Abstract
ABSTRACT Transition-metal catalyzed synthetic methods are the most important tools of modern organic synthesis. Particularly, direct C–H functionalization by utilizing readily available π-components is an attractive methodology to synthesize natural products and bioactive skeletons in a single step. It is an atom- and step-economical method which is highly urged as one of the green processes. In this regard, this thesis describes three new reactions that focus on the transition-metal catalyzed C–H bond functionalization reactions with allenes. For convenience and better understanding, the thesis is divided into three chapters. These three chapters describe Rh(III) and Co(III)-catalyzed C–H bond functionalization and annulation reactions with allenes. Chapter 1 describes a Rh(III)-catalyzed [4+1] annulations of 2-hydroxy- and 2-aminobenzaldehydes with allenes. The reaction proceeds via aldehyde C–H bond activation, allene insertion and nucleophilic addition followed by β-hydride elimination to afford functionalized 3-coumaranones and 3-indolinones in excellent yields. Chapter 2 deals about Co(III)-catalyzed phenolic OH-assisted C–H bond functionalization of 2-vinylphenols with allenes to afford [5+1] annulation product. The reaction proceeds via vinylic C–H bond activation, allene insertion and unusual intramolecular regioselective phenoxide addition followed by β-hydride elimination to afford 2,2-disubstituted 2H-chromenes in excellent yields. This is the first time that allenes have been used as the coupling partners in cobalt-catalyzed C–H activation reactions. Chapter 3 explains about a Co(III)-catalyzed [3+3] annulation of N-Phenylacetamides with Allenes. The reaction proceeds via aromatic C–H bond activation, allene insertion, followed by β-hydride elimination and 1,4-addition reaction affords a functionalized 1,2-dihydroquinolines in excellent yields. Isolation of acyclic diene intermediate clearly explains the reaction pathway.