C_(sp~2)-H Bond Activation Triggered by Formation of Metallacycles: Rhodium(I)-Catalyzed Cyclopropanation/Cyclization of Allenynes

摘要

Transition-metal-catalyzed selective C-H activation has become one of the most powerful and straightforward strategies for the construction of complex molecules in synthetic organic chemistry. Aliphatic Q_(sp~3)-H bonds have no π electrons that can readily interact with transition metal complexes, and site-selective activation of Q_(sp~3)-H bonds therefore remains as one of the most challenging topics for synthetic chemists. Most of the C_(sp~3)-H activations that have been reported so far need the assistance of directing groups (DG), most of which contain a nitrogen or oxygen atom at the appropriate position in the substrate [Scheme 1, Eq. (1)] for site-selective activation. However, such directing groups might be incorporated in the products, and they often cannot be easily removed or be converted into other valuable functional groups. Thus, new strategies for Q_(sp~3)-H activations that do not incorporate directing groups in the product are needed.' Herein we report a Q_(sp~3)-H bond activation that is triggered by the formation of a metallacycle intermediate [Scheme 1,Eq. (2)].