Reactions of triplet carbenes with sulfides and disulfides: Ylide vs. radical formation

The reactions between triplet diphenylcarbene and fluorenylidene with a variety of sulfides and disulfides were investigated with use of electron paramagnetic resonance (EPR) spectroscopy, laser flash photolysis, and product studies. Diphenylcarbene reacted with these substrates by a radical-like displacement mechanism. Rate constants were ca. IO6 M-l s-I, and the resulting thio-substituted diphenylmethyl radicals were identified by their EPR and optical spectra. By contrast, the analogous reactions of fluorenylidene had rate constants of 108-109 M-' s-I and proceeded by an ylide mechanism. Product studies were consistent with these results but were not sufficient in themselves to reveal these mechanistic differences.