1,3-dipolar cycloadditions, 116 The formation of 1,3-dithiolanes from aromatic thioketones and diazomethane - The mechanism of the Schonberg reaction

Reactions of diaryl thioketones with diazomethane at room temperature affor d 4,4,5,5-tetraaryl-1,3-dithiolanes; the scope of this surprising 2:1 inter action has been studied for decades (Schonberg Reaction). The clue to the m echanism was our observation that the stoichiometry is 1:1 at -78 degrees C , and 2,5-dihydro-2,2-diaryl-1,3,4-thiadiazoles are formed as primary [2+3] cycloadducts. They lose N-2 at -45 degrees C in first-order reactions gene rating diaryl thioketone S-methylides which can be intercepted by thioketon es (-->1,3-dithiolanes), multiple CC bonds, or acids HX. In the absence of trapping reagents, the elusive intermediates either dimerize furnishing 2,2 ,3,3-tetraaryl-1,4-dithianes or give rise to 2,2-diarylthiiranes by electro cyclization. Beyond thiobenzophenone and diazomethane, our main model react ion, the studies involve fluorene-g-thione, 4,4-dimethoxy- and 4,4-dichloro thiobenzophenone. The ring of 2,5-dihydro-2,2-diphenyl-1,3,4-thiadiazole (8 ) is opened by LDA at -78 degrees C and derivatives of anion 12 are obtaine d. - In summa: The Schonberg reaction consists of two 1,3-dipolar cycloaddi tions, linked by a 1,3-dipolar cycloreversion.