Studies Of Rhodium And Cobalt-Catalyzed C-H Bond Activation

Directing groups assisted transition-metal catalyzed C-H activation has been widely employed in the formation of carbon-carbon/carbon-hetero bonds and the synthesis of heterocyclic motif owing to its atom-economic advantages.Particularly,significant progress in this field has been accomplished in the past thirty years,with the development of the C-H functionalization from C(sp2)-H to C(sp3)-H bonds.Despite these impressive advances,noble metals(palladium,rhodium,ruthenium)remain to be the the most commonly used catalysts in the C-H activation reactions.Due to the high price and toxicity of noble metals,there is a continued strong interest in the development of cheap metals as the catalysts in the C-H activation.This dissertation mainly focused on the transition metal-catalyzed chelation-assisted C-H bond functionalization.The contents were listed as follows:1.Amide-Assisted Acetoxylation of Vinyl C(sp2)-H Bonds by Rhodium CatalysisDirecting-groups assisted transition-metal catalyzed aryl C(sp2)-H acetoxylation has been well developed.However,the vinyl C(sp2)-H acetoxylation was rarely reported.We reported rhodium-catalyzed direct acetoxylation of enamides by using amide as directing group.Cu(OAc)2·H2O is employed as both oxidant and acetate source.The reaction is highly regioselective to give the exclusive acetoxylated Z-enamides.The intermolecular kinetic isotopic effect studies indicated that the vinyl C(sp2)-H cleavage of enamides might not involved in the rate-determining step.2.Cobalt(?)-catalyzed synthesis of pyrroles from enamides and alkynesPyrroles are important five-membered heterocycles and can be found as key structural units in biologically active natural products and diverse therapeutic agents.The facile access of pyrroles independently from the annulation of alkynes with enamides by noble metal catalysis(Pd,Rh,Ru)has been well developed.Despite these distinguished advances,there is still a primary demand for alternative approaches that involve cheap and environmentally friendly catalysts.We reported an efficient cobalt-catalyzed synthesis of pyrroles from enamides and alkynes.Only CuO was compatible with the substrates and other oxidants led to the decomposition of enamides.Comparing with the palladium catalytic system,this cobalt catalytic system showed excellent regioselectivity.3.Cobalt(?)-catalyzed cross-coupling of enamides with allyl acetates/maleimidesCobalt(?)-catalyzed cross-coupling of enamides with allyl acetates/maleimides was developed.This reaction proceeds efficiently under mild conditions and displays good functional group compatibility.Besides allyl acetates,2-vinyloxirane and vinylcyclopropane can also serve as effective allyl reagents.Maleimides were also effective coupling partners to introduce the useful succinimide motif into enamides under the established conditions.4.Cobalt(?)-catalyzed annulation of esters and alkynes:a facile route to indenonesAn efficient protocol for the synthesis of indenones has been described via the annulation of benzoic esters and internal alkynes by employing the cobalt catalyst.A significant electronic effect of arenes was observed in this annulation reaction and the esters with electron-withdrawing groups were sluggish.A key intermediate was observed by the MALDI-TOF analysis experiment.The intermolecular kinetic isotopic effect studies revealed that the initial C-H bond activation may be involved in the rate-limiting step.