A practical amine-free synthesis of symmetric ureas and thioureas by self-condensation of iso(thio)cyanates

Isocyanates and isothiocyanates are readily transformed into the correspond ing symmetric N,N'-disubstituted ureas and thioureas upon treatment with py ridine-water with no formation of side products. Evidence is shown for an a mine-free mechanistic pathway, probably involving (thio)carbamic anhydrides as reaction intermediates. The methodology is compatible with in situ gene ration of the isocyanate precursor from an acyl azide via Curtius rearrange ment and with the presence of ester and amide functional groups in the mole cule. Examples given include alkyl, aryl and carbohydrate substrates. This procedure allows the high yielding preparation of (thio)ureas in those case s where the related amine is not accessible.