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 , a thiocarbonyl ylide prepared by N2 extrusion from dihydrothiadiazole at 80 °C, combined with 2,3-bis(trifluoromethyl)fumaronitrile () to give a zwitterion (); the latter failed to close the thiolane ring by 1,5-cyclization, but formed the seven-membered ketene imine by 1,7-cyclization. X-ray analysis of revealed an angle-deformed cumulated bond system and a transoid relation of the CF3 groups. The relatively stable 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 transoid ⇌cisoid is preferred which involves a topomerization at the Cdouble bond; length as m-dashN bond; lateral inversion and rotation are discussed. At 80 °C in solution, ketene imine slowly suffered fragmentation to give trans- and cis-1,2-bis(trifluoromethyl)cyclopropane-1,2-dicarbonitrile ()+thioketone by intramolecular substitution. The reaction of with ethenetetracarbonitrile furnished a tetracyanothiolane , whereas and dimethyl 2,3-dicyanofumarate () afforded thiolanes of the same trans,cis-ratio as with dimethyl 2,3-dicyanomaleate (); a preceding (E,Z)-equilibration of thwarts mechanistic conclusions. When the solvent contained water or methanol, short-lived ketene imines and were intercepted.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 , a thiocarbonyl ylide prepared by N2 extrusion from dihydrothiadiazole at 80 °C, combined with 2,3-bis(trifluoromethyl)fumaronitrile () to give a zwitterion (); the latter failed to close the thiolane ring by 1,5-cyclization, but formed the seven-membered ketene imine by 1,7-cyclization. X-ray analysis of revealed an angle-deformed cumulated bond system and a transoid relation of the CF3 groups. The relatively stable 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 transoid ⇌cisoid is preferred which involves a topomerization at the Cdouble bond; length as m-dashN bond; lateral inversion and rotation are discussed. At 80 °C in solution, ketene imine slowly suffered fragmentation to give trans- and cis-1,2-bis(trifluoromethyl)cyclopropane-1,2-dicarbonitrile ()+thioketone by intramolecular substitution. The reaction of with ethenetetracarbonitrile furnished a tetracyanothiolane , whereas and dimethyl 2,3-dicyanofumarate () afforded thiolanes of the same trans,cis-ratio as with dimethyl 2,3-dicyanomaleate (); a preceding (E,Z)-equilibration of thwarts mechanistic conclusions. When the solvent contained water or methanol, short-lived ketene imines and were intercepted.