Spectroscopic characterization by laser flash photolysis of electrophilic intermediates derived from 4-aminostilbenes. Stilbene “nitrenium” ions and quinone methide imines†
This paper reports the observation in water–acetonitrile solutions of two electrophiles derived from 4-amino- and 4-acetylaminostilbene carcinogens—the ‘nitrenium’ ion ArCHCHC6H4–N+R and the quinone methide imine ArCHOH–CHC6H4NR. One example where R = acetyl (Ar = phenyl) is included, with the photochemical precursor N-(sulfonatooxy)-N-acetyl-4-aminostilbene (1). There are six examples where R = H, with substituents in Ar of 4-H, 4-Br, 3-Me, 4-Me, 4-NMeAc, 4-MeO; an azidostilbene 12 is the precursor for this series. The ‘nitrenium’ ions are characterized by a strong absorption in the region from 500–600 nm. Lifetimes range from 40 ns (R = acetyl, Ar = C6H5) to 160 ns (R = H, Ar = C6H5) to 16 µs (R = H, Ar = 4-MeOC6H4). These cations react with water at the β-carbon, i.e. through the resonance contributor ArCH+–CHC6H4NH, and the substituent effect in the R = H series shows a close parallel to reactions of other benzylic-cations. However, both the spectra of the cations and the unusually long lifetimes show that there must be substantial delocalization of the positive charge throughout the structure. The quinone methide imines, the immediate products of water addition, are observed at 300–400 nm following the decay of the cation. The imine with R = H (all Ar) exists as both the neutral compound and the conjugate acid, with a pKa of 8.3 being obtained from both the spectral change and the rate–pH profile. The latter shows a plateau region in acid (lifetime of 4 ms) due to reaction of the cationic form with water. A plateau region in base is also assigned to the reaction of the cation, this time with hydroxide ion. The acylimine (R = Ac) shows an H+-dependent reaction below pH 8 where a sharp break to an OH–-dependent reaction occurs. The latter is assigned to the neutral form reacting directly with hydroxide ion. The acid region represents equilibrium protonation followed by rapid reaction with water. The absence of a break in the rate–pH profile suggests that the cation has a pKa value less than 2. Application of previously measured azide-trapping data provides an estimate of pKa = 1.6 with the lifetime of the cation of 7 ns. This study reinforces conclusions made previously in a study of the ground state solvolysis of the N-acetyl derivative 1. The aminostilbene system is an unusual arylamine carcinogen in both the nature of the reaction of the ‘nitrenium’ ion, and in the formation of a subsequent electrophile, the quinone methide imine. The present study reports the first kinetic information regarding the latter class in water. At pH 7, both the NH and N-acetyl derivatives are quite long-lived. Both react at this pH through their conjugate acid form, which in one resonance form, can be regarded as benzylic-type cation.