RING FLATTENING IN THIANTHRENE SULFONIUM BIS(CARBOMETHOXY)METHYLIDES

Abstract Bis(carbomethoxy)methylides derived from thianthrene 5,5-dioxide and 2,7-dichlorothianthrene 5,5-dioxide exhibit hindered S+ —C− bond rotation. Variable temperature NMR studies are described for these ylides as well as 9,9-dimethylthioxanthenium bis(carbomethoxy)methylide. With coalescence temperatures of ≍30°C (200 MHz), all have δG‡ ≍ 15 kcal/mol.