1,3-Dipolar Cycloaddition of Nitrile Imine with Carbon Dioxide: Access to 1,3,4-Oxadiazole-2(3<i>H</i>)‑ones

Efficient synthesis of 1,3,4-oxadiazole-2­(3<i>H</i>)-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(<i>3H</i>)-one is highlighted by the convenient synthesis of a commercial herbicide Oxadiazon and a MAO B inhibitor

1,3-Dipolar Cycloaddition of Nitrile Imine with Carbon Dioxide: Access to 1,3,4-Oxadiazole-2(3<i>H</i>)‑ones

Efficient synthesis of 1,3,4-oxadiazole-2­(3<i>H</i>)-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(<i>3H</i>)-one is highlighted by the convenient synthesis of a commercial herbicide Oxadiazon and a MAO B inhibitor

CO₂ Adducts of Phosphorus Ylides: Highly Active Organocatalysts for Carbon Dioxide Transformation

A series of phosphorus ylide (P-ylide) CO₂ adducts were synthesized and first used as organocatalysts for CO₂ transformation. Detailed studies on the cycloaddition reaction of CO₂ with terminal epoxides show that P-ylide CO₂ adducts are efficient metal-free and halogen-free organocatalysts to mediate this reaction under ambient conditions (25 °C, 1 atm of CO₂). More importantly, the reactions proceeded with a broad scope, high efficiency, and good functional group tolerance and the corresponding cyclic carbonate products were obtained in good to excellent yields (46–99%). Meanwhile, the kinetic study by in situ FTIR methods suggested an intermolecular cooperation effect for effectively accelerating the ring opening of terminal epoxides. Furthermore, from an investigation of the catalytic diversity of P-ylide CO₂ adducts, CO₂ also could be converted to functionalized cyclic α-alkylidene carbonates, oxazolidinone, and N-methylated and N-formylated amines by organocatalytic reactions

Fast CO₂ Sequestration, Activation, and Catalytic Transformation Using <i>N</i>‑Heterocyclic Olefins

<i>N</i>-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO₂ sequestration, affording a CO₂ adduct (NHO–CO₂). X-ray single crystal analysis revealed the bent geometry of the binding CO₂ in the NHO–CO₂ adducts with an O–C–O angle of 127.7–129.9°, dependent on the substitute groups of <i>N</i>-heterocyclic ring. The length of the C_carboxylate–C_NHO bond is in the range of 1.55–1.57 Å, significantly longer than that of the C_carboxylate–C_NHC bond (1.52–1.53 Å) of the previously reported NHC–CO₂ adducts. The FTIR study by monitoring the ν­(CO₂) region of transmittance change demonstrated that the decarboxylation of NHO–CO₂ adducts is easier than that of the corresponding NHC–CO₂ adducts. Notably, the NHO–CO₂ adducts were found to be highly active in catalyzing the carboxylative cyclization of CO₂ and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO₂ pressure), selectively giving α-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10–200 times that of the corresponding NHC–CO₂ adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO–CO₂ adduct was tentatively ascribed to its low stability for easily releasing the CO₂ moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle

Fast CO₂ Sequestration, Activation, and Catalytic Transformation Using <i>N</i>‑Heterocyclic Olefins

<i>N</i>-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO₂ sequestration, affording a CO₂ adduct (NHO–CO₂). X-ray single crystal analysis revealed the bent geometry of the binding CO₂ in the NHO–CO₂ adducts with an O–C–O angle of 127.7–129.9°, dependent on the substitute groups of <i>N</i>-heterocyclic ring. The length of the C_carboxylate–C_NHO bond is in the range of 1.55–1.57 Å, significantly longer than that of the C_carboxylate–C_NHC bond (1.52–1.53 Å) of the previously reported NHC–CO₂ adducts. The FTIR study by monitoring the ν­(CO₂) region of transmittance change demonstrated that the decarboxylation of NHO–CO₂ adducts is easier than that of the corresponding NHC–CO₂ adducts. Notably, the NHO–CO₂ adducts were found to be highly active in catalyzing the carboxylative cyclization of CO₂ and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO₂ pressure), selectively giving α-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10–200 times that of the corresponding NHC–CO₂ adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO–CO₂ adduct was tentatively ascribed to its low stability for easily releasing the CO₂ moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle

Fast CO₂ Sequestration, Activation, and Catalytic Transformation Using <i>N</i>‑Heterocyclic Olefins

