Development of iridium-catalyzed oxidative C-H arylation reactions

Abstract

In contrast to the well-studied stoichiometric cyclometalation of half-sandwich iridium(III)-complexes with chelation group containing arenes via C-H cleavage, the development of iridium(III)-catalyzed direct C-H functionalization is still in infancy and the reported catalytic procedures are largely limited to C-N bond formation. In this context, herein we describe the development of iridium(III)-catalyzed oxidative C-H arylation reactions.

First, we have developed $Cp^\ast$Ir(III)-catalyzed mild and external oxidant free C-H arylation of arenes and alkenes using aryldiazonium salts as both an aryl precursor and an oxidant via $C-N_2$ bond cleavage. A series of mechanistic studies disclosed that the reaction proceeds via the formation of an Ir(V)-aryl intermediate, which is proposed to be the turnover-limiting step. Synthetic utility of the developed arylation procedure was also demonstrated for the (Z)-selective arylation of enamides and C8-selective arylation of quinoline N-oxides.

Second, $Cp^\ast$Ir(III)-catalyzed cross-coupling of C($sp^2$)-H bonds with arylsilanes, aryl nucleophiles, has also been developed. This arylation procedure clearly demonstrates that $Cp^\ast$Ir(III)-catalyst system can mediate the facile carbon-aryl bond formation under ambient reaction conditions in the presence of an oxidant. In addition, this is the first example of the iridium(III)-catalyzed oxidative C-H arylation being compatible with an external oxidant. Mechanistic investigations disclosed that either in-situ generated copper- or silver-aryl species might be involved in the transmetalation step, leading to the formation of isolable iridium(III)-aryl complex. In addition, it was also proved that a catalytic cycle involves the formation of a high-valent iridium-aryl species. The arylation proceeded smoothly over a range of benzamides and enamides.