Topochemically controlled solid-state methyl rearrangement in thiocyanurates

4,6-Dimethoxy-3-methyl-1,3,5-triazine-2(3H)-thione crystallizes in two poly morphic forms, needles and plates. In the needle-shaped crystals (9a) the m olecules occupy the crystallographic mirror plane, thus the layers are stac ked along the b axis. The molecules of the other polymorph [plate-shape cry stals, (9b)] are packed in a herringbone packing mode. Upon heating, (9b) u ndergoes a phase transition to form (9a). At 378 K the needles undergo O -- > S topochemically controlled methyl transfer in the solid state to produce 1-methyl-4-methoxy-6-methylthio-1,3,5-triazine-2(1H)-one in 75% yield. The enthalpy of the rearrangement is estimated to be -39.1 kJ mol(-1). 1-Methy l-6-methoxy-4-methylthio-1,3,5-triazine-2(1H)-thione crystallizes in space group P2(1) with two crystallographically independent molecules in the asym metric unit. Compound (9b) undergoes O --> S methyl transfer in the solid s tate at 373 K. The rearrangement is topochemically assisted and the product , 1-methyl-2,4-bismethylthio-1,3,5-triazine-6(1H)-one, is obtained in quant itative yield. The enthalpy of the rearrangement is estimated to be 5-58.8 kJ mol(-1). The crystal structures of the compounds as well as their DSC th ermographs are described and discussed. Energy calculation by ab initio met hods shows that the driving force for the reactions is the difference betwe en the molecular energies of the pre-rearranged compounds and their product s, 54.2 and 59.3 kJ mol(-1) in the two cases, respectively.