Using carbon dioxide (CO2) as a feedstock for commodity synthesis is an attractive means of reducing greenhouse gas emissions and a possible stepping-stone towards renewable synthetic fuels. A major impediment to synthesizing compounds from CO2 is the difficulty of forming carbon-carbon (C-C) bonds efficiently: although CO2 reacts readily with carbon-centred nucleophiles, generating these intermediates requires high-energy reagents (such as highly reducing metals or strong organic bases), carbon-heteroatom bonds or relatively acidic carbon-hydrogen (C-H) bonds. These requirements negate the environmental benefit of using CO2 as a substrate and limit the chemistry to low-volume targets. Here we show that intermediate-temperature (200 to 350 degrees Celsius) molten salts containing caesium or potassium cations enable carbonate ions (CO3(2-)) to deprotonate very weakly acidic C-H bonds (pKa > 40), generating carbon-centred nucleophiles that react with CO2 to form carboxylates. To illustrate a potential application, we use C-H carboxylation followed by protonation to convert 2-furoic acid into furan-2,5-dicarboxylic acid (FDCA)--a highly desirable bio-based feedstock with numerous applications, including the synthesis of polyethylene furandicarboxylate (PEF), which is a potential large-scale substitute for petroleum-derived polyethylene terephthalate (PET). Since 2-furoic acid can readily be made from lignocellulose, CO3(2-)-promoted C-H carboxylation thus reveals a way to transform inedible biomass and CO2 into a valuable feedstock chemical. Our results provide a new strategy for using CO2 in the synthesis of multi-carbon compounds.

The nucleophilic addition of organometallic reagents to polar electrophiles, such as aldehydes, imines, and Michael acceptors, is a fundamental CÀC bond-forming reaction in organic synthesis. The generation of nucleophilic organometallic reagents, however, generally requires stoichiometric amounts of strong bases and/or reducing metals, such as Mg and Li, and stoichiometric salt waste is therefore inevitably produced. Thus, the development of atom-economical processes [1] involving the catalytic generation of nucleophilic organometallic species and their addition to polar electrophiles without additional activating reagents is highly desirable. Transition-metal-catalyzed C À H bond functionalization could be an attractive method for addressing these issues, [2] but addition reactions of CÀH bonds to polar CÀX multiple bonds (X = N, O, or C) have been investigated much less [3] than related reactions with nonpolar alkenes and alkynes. [2] Quite recently, it was disclosed that Cp*Rh III complexes (Cp* = pentamethylcyclopentadienyl) [4] catalyze addition reactions of arene C À H bonds to imines, [5] aldehydes, [6] Michael acceptors, [7] and other polar electrophiles. [8] Although Cp*Rh III -catalyzed processes are useful and versatile, the need for expensive and precious rhodium sources is economically and environmentally disadvantageous. Therefore, studies are needed for the development of an inexpensive base metal catalyst as an alternative to the cationic Cp*Rh III complexes. [9] Herein, we describe the utility of a cationic high-valent cobalt complex and the structure-activity rela-tionship of various Cp*Co III complexes (Scheme 1) for the catalytic generation of nucleophilic organometallic species. We found that the [Cp*Co III (arene)](PF 6 ) 2 complex 1 a (5-10 mol %) promoted the addition of 2-aryl pyridines to imines, enones, and a,b-unsaturated N-acyl pyrroles as ester and amide surrogates.

A unique synthetic utility of a Cp*Co(III) catalyst in comparison with related Cp*Rh(III) catalysts is described. A C2-selective indole alkenylation/annulation sequence proceeded smoothly with catalytic amount of a [Cp*Co(III)(C6H6)](PF6)2 complex and KOAc. Intramolecular addition of an alkenyl-Cp*Co species to a carbamoyl moiety gave pyrroloindolones in 58-89% yield in one pot. Clear difference was observed between the catalytic activity of the Cp*Co(III) complex and those of Cp*Rh(III) complexes, highlighting the unique nucleophilic activity of the organocobalt species. The Cp*Co(III) catalysis was also suitable for simple alkenylation process of N-carbamoyl indoles, and broad range of alkynes, including terminal alkynes, were applicable to give C2-alkenylated indoles in 50-99% yield. Mechanistic studies on C-H activation step under Cp*Co(III) catalysis with the aid of an acetate unit as well as evaluation of the difference between organo-Co(III) species and organo-Rh(III) species are also described.

The readily available carbonyl(pentamethylcyclopentadienyl)cobalt diiodide complex [Cp*Co(CO)I 2 ] was successfully utilized as the precursor of a cationic cobaltA C H T U N G T R E N N U N G (III) active catalyst for directed C À H bond functionalization. The complex Cp*Co(CO)I 2 (2.5-1.25 mol%), in combination with silver hexafluoroantimonate (AgSbF 6) and potassium acetate (KOAc), efficiently catalyzed the directed C-2 selective amidation of indoles with sulfonyl azides, and the corresponding products were obtained in 85-98 % yield.