2,2,6,6-Tetramethylcyclohexanethione S-methylide, a highly hindered thiocarbonyl ylide: two-step cycloadditions

The switching from the concerted 1,3-dipolar cycloaddition to a two-step pathway via zwitterionic intermediates requires a major energy difference between HOMO-LUMO energies of 1,3-dipole and dipolarophile, as well as sterically demanding reactants. In contrast to previously studied models, the title compound 1C, a thiocarbonyl ylide prepared by N₂ extrusion from dihydrothiadiazole 7C at 80 °C, combined with 2,3-bis(trifluoromethyl)fumaronitrile (11) to give a zwitterion (gauche-10); the latter failed to close the thiolane ring by 1,5-cyclization, but formed the seven-membered ketene imine 9C by 1,7-cyclization. X-ray analysis of 9C revealed an angle-deformed cumulated bond system and a transoid relation of the CF₃ groups. The relatively stable 9C allowed ¹⁹F NMR recordings from −90 to +90 °C; temperature-dependent line broadening resulted from equilibration with ≤1% of an unknown isomer. Among various possible angle-strained rate processes, an inversion transoid19?cisoid20 is preferred which involves a topomerization at the CN bond; lateral inversion and rotation are discussed. At 80 °C in solution, ketene imine 9C slowly suffered fragmentation to give trans- and cis-1,2-bis(trifluoromethyl)cyclopropane-1,2-dicarbonitrile (13)+thioketone 6C by intramolecular substitution. The reaction of 1C with ethenetetracarbonitrile furnished a tetracyanothiolane 3C, whereas 1C and dimethyl 2,3-dicyanofumarate ((E)-26) afforded thiolanes of the same trans,cis-ratio as 1C with dimethyl 2,3-dicyanomaleate ((Z)-26); a preceding (E,Z)-equilibration of 26 thwarts mechanistic conclusions. When the solvent contained water or methanol, short-lived ketene imines 4C and 31 were intercepted.