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

Isocianates and isothiocyanates are readily transformed into the corresponding symmetric N,N'-disubstituted ureas and thioureas upon treatment with pyridine-water with no formation of side products. Evidence is shown for an amine-free mechanistic pathway, probably involving (thio)carbamic anhydrides as reaction intermediates. The methodology is compatible with in situ generation of the isocyanate precursor from and acyl azide via Curtius rearrangement and with the presence of ester and amide functional groups in the molecule. Examples given include alkyl, aryl and carbohydrate substrates. This procedure allows the high yielding preparation of (thio)ureas in those cases where the related amine is not accessible