Chemoselectivity in the Transannulaction of 1,2,3‐Thiadiazoles and Alk‐2‐enenitriles: Specific Synthesis of 3‐(Alk‐1‐enyl)isothiazoles

Abstract

1,2,3‐Thiadiazoles are precursors of thioacylcarbenes and thioketenes and show various reactions with unsaturated compounds. The rhodium‐catalyzed reactions of 1,2,3‐thiadiazoles and alk‐2‐enenitriles, including alkyl, aryl, and heteroaryl acrylonitriles, have been investigated, showing chemospecific reactivity to generate 3‐vinylisothiazole derivatives via the (3+2) transannulation between the cyano group of alk‐2‐enenitriles and generated thioacylcarbenes. (Z)‐Cinnamonitriles are less active than (E)‐cinnamonitriles due to the strong coordination of the (Z)‐C=C bond and the Rh catalyst. The current reaction provides a method for the synthesis of 3‐vinylisothiazole derivatives.

Catalytic (3 + 2) umpolung annulations of α-thioacyl carbenes with aryl isothiocyanates

“…The catalytic (3 + n) annulations of a-thioacyl carbenes, derived from 1,2,3-thiadiazoles under Rh(I) or Ir(I) catalysis, have served as an efficient tool for constructing thiacycles. [1][2][3][4][5][6] The advances in annulations with substrates containing C-X (X = heteroatom) multiple bonds indicated that the reactivity of a-thioacyl carbenes differed significantly as the electronic properties of substrates changed. [4][5][6] For example, in their (3 + 2) annulations with 1-phosphaalkynes, 4 a-thioacyl carbenes showed normal reactivity (nucleophilic sulfur and electrophilic carbenic carbon).…”

“…[1][2][3][4][5][6] The advances in annulations with substrates containing C-X (X = heteroatom) multiple bonds indicated that the reactivity of a-thioacyl carbenes differed significantly as the electronic properties of substrates changed. [4][5][6] For example, in their (3 + 2) annulations with 1-phosphaalkynes, 4 a-thioacyl carbenes showed normal reactivity (nucleophilic sulfur and electrophilic carbenic carbon). 1,2 However, when nitriles were reacted, 3b,c,5c complete umpolung reactivity (electrophilic sulfur and nucleophilic carbenic carbon) was observed.…”

“…The failed attempts in exploring the potential annulations of a-thioacyl carbenes with imines (CQN bond) by Xu's group 6 and with thiocarbonyl compounds (CQS bond) by us (Scheme S1 in the ESI †) prompted us to exploit their reactions with isothiocyanates, which feature two perpendicular CQS and CQN planes. We imagined that the diminished steric hindrance and nucleophilicity of the CQN or CQS bonds of isothiocyanates might facilitate the desired annulations.…”

“…Based on previous mechanistic studies, 5 c , d ,6,17 we put forward a tentative mechanism for the umpolung (3 + 2) transannulation (Scheme 4). Ligand exchange of [Rh(COD)Cl] 2 with (±)-BINAP followed by denitrogenative oxidative addition of 1,2,3-thiadiazoles 1 forms 1-thia-2-rhodacyclobutene species ( B ) (step a), which are suggested to resonate with thioacyl-coordinated Rh( i ) carbenes ( A ).…”

Lewis Acid‐Mediated Ring Contraction of 2,4‐Diaryl‐2,3‐dihydro‐1H‐1,5‐benzodiazepines: Access to 2‐Aryl‐1‐styrylbenzimidazoles

“…[16 -19] 1,2-Diaryl-2,3-dihydro-1H-1,5-benzodiazepines were readily prepared from o-phenylenediamine with chalcones [10] and 1phenylbut-2-en-1-one [20] in the presence of piperidine followed by treatment with acetic acid. When we conducted annulation of 1,2,3-thiadiazoles with alk-2enenitriles, [21] we also attempted annulation of 1,2,3thiadiazoles with imines, including 2,3-dihydro-1H-1,5-benzodiazepines as a class of cyclic imines. Although no annulation products were observed, a benzimidazole ring contraction product was isolated in low yield under the action of the Rh catalyst.…”