NMR spectroscopic studies on kinetics and thermodynamics of reversible dissociation reactions. 6. Ionization of tropylium azide in liquid sulfur dioxide and selective migration of the azide ligand in less polar solvents

The equilibrium between the covalent azidocycloheptatriene and ionic tropylium azide has been directly observed in the low-temperature ^lH and ^l3C NMR spectra in solvents containing SO₂. The kinetic parameters of the ionization (ΔH≠_i= 28.8 ± 6.3 kJ, ΔH≠_j-77 ± 25 J/grad-mol,ΔH≠_i232= 46.5 ± 0.5 kJ/mol) have been determined by ¹³CNMR line shape analysis. Nonrandom migration of the azide ligand occurs in less polar media as was shown by (a) saturation transfer in the ¹³C NMR spectrum at carefully selected temperatures; (b) ¹³C NMR line shape analysts; (c) ¹C NMR line shape analysis of hexadeuterioazidocycloheptatriene. 1, 3-Shift ([3, 3] sigmatropic migration) dominates but also nonnegligible participation of 1, 2-shift ([1, 7] sigmatropic migration) is observed (e.g., in CD₃CN: ΔH≠₂₉₈^{1, 3}= 59.85 ± 0.13 kJ/mol, ΔH≠₂₉₈^{1, 2}= 61.4 ± 0.2 kJ/mol). It has been shown that the rate of the selective migration increases with increasing solvent polarity. Thus tropylium azide represents a borderline example between sigmatropic and ionic migration.