Infrared spectrometric study of the interaction between 2-dimethylamino-3,3-dimethyl-1 azirine and some phenol derivatives

The H-bonded complexes between 2-dimethylamino-3,3-dimethyl-1-azirine (TMAAZ) and some phenol derivatives have been studied by IR spectrometry in carbon tetrachloride. The equilibrium constants at 300, 315 and 328 K and the − ΔH°, − ΔνOH and ΔνCN values have been determined. The comparison with previously studied OH … N complexes shows that TMAAZ forms stronger complexes than would be expected from the pKa value. These facts are discussed in terms of steric, hybridization, solvation and charge transfer effects. From a pKa of 8, the IR spectrum shows the appearance of ion pairs N+-H …−O. A predominance of protonated species can be calculated for a pKa of 6. This can explain why 1,2-ring cleavage whose first step is probably the formation of an azirinium cation, occurs for acids characterized by pKa values lower than 5 but not for enolizable ketones having pKa values higher than 9.