New strategies for ruthenium catalyzed C-C bond formation

Transition metal-catalyzed C-H bond activation allows direct functionalization of the ubiquitous C-H bonds in organic molecules to increase the molecular complexity. Since Murai's pioneering work in ruthenium catalyzed regioselective arene-alkene coupling reaction, a number of transition metal catalysts have been developed for C-C bond formation via C-H bond activation. However, metal-catalyzed C-H functionalization faces a number of long-standing challenges such as the control over regio- and stereoselectivity and harsh reaction conditions. Presented herein is our research on the development of ruthenium(II)-based catalysts for new and improved methods in C-C bond formations by formal activation of sp2 C-H bonds and subsequent coupling with alkyne substrates.;Chapter 1 introduces the background of alkyne hydroarylation initiated by transition metal-catalyzed C-H bond activation and the significance to develop new strategies to overcome the limitations of current methods.;In Chapter 2 and Chapter 3, ruthenium(II)-N-heterocyclic carbene (NHC) catalyst systems were developed for efficient [3+2] carbocyclization between N-H aromatic ketimines or aromatic ketones and internal alkynes under very mild conditions. This process incorporates the ortho-directing imine and ketone groups for C-H bond activation into the overall transformation in a tandem manner and enables efficient access to indenyl amines and alcohols in high yields.;Chapter 4 describes the development of bis-cyclometalated ruthenium(II) complexes with readily available N-H aromatic ketimine and ketone ligands as a new class of catalyst precursors for C-C coupling reactions. The catalytic activity of the bis(imine) complex is evaluated in several catalytic coupling reactions of alkene and alkyne substrates. The coupling reactions are proposed to proceed by Ru(II)/Ru(IV) catalytic cycles involving C-C bond formation by oxidative cyclization.;Chapter 5 details the development of a decarboxylative alkyne hydroarylation process to synthesize arylalkenes with controlled and versatile regiochemistry of aromatic substituents. Following a tandem sequence of C-H bond activation and alkyne coupling, the subsequent decarboxylation is facilitated by the newly installed ortho-alkenyl moiety and is compatible with various aromatic substituents at para-, meta- and ortho-positions. This new decarboxylation strategy eliminates the prerequisite of substrate activation by ortho-substitution and allows a broad scope of substituted benzoic acids to serve as aromatic building blocks for alkyne hydroarylation.