Functionalized Cycloolefin Ligand as a Solution to Ortho-Constraint in the Catellani-Type Reaction

The Catellani reaction, i.e., the Pd/norbornene (NBE) catalysis, has been evolved into a versatile approach to multisubstituted arenes via the ortho-functionalization/ipso-termi-nation process of a haloarene. Despite significant advances over the past 25 years, this reaction still suffered from an intrinsic limitation in the substitution pattern of haloarene, referred to as "ortho- constraint". When an ortho substituent is absent, the substrate often fails to undergo an effec t i v e mono ortho-functionalization process, and either ortho-difunctionalization products or NBE-embedded byproducts predominate. To tackle this challenge, structurally modified NBEs (smNBEs) have been developed, which were proved effective for the mono ortho-aminative,-acylative, and-arylative Catellan i reactions of ortho-unsubstituted haloarenes. Howe v e r , this strategy is incompetent for solving the ortho-constraint in Catel l a n i reactions with ortho-alkylation, and to date there lacks a general solution to this challenging but synthetically usef u l transformation. Recently, our group developed the Pd/olefin catalysis, in which an unstrained cycloolefin ligand served as a covalent catalytic module to enable the ortho-alkylative Catella n i reaction without NBE. In this work, we show that this chemistry could afford a new solution to ortho-constraint in the Catella n i reaction. A functionalized cycloolefin ligand bearing an amide group as the internal base was designed, which allowed for mono ortho-alkylative Catellani reaction of iodoarenes suffering from ortho-constraint before. Mechanistic study revealed that this ligand is capable of both accelerating the C-H activation and inhibiting side reactions, which accounts for its superior performance. The present work showcased the uniqueness of the Pd/olefin catalysis as well as the power of rational ligand design in metal catalysis.