Computational insights into cycloadditions of thioisomünchnones with acetylenes: how does sulfur escape from cycloadducts?

• Published in: Tetrahedron Vol. 72; no. 31; pp. 4665 - 4670
• Main Authors: García de la Concepción, Juan, Ávalos, Martín, Babiano, Reyes, Cintas, Pedro, Jiménez, José L., Light, Mark E., Palacios, Juan C.
• Format: Journal Article
• Language: English
• Published: OXFORD Elsevier Ltd 04.08.2016; Elsevier
• Subjects: alkynes; chemical structure; Chemistry; Chemistry, Organic; Cycloaddition; cycloaddition reactions; Mesoionic; Physical Sciences; Science & Technology; Sigmatropic rearrangement; sulfur; Thiazolopyridone; Thiirane; thiophenes; Mesoionic; Cycloaddition; Thiazolopyridone; Sigmatropic rearrangement; Thiirane; ISOTHIOCYANATES; 1,3-DIPOLAR CYCLOADDITIONS; DESIGN; MOLECULAR-ORBITAL METHODS; MESOIONIC COMPOUNDS; MUNCHNONES; THIONATION; DENSITY FUNCTIONALS; AROMATICITY; CHEMISTRY
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/j.tet.2016.06.041

The spontaneous loss of sulfur or isocyanate from transient 7-thia-2-azabicyclo[2.2.1]hept-5-en-3-ones, which are initially formed by 1,3-dipolar cycloadditions of thioisomünchnones with acetylenic dipolarophiles, is the key step in the chemoselective syntheses of pyridin-2-ones or thiophenes. The way by which sulfur is released has been the subject of previous studies pointing to a concerted retro-cheletropic mechanism as a more favorable route than the alternative stepwise pathway. The latter however, is apparently prevalent for elimination of isocyanate. Working with a conformationally-restricted bicyclic thioisomünchnone that undergoes facile cycloaddition with acetylenes, sulfur elimination has now been interrogated by experiment and theoretical calculations at the M06-2X and M11 methods in combination with the 6-311++G(d,p) basis set, which unveil rather a sigmatropic shift via the intermediacy of thiirane species. These results provide new vistas and synthetic opportunities in mesoionic cycloadditions. [Display omitted]