<i>N</i>-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO₂ sequestration, affording a CO₂ adduct (NHO–CO₂). X-ray single crystal analysis revealed the bent geometry of the binding CO₂ in the NHO–CO₂ adducts with an O–C–O angle of 127.7–129.9°, dependent on the substitute groups of <i>N</i>-heterocyclic ring. The length of the C_carboxylate–C_NHO bond is in the range of 1.55–1.57 Å, significantly longer than that of the C_carboxylate–C_NHC bond (1.52–1.53 Å) of the previously reported NHC–CO₂ adducts. The FTIR study by monitoring the ν­(CO₂) region of transmittance change demonstrated that the decarboxylation of NHO–CO₂ adducts is easier than that of the corresponding NHC–CO₂ adducts. Notably, the NHO–CO₂ adducts were found to be highly active in catalyzing the carboxylative cyclization of CO₂ and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO₂ pressure), selectively giving α-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10–200 times that of the corresponding NHC–CO₂ adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO–CO₂ adduct was tentatively ascribed to its low stability for easily releasing the CO₂ moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle

Fast CO₂ Sequestration, Activation, and Catalytic Transformation Using <i>N</i>‑Heterocyclic Olefins

<i>N</i>-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO₂ sequestration, affording a CO₂ adduct (NHO–CO₂). X-ray single crystal analysis revealed the bent geometry of the binding CO₂ in the NHO–CO₂ adducts with an O–C–O angle of 127.7–129.9°, dependent on the substitute groups of <i>N</i>-heterocyclic ring. The length of the C_carboxylate–C_NHO bond is in the range of 1.55–1.57 Å, significantly longer than that of the C_carboxylate–C_NHC bond (1.52–1.53 Å) of the previously reported NHC–CO₂ adducts. The FTIR study by monitoring the ν­(CO₂) region of transmittance change demonstrated that the decarboxylation of NHO–CO₂ adducts is easier than that of the corresponding NHC–CO₂ adducts. Notably, the NHO–CO₂ adducts were found to be highly active in catalyzing the carboxylative cyclization of CO₂ and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO₂ pressure), selectively giving α-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10–200 times that of the corresponding NHC–CO₂ adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO–CO₂ adduct was tentatively ascribed to its low stability for easily releasing the CO₂ moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle

Fast CO₂ Sequestration, Activation, and Catalytic Transformation Using <i>N</i>‑Heterocyclic Olefins

<i>N</i>-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO₂ sequestration, affording a CO₂ adduct (NHO–CO₂). X-ray single crystal analysis revealed the bent geometry of the binding CO₂ in the NHO–CO₂ adducts with an O–C–O angle of 127.7–129.9°, dependent on the substitute groups of <i>N</i>-heterocyclic ring. The length of the C_carboxylate–C_NHO bond is in the range of 1.55–1.57 Å, significantly longer than that of the C_carboxylate–C_NHC bond (1.52–1.53 Å) of the previously reported NHC–CO₂ adducts. The FTIR study by monitoring the ν­(CO₂) region of transmittance change demonstrated that the decarboxylation of NHO–CO₂ adducts is easier than that of the corresponding NHC–CO₂ adducts. Notably, the NHO–CO₂ adducts were found to be highly active in catalyzing the carboxylative cyclization of CO₂ and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO₂ pressure), selectively giving α-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10–200 times that of the corresponding NHC–CO₂ adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO–CO₂ adduct was tentatively ascribed to its low stability for easily releasing the CO₂ moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle

Enantioselective Rhodium-Catalyzed Isomerization of 4‑Iminocrotonates: Asymmetric Synthesis of a Unique Chiral Synthon

An enantioselective isomerization of 4-iminocrotonates catalyzed by a rhodium­(I)/phosphoramidite complex is described. This reaction uses widely available amines to couple with 4-oxocrotonate to provide a convenient access to a central chiral building block in good yield and high enantioselectivity. Although the mechanism of this new transformation remains unclear, both Rh and the phosphoramidite play a central role

Fast CO₂ Sequestration, Activation, and Catalytic Transformation Using <i>N</i>‑Heterocyclic Olefins

<i>N</i>-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO₂ sequestration, affording a CO₂ adduct (NHO–CO₂). X-ray single crystal analysis revealed the bent geometry of the binding CO₂ in the NHO–CO₂ adducts with an O–C–O angle of 127.7–129.9°, dependent on the substitute groups of <i>N</i>-heterocyclic ring. The length of the C_carboxylate–C_NHO bond is in the range of 1.55–1.57 Å, significantly longer than that of the C_carboxylate–C_NHC bond (1.52–1.53 Å) of the previously reported NHC–CO₂ adducts. The FTIR study by monitoring the ν­(CO₂) region of transmittance change demonstrated that the decarboxylation of NHO–CO₂ adducts is easier than that of the corresponding NHC–CO₂ adducts. Notably, the NHO–CO₂ adducts were found to be highly active in catalyzing the carboxylative cyclization of CO₂ and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO₂ pressure), selectively giving α-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10–200 times that of the corresponding NHC–CO₂ adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO–CO₂ adduct was tentatively ascribed to its low stability for easily releasing the CO₂ moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle