Substrate‐Controlled and Site‐Selective [3+2] Cycloadditions of N‐Heterocyclic Carbene Derived Ambident Dipoles

Abstract

2-aryl thiocarbamoyl benzimidazolium and imidazolinium inner salts derived from benzimidazole and imidazoline carbenes are unique ambident C-C-S and C-C-N 1,3-dipolar systems, which undergo highly efficient and site-selective cycloaddition reactions with dimethyl acetylenedicarboxylate or dibenzoylacetylene to furnish spiro(imidazole-2,3'-thiophene) derivatives in excellent yields. When treated with ethyl propiolate, methyl acrylate or acrylonitrile, spiro(imidazole-2,3'-pyrrole) derivatives were formed in good yields. Theoretical studies revealed an asynchronous concerted mechanism for both the C-C-S and C-C-N 1,3-dipolar cycloaddition reactions. The site selectivity in the [3+2] cycloaddition reaction of ambident 1,3-dipoles was predictably regulated by both the electronic and steric effects of dipolarophiles.

Reaction of amidrazones with 1,4-diphenylbut-2-yne-1,4-dione

“…6 2-Aryl thiocarbamoyl benzimidazolium salts derived from benzimidazole and imidazoline carbenes undergo cycloaddition reactions with DBD to furnish spiro(imidazole-2,3'-thiophenes). 7 Protonation of the highly reactive 1 : 1 intermediates produced in the reaction between alkyl isocyanides and DBD leads to vinylnitrilium cations, which undergo carbon-centred Michael type addition with the conjugate base of the NH-acid to produce highly functionalised aminofuran derivatives. 8 It is reported that amidrazones condense only with dicarbonyl compounds to yield 1,2,4-triazines.…”

Exploring the nucleophilicity of N,N′‐diamidocarbenes: Heteroallenes and related compounds as coupling reagents

“…Subsequent efforts were directed toward analyzing the reaction of 1 with isothiocyanates, substrates that typically afford imidothioates when exposed to NHCs . Stirring a C 6 H 6 solution of 1 with an equimolar amount of 4‐nitrophenyl isothiocyanate at room temperature for 1 h ([ 1 ] 0 = 0.13 M) resulted in the formation of dihydropyrimidinium imidothioate 3 (Scheme ), as evidenced by the FT‐IR carbonyl stretching frequencies observed at 1769 and 1740 cm −1 and the 1:1 DAC:isothiocyanate stoichiometry gleaned from 1 H NMR spectroscopic analysis of the crude reaction mixture.…”

“…These reactions are generally rapid, proceed with complete atom economy, and give high yields of products that are attractive for the synthesis of heteroatom rich compounds . Moreover, such carbene–heteroallene coupling reactions have been used in applications that range from small molecule activation and catalysis to the synthesis of novel polymers and heterocycles …”

Triazaspirocycles: Occurrence, Synthesis, and Applications

“…Formation of NHCs can proceed via the corresponding metal-carbene complex [49], from the elimination of a haloform (CHX 3 ) [50], or from deprotonation of an imidazolium species [51, 52]. Elimination of carbamates, analogous to the urea elimination observed by Ulrich et al has been described as well in the synthesis of NHCs [53].…”