Reaction of a highly spiro-activated cyclopropane with pyridines. A novel intramolecular charge transfer interaction of the product

Abstract

The reaction of 3,3,10,10-tetramethyldispiro[5.0.5.1]trideca-1,5,8,12-tetraone (1) with pyridines afforded a yellow crystalline product under very mild conditions. The structure of the product was assigned as a derivative of the pyridinium β-keto-enolate (2a) in which the cyclopropane ring of (1) is cleaved by heterolysis giving a β-ketoenolate anion. In its electronic spectrum the absorption maximum at longer wavelength (charge transfer band) for (2a) showed solvatochromic shifts. The transition energes (E_ct) of (2a) measured in various solvents give linear plots against the Kosower Z valves and Taft–Kamlet α-scales, with slopes of 0.62 and 15.70, respectively. Also, there is a linear relationship between E_ct and the Hammett α-value for each substituent of the heteroaromatic ring in (2a–g), the ρ-value being –16.8. The structure and properties of (2a) have been described from these results.