Cationic Ruthenium(II) Complex-Catalyzed ortho Alkenylation and Benzoxylation of Aromatics via C-H Bond Activation

Abstract

The transition metal catalyzed chelation assisted coupling of an unreactive C-H bond of aromatics with nucleophiles via C H bond activation is an efficient method to construct chemical bonds in organic synthesis. By employing this method, various chemical bonds such as C C, C N, C X (X = Halogens) and C O are efficiently constructed in a highly atom economical and environmentally friendly manner. C-H Bond of aromatics can be activated by several ways in the presence of metal catalysts. However, the control of regioselectivity is key problem in most of cases. But, the regioselective can be controlled by activating the C H bond via chelation assisted metalation pathway. Heteroatoms such as nitrogen or oxygen containing chelating groups are needed for this reaction. The C H bond activation by using strong nitrogen containing chelating groups is quite easy and well documented in the literature. But, activation in the presence of weak oxygen containing directing groups such as ketones, aldehydes, esters, sulfoxide and nitrile are very challenging. In this thesis work, we would like to discuss a less expensive ruthenium catalyzed ortho-alkenylation of weak oxygen containing directing group substituted aromatics with alkenes. The reaction of aryl ketones with olefins in the presence of a ruthenium catalyst gave ortho-alkenylated aromatic ketones in a highly regio- and stereoselective fashion. Further, weakly coordinating CHO as well as ester substituted aromatics and heteroaromatics underwent alkenylation reaction with alkenes to provide ortho-alkenylated aromatic and heteroaromatic aldehydes or esters in a highly regio- and stereoselective manner. It is interesting to note that this catalytic reaction was conducted under the air atmosphere and only catalytic amount of terminal oxidant Cu(OAc)2 has been used, the remaining amount of copper source being reoxidized by air. In addition, we would like to discuss a weakly coordinating S=O assisted hydroarylation of aromatic sulfoxides with alkynes in the presence of a ruthenium catalyst leading to trisubstituted alkenes in good to excellent yields in a highly regio- and stereoselective manner. Further, we would also like to discuss a ruthenium catalyzed ortho-benzoxylation of substituted acetanilides or N-alkyl benzamides with aromatic and aliphatic acids in the presence of (NH4)2S2O8 oxidant in a highly regioselective manner.