal catalyzed C-H bond functionalization.To the best of our knowledge,there is no report on paraformaldehyde as the C2 source.Herein we developed Rh（Ⅲ）-catalyzed acetylation of sp² C-H bonds has been realized using paraformaldehyde as an acetylating reagent（Scheme 1）.This procedure features simultaneous formation of two C-C bonds,paraformaldehyde as a C2 source.external oxidants free,and water as the sole byproducts,thus offering an environmentally benign acetylation of arenes.Based on the preliminary mechanistic experiments and literature reported,a plausible reaction mechanism is proposed in（Scheme 2）.2.Rh-Catalyzed [4+2] Annulation of 3-Phenyl-5-isoxazolone and Maleimides via C-H ActivationRh-catalyzed [4+2] annulation of 3-phenyl-5-isoxazolone and maleimides via C-H activation has been developed（Scheme 3）.The [4+2] cycloaddition product is obtained by imine-directed ortho-C-H bond activation/decarboxylation with maleimide,through the N-O bond of 3-phenyl-5-isoxazolonecleavage and then decarboxylation.This approach features mild conditions,easy operation,high regioselectivity and CO₂ as only by-product.One C-C bond and C-N bond were formed in one pot manners,providing potential chance for further synthesis of valuable isoquinoline derivatives.