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

Abstract

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 N 2 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 CF 3 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 transoid 19 ⇌ cisoid 20 is preferred which involves a topomerization at the C N 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.