Functionalization of Unactivated Alkenes through Iridium-Catalyzed Borylation of Carbon−Hydrogen Bonds. Mechanism and Synthetic Applications

Abstract

This paper describes an efficient carbon-carbon bond formation reaction, which is based on carbon-hydrogen bond functionalization of unactivated alkenes. This process is based on in situ generation of allylic and vinylic boronates by iridium-catalyzed borylation of alkenes followed by carbon-carbon bond formation reactions. The selectivity of the carbon-hydrogen bond functionalization can be efficiently controlled for cyclic alkenes. By using additives, such as methylimidazole and DBU, the iridium-catalyzed borylation led to formation of allyl boronates, which reacted with aldehydes in a one-pot sequence affording stereodefined homoallylic alcohols. Cycloalkenes without additives as well as acyclic substrates gave vinylic boronates, which were coupled with organohalides in a Suzuki-Miyaura sequence. By this process allylic and vinylic silabutadiene derivatives can be prepared from allylsilanes with excellent regio- and stereoselectivity. The mechanism of the carbon-hydrogen bond functionalization based on the borylation reaction was explored by isotope labeling experiments, measuring the kinetic isotope effect and study of the effects of the additives on the selectivity of the process. It was concluded that the reactions proceed via a dehydrogenative borylation mechanism, which shows analogous features with the palladium-catalyzed Heck coupling reaction.