Intramolecular Palladium-Catalyzed Alkane C−H Arylation from Aryl Chlorides

摘要

The first examples of efficient and general palladium-catalyzed intramolecular C(sp3)−H arylation of (hetero)aryl chlorides, giving rise to a variety of valuable cyclobutarenes, indanes, indolines, dihydrobenzofurans, and indanones, are described. The use of aryl and heteroaryl chlorides significantly improves the scope of C(sp3)−H arylation by facilitating the preparation of reaction substrates. Careful optimization studies have shown that the palladium ligand and the base/solvent combination are crucial to obtaining the desired class of product in high yields. Overall, three sets of reaction conditions employing PtBu₃, PCyp₃, or PCy₃ as the palladium ligand and K₂CO₃/DMF or Cs₂CO₃/pivalic acid/mesitylene as the base/solvent combination allowed five different classes of products to be accessed using this methodology. In total, more than 40 examples of C−H arylation have been performed successfully. When several types of C(sp3)−H bond were present in the substrate, the arylation was found to occur regioselectively at primary C−H bonds vs secondary or tertiary positions. In addition, in the presence of several primary C−H bonds, selectivity trends correlate with the size of the palladacyclic intermediate, with five-membered rings being favored over six- and seven-membered rings. Regio- and diastereoselectivity issues were studied computationally in the prototypal case of indane formation. DFT(B3PW91) calculations demonstrated that C−H activation is the rate-determining step and that the creation of a C−H agostic interaction, increasing the acidity of a geminal C−H bond, is a critical factor for the regiochemistry control.