Wen-Zhen Zhang

CO2, COS and CS2 adducts of N-heterocyclic olefins and their application as organocatalysts for carbon dioxide fixation

Various CO2, COS and CS2 adducts of N-heterocyclic olefins (NHOs) were synthesized and characterised by single crystal X-ray crystallography. The length of the C–O bond in NHO–COS adducts is slightly shorter than that in the corresponding NHO–CO2 adducts, while the C–S bond length in NHO–COS adducts is significantly longer than the corresponding C–S distance in the NHO–CS2 adducts, suggesting that the negative charge of the NHO–COS adducts is preferentially delocalized on the sulfur atom. The length of the CCS2–CNHO bond is significantly shorter than that of the CCOS–CNHO or CCO2–CNHO bond in the corresponding NHO–COS or NHO–CO2 adducts, implying the relatively high thermal stability of NHO–CS2 adducts. The NHO–CO2, NHO–COS and NHO–CS2 adducts were found to be efficient in catalyzing the cycloaddition reaction of CO2 and epoxides to selectively afford the corresponding cyclic carbonates. Among them, NHO–CO2 adducts were found to be more active, while NHO–CS2 adducts exhibited the lowest activity for this reaction, probably due to their high stability for difficult release of the highly active NHO.

Carboxylative cyclization of substituted propenyl ketones using CO2: transition-metal-free synthesis of α-pyrones

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.

Sequential carboxylation/intramolecular cyclization reaction of o-alkynyl acetophenone with CO2

A novel sequential carboxylation/intramolecular cyclization reaction of o-alkynyl acetophenone with CO2 was developed for producing the synthetically important 1(3H)-isobenzofuranylidene acetic acids and esters in good yield and exclusive selectivity toward 5-exo oxygen cyclization under very mild reaction conditions (room temperature and CO2 balloon). This efficient reaction system showed wide functional group compatibility. Also, the computational study successfully explained the exclusive product selectivity toward the 5-exo oxygen cyclization product.

Access to α-Arylglycines by Umpolung Carboxylation of Aromatic Imines with Carbon Dioxide.

A straightforward and transition-metal-free approach for the efficient synthesis of α-arylglycine derivatives from aromatic imines and carbon dioxide was enabled by an umpolung carboxylation reaction. Various substituted diphenylmethimines underwent the carboxylation smoothly with carbon dioxide in the presence of potassium tert-butoxide and 18-crown-6 to give the corresponding carboxylated products in good to high yields. Besides the enhancement of the solubility of potassium tert-butoxide in THF, 18-crown-6 also plays key roles in suppressing the reverse protonation or 1, 3-proton shift isomerization as well as by stabilizing the carboxylated intermediate.

Double carboxylation of o-alkynyl acetophenone with carbon dioxide

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.

Sequential Ag-catalyzed carboxylative coupling/Ru-catalyzed cross-metathesis reactions for the synthesis of functionalized 2-alkynoates

Abstract Sequential reactions can provide efficient access to a variety of important organic compounds that would be otherwise difficult to obtain using conventional methods and readily available starting materials. Based on the importance of 2-alkynoates in organic synthesis, the current research aimed to develop a method for the convenient synthesis of functionalized 2-alkynoates from terminal alkynes, CO2, terminal alkene-derived bromides, and methyl acrylate using a combination of the carboxylative coupling and cross-metathesis reactions. The initial ligand-free silver-catalyzed carboxylative coupling reactions of a variety of different aryl-substituted terminal alkynes and CO2 with 5-bromopentene and 6-bromohexene provided a series of 4-pentenyl 2-alkynoates and 5-hexenyl 2-alkynoates, respectively, in good yield. These resulting 2-alkynoates were further transformed into methyl (E)-6-acetylenecarboxy-2-hexenoates and (E)-7-acetylenecarboxy-2-heptenoates in moderate to good yields by their cross-metathesis reactions with methyl acrylate in the presence of the Grubbs-Hoveyda catalyst. All of the new products characterized spectroscopically.