C−H Activation with Elemental Sulfur: Synthesis of Cyclic Thioureas from Formaldehyde Aminals and S8

The C−H activation of cyclic formaldehyde aminals LCH2 (L=RN-CH2CH2CH2-NR and RNCH2CH2NR, R=Me, Et, iPr, tBu, or Ph) with S8 proceeds at unusually low temperatures (T<160 °C) and results in the formation of the respective thioureas LC=S and H2S. The reaction constitutes a new, solvent-free method for the synthesis of thioureas that eliminates the toxic and highly flammable CS2. For R=tBu, the ionic carbenium thiocyanates [LCH]+ SCN− dominate the product spectrum and the respective thioureas are obtained in low yield. The reactivity of the analogous sulfur and oxygen ring systems towards S8 was investigated. 1,3-Dithiolane is cleanly converted into 1,3-dithiolane-2-thione (S8, 14 d, 190 °C) and resembles the cyclic formaldehyde aminals in this respect. 1,3-Dioxolane (L=OCH2CH2O) is completely inert towards sulfur even under forceful reaction conditions (190 °C, 14 d). The formation of thioureas from aminals was investigated at the CBS-4 and B3LYP/6-31G(d) levels of theory.