Acta Crystallographica Section B

Structural Science

Volume 57, Part 3 (June 2001)

Acta Cryst. (2001). B57, 428-434 [doi:10.1107/S0108768101005171]

Topochemically controlled solid-state methyl rearrangement in thiocyanurates

M. Greenberg, V. Shteiman and M. Kaftory

Abstract: 4,6-Dimethoxy-3-methyl-1,3,5-triazine-2(3H)-thione crystallizes in two polymorphic forms, needles and plates. In the needle-shaped crystals (9a) the molecules occupy the crystallographic mirror plane, thus the layers are stacked along the b axis. The molecules of the other polymorph [plate-shape crystals, (9b)] are packed in a herringbone packing mode. Upon heating, (9b) undergoes 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-Methyl-6-methoxy-4-methylthio-1,3,5-triazine-2(1H)-thione crystallizes in space group P2₁ with two crystallographically independent molecules in the asymmetric unit. Compound (9b) undergoes O --> S methyl transfer in the solid state 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 quantitative yield. The enthalpy of the rearrangement is estimated to be -58.8 kJ mol^-1. The crystal structures of the compounds as well as their DSC thermographs are described and discussed. Energy calculation by ab initio methods shows that the driving force for the reactions is the difference between the molecular energies of the pre-rearranged compounds and their products, 54.2 and 59.3 kJ mol^-1 in the two cases, respectively.

Online 25 May 2001


	Structure factor file (CIF format) (42.7 kbytes) Contains datablock 9a

	Structure factor file (CIF format) (75.0 kbytes) Contains datablock 9b

	Structure factor file (CIF format) (157.1 kbytes) Contains datablock 11

	Structure factor file (CIF format) (44.3 kbytes) Contains datablock 12

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