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STRUCTURE, BOND-ENERGY, AND REACTIVITY OF CYCLIC SULFURANES
A series of 10-S-3 type sulfuranes, tetraazathiapentalenes (10 a-i, 11 a-f, 12, and 17) fused with pyrimidine ring and/or pyridine ring, were prepared by oxidation of the corresponding thioureas. The restricted internal rotation of the pyrimidine ring was observed by temperature dependent H-1 NMR spectrum which gave the kinetic parameters of Delta G(298)(double dagger)=16.6 kcal/mol, Delta H-double dagger=15.9 kcal/mol, and Delta S-double dagger=-2.4 eu at 25 degrees C for the rotational barrier in 10a, The substituent effect to the kinetic data can be reasonably explained in terms of electronic balance of the N-S-N hypervalent bond or the different degree of contribution of resonance canonical structures. Comparison of the rotational barrier of 10a with that of model compounds (18 and 24) suggests that hypervalent N-S-N stabilization in 10a is more than 6 kcal/mol. Methylation of these 10-S-3 type sulfuranes (10a, 10d, and 11a) was investigated to clarify the electronic effect in these molecules. The crystal structure of the neutral symmetric sulfurane 10a reveals to be a planar molecule in which the sulfur atom was found to be in the same plane of the two pyrimidine rings. The S-N bond lengths 1.948(3) and 1.938(3) Angstrom are longer than the sum of the covalent bond radii, consistent with a bond order less than unity. The crystal structures of the dicationic symmetric sulfurane 17 and the unsymmetrical sulfurane 11a also show to be planar molecules. The S-N distances in 11a were different each other due to the electronic imbalance between the apical ligands. Semi-empirical calculation (AM 1) of these systems (10a and 17) indicate the expected electronic features of the hypervalent molecules (10-S-3 sulfuranes) and the small contribution of pi-overlapping in the N-S-N bonds.
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Reference:
Isothiazole – Wikipedia,
,Isothiazole – ScienceDirect.com