Solvent Effects on Valence Isomerization Equilibria

Valenee isomerization reaetions intereonvert so-called valenee isomers by simple reorganization of some of the bonding eleetrons, without any atom migration. Since both valenee isomers may have different straetures and thus different physical properties e.g. different dipole moments) it is to be expected that solvents should influence the equilibrium between non-degenerate isomers. 

For example, NMR measurements showed that the azido/tetrazole equilibrium of thiazolo[2,3-e]tetrazole (38b) is eonsiderably affected by the medium [lOOj. 

Whereas in the gas phase and in nonpolar solvents sueh as tetraehloromethane and benzene, the 2-azidothiazole (38a) is the more stable isomer, in polar solvents such as dimethyl sulfoxide and hexamethylphosphorie triamide the bieyelie valenee isomer (38b) is the dominant speeies [100]. This result is in line with the faet that the dipole moment of phenyl azide g = 5.2 10 Cm) is smaller than that of cyclie 1,2,3-benzotriazole (// = 13.7 10 Cm). Similar results have been obtained for the valenee isomers of 3-azidopyrazine-l-oxide [204]. 

Another remarkable example is 2,4,6-tri-tert-butyl-A-thiosulflnylanihne (39a), which is in equilibrium with its valence isomer (39b) in solution, while in the solid state only (39b) exists [205]. The equilibrium ratio (39b) (39a) is subjeet to a considerable 

In the case of the oxepin/benzene oxide valenee isomerization (40a) (40b), it has been found by UV/Vis measurements that, with zw-oetane as the solvent, only about 30 cmol/mol benzene oxide is present, whereas in water/methanol (85 15, cL/L) 

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