Efficient synthesis of 1,3,4-oxadiazole-2(3H)-one was achieved by CsF/18-crown-6 mediated 1,3-dipolar cycloaddition of nitrile imine and 2.0 MPa of CO2. CsF/18-crown-6 played a key role in enhancing the reactivity of CO2 as a 1,3-dipolarophile. The practical utility of this transition-metal-free approach to 1,3,4-oxadiazole-2(3H)-one is highlighted by the convenient synthesis of a commercial herbicide Oxadiazon and a MAO B inhibitor.

Isatoic anhydrides, a class of valuable synthetic intermediates and RNA structure probing reagents, are usually prepared with highly toxic phosgene or stoichiometric oxidants. Herein we report a highly selective palladium-catalyzed cyclization reaction for the efficient synthesis of isatoic anhydrides from readily available o-iodoanilines, CO2, and CO. The reaction proceeds under mild conditions and is redox-neutral. Both CO2 and CO are indispensable C1 building blocks for this catalytic reaction.Keywords: anhydrides; carbon dioxide; carbonylation; homogeneous catalysis; palladium;

Carbon dioxide is a green carboxylative reagent due to its non-toxic and renewable properties. Described herein is a carboxylative cyclization of substituted 1-propenyl ketones via γ-carboxylation using CO2, which provides an efficient, transition-metal-free and straightforward access to important α-pyrone compounds from easily available substrates and CO2.

The oxazolidine-2,4-dione motif is found frequently in biologically important compounds. A tandem phosphorus-mediated carboxylative condensation of primary amines and α-ketoesters/base-catalyzed cyclization reaction have been developed. These processes provide a novel and convenient access to various oxazolidine-2,4-diones in a one-pot fashion using atmospheric carbon dioxide and readily available substrates under very mild and transition-metal-free conditions.

A copper(I) catalyzed reaction of 2-alkynylaniline with CO2 using DBU as base to produce 4-hydroxyquinolin-2(1H)-one derivatives in moderate to good yield is presented. In this reaction, a unique bond cleavage pattern for CO2 was observed that one of the C–O double bonds of CO2 was totally broken and rearranged into two moieties in the absence of the reductive reagent. This efficient reaction system showed the wide generality of substrates including nitro, bromo, cyano and methoxylcarbonyl groups. A possible mechanism containing isocyanate intermediate is proposed.

DFT calculations on the Ag-catalyzed carboxylation of phenyl acetylene with CO2 indicate that the true catalytically active species is a CsCO3–-coordinated Ag complex rather than neutral PhC≡CAg conventionally considered for such a process. The energy barrier for the insertion of CO2 into the C–Ag bond of PhC≡CAg (28.8 kcal/mol) is higher than that of PhC≡CAgI– and PhC≡CAgCsCO3– anions (19.0 and 23.6 kcal/mol, respectively). Such an anion as a key intermediate is the predominant feature of the carboxylation process. The electronic effect plays a crucial role in stabilizing such transition states. In addition, the presence of an organic ligand slightly hampers generation of the active species and, therefore, reduced the yield of the final carboxylation product, which was observed experimentally.

A variety of arylboronic esters were efficiently carboxylated with CO2 using a simple AgOAc/PPh3 catalyst, affording the corresponding carboxylic acids in good yield. This simple and efficient silver(I) catalytic system showed wide functional group compatibility.

A series of oxacyclic diene compounds, especially eight-membered products bearing a single oxygen atom which have not been reported previously, were successfully synthesized via ring-closing enyne metathesis using the second-generation Grubbs catalyst. In contrast to the construction of the five-membered rings, completely opposite substrate selectivity that methyl substituted internal alkyne showed much higher reactivity than terminal alkyne was observed in building eight-membered ring derivatives.

A highly selective synthesis of a variety of functionalized allylic 2-alkynoates was realized via the carboxylative coupling of terminal alkynes, allylic chlorides, and CO2 catalyzed by the N-heterocyclic carbene copper(I) complex (IPr)CuCl. The catalyst can be easily recovered without any loss in activity and product selectivity.

A copper-catalyzed double carboxylation of o-alkynyl acetophenone using carbon dioxide to afford 1(3H)-isobenzofuranylidene dicarboxylic esters in good yields is described. The reaction proceeds via a carboxylation/intramolecular cyclization/carboxylation sequence. Alkyl-substituted substrates show much higher double carboxylation product selectivities than aryl-substituted substrates.

A copper(I) catalyzed reaction of 2-alkynylaniline with CO2 using DBU as base to produce 4-hydroxyquinolin-2(1H)-one derivatives in moderate to good yield is presented. In this reaction, a unique bond cleavage pattern for CO2 was observed that one of the C–O double bonds of CO2 was totally broken and rearranged into two moieties in the absence of the reductive reagent. This efficient reaction system showed the wide generality of substrates including nitro, bromo, cyano and methoxylcarbonyl groups. A possible mechanism containing isocyanate intermediate is proposed.

The oxazolidine-2,4-dione motif is found frequently in biologically important compounds. A tandem phosphorus-mediated carboxylative condensation of primary amines and α-ketoesters/base-catalyzed cyclization reaction have been developed. These processes provide a novel and convenient access to various oxazolidine-2,4-diones in a one-pot fashion using atmospheric carbon dioxide and readily available substrates under very mild and transition-metal-free conditions.

A variety of arylboronic esters were efficiently carboxylated with CO2 using a simple AgOAc/PPh3 catalyst, affording the corresponding carboxylic acids in good yield. This simple and efficient silver(I) catalytic system showed wide functional group compatibility.