Recent Advances in the Generation of Nitrilium Betaine 1,3-Dipoles

Synthesis and reactivity of N-heterocyclic carbene (NHC)-supported heavier nitrile ylides

The synthesis and isolation of stable heavier analogues of nitrile ylide as N-heterocyclic carbene (NHC) adducts of phosphasilenyl-tetrylene [(NHC)(TerAr)Si(H)PE14(TerAr)] (E14 = Ge 1, Sn 2; TerAr = 2,6-Mes2C6H3, NHC = IMe4) are reported. The delocalized Si-P-E14 π-conjugation was examined experimentally and computationally. Interestingly, the germanium derivative 1 exhibits a 1,3-dipolar nature, leading to an unprecedented [3 + 2] cycloaddition with benzaldehyde, resulting in unique heterocycles containing four heteroatoms from group 14, 15, and 16. Further exploiting the nucleophilicity of germanium, activation of the P-P bond of P4 was achieved, leading to a [(NHC)(phosphasilenyl germapolyphide)] complex. Moreover, the [3 + 2] cycloaddition and the σ-bond activation by 1 resemble the characteristics of the classic nitrile ylide.

Rhodium-Catalyzed [3+2]-Cycloaddition of in-situ Generated Nitrile Ylides with Nitrosoarenes

Herein we report the rhodium-catalyzed one-pot three-component reaction of diazo compounds, nitriles, and nitrosoarenes to construct 2,5-dihydro-1,2,4-oxadiazole derivatives. Mechanistic studies indicate that the transformation may proceed through the formation of nitrile ylides intermediates, which then undergo [3+2]-cycloaddition with nitrosoarenes. The strategy exhibits several synthetic advantages, including operational simplicity, good functional group tolerance, and scalability.

Studies on the [4 + 2] cycloaddition and allylic substitution of indole-fused zwitterionic π-allylpalladium

The zwitterionic π-allylpalladium species, also known as dipoles, are important synthons widely used in various reactions including cycloaddition and allylic substitution. This study reported the development of a new indole-fused zwitterionic π-allylpalladium precursor compound and its application in [4 + 2] cycloaddition and allylic substitution reactions. As a result, the synthesis of pyrrolo[3,2,1-ij]quinazolin-3-one and 7-vinyl indole compounds was achieved with moderate to good yields. Notably, the allylic substitution reaction exhibited excellent regio- and stereoselectivity.

Regioselective Synthesis of 5-Trifluoromethyl 1,2,4-Triazoles via [3 + 2]-Cycloaddition of Nitrile Imines with CF3CN

We herein describe a general approach to 5-trifluoromethyl 1,2,4-triazoles via the [3 + 2]-cycloaddition of nitrile imines generated in situ from hydrazonyl chloride with CF₃CN, utilizing 2,2,2-trifluoroacetaldehyde O-(aryl)oxime as the precursor of trifluoroacetonitrile. Various functional groups, including alkyl-substituted hydrazonyl chloride, were tolerated during cycloaddition. Furthermore, the gram-scale synthesis and common downstream transformations proved the potential synthetic relevance of this developed methodology.

Environmentally Benign and User-Friendly In Situ Generation of Nitrile Imines from Hydrazones for 1,3-Dipolar Cycloaddition

Nitrile imines are highly reactive and versatile dipoles and conventionally generated in situ from unstable hydrazonyl halides under basic conditions. Herein, we report the first green and user-friendly protocol for in situ generation of nitrile imines from Oxone-KBr oxidation of hydrazones and base-promoted dehydrobromination. The nitrile imines were demonstrated for 1,3-dipolar cycloaddition with various dipolarophiles, including alkene and alkyne groups. With its green nature, ease of operation, and air and moisture tolerance, we expect our method will find wide applications in organic synthesis.

Quadruple Functionalized Pyrazole Pharmacophores via One-pot Regioselective [3 + 2] Cycloaddition of Fluorinated Nitrile Imines and Dicyanoalkenes

Here we present a quadruple functionalization approach for the modular construction of fully substituted N 1 -aryl 3-di/trifluoro-methyl-4/5-cyanopyrazole pharmacophores from readily available hydrazonyl chlorides and dicyanoalkenes. The realization of this [3 + 2] cycloaddition reaction hinges upon the employment of N -aryl di/trifluoromethyl nitrile imines as the 1,3-dipoles to bypass external synthetic steps and dicyanoalkenes as the dipolarophiles to tune the regioselectivity. This one-pot strategy offers access to a divergent library of cyano analogues of prevalent 3-di/trifluoromethyl pyrazole pharmacophores, among which several compounds have shown potent inhibitory activity towards cyclooxygenase 2 (COX-2) compared with marketed drug Celecoxib.

From α-keto acids to nitrile oxides enabled by copper nitrate: a facile access to fused isoxazolines

An unprecedented generation of nitrile oxides was developed from α-keto acids and copper nitrate through a novel carbon–carbon bond cleavage mode, affording pharmacologically interesting fused isoxazolines via 1,3-dipolar cycloadditions.

Amide Bond Formation via the Rearrangement of Nitrile Imines Derived from N-2-Nitrophenyl Hydrazonyl Bromides

We report how the rearrangement of highly reactive nitrile imines derived from N-2-nitrophenyl hydrazonyl bromides can be harnessed for the facile construction of amide bonds. This amidation reaction was found to be widely applicable to the synthesis of primary, secondary, and tertiary amides and was used as the key step in the synthesis of the lipid-lowering agent bezafibrate. The orthogonality and functional group tolerance of this approach was exemplified by the N-acylation of unprotected amino acids.