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 e lectrophiles derived from 4-amino- and 4-acetylaminostilbene carcinogens-th e 'nitrenium' ion ArCH=CHC6H4-N+R and the quinone methide imine ArCHOH-CH=C 6H4=NR. One example where R = acetyl (Ar = phenyl) is included, with the ph otochemical 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 mu s (R = H, Ar = 4-MeOC6H4). These cations re act with water at the beta-carbon, i.e. through the resonance contributor A rCH+-CH=C6H4=NH, and the substituent effect in the R = H series shows a clo se parallel to reactions of other benzylic-cations. However, both the spect ra 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, wi th a pK(a) 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) d ue 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 s harp break to an OH--dependent reaction occurs. The latter is assigned to t he neutral form reacting directly with hydroxide ion. The acid region repre sents equilibrium protonation followed by rapid reaction with water. The ab sence of a break in the rate-pH profile suggests that the cation has a pK(a ) value less than 2. Application of previously measured azide-trapping data provides an estimate of pK(a) = 1.6 with the lifetime of the cation of 7 n s. This study reinforces conclusions made previously in a study of the grou nd state solvolysis of the N-acetyl derivative 1. The aminostilbene system is an unusual arylamine carcinogen in both the nature of the reaction of th e 'nitrenium' ion, and in the formation of a subsequent electrophile, the q uinone methide imine. The present study reports the first kinetic informati on regarding the latter class in water. At pH 7, both the NH and N-acetyl d erivatives are quite long-lived. Both react at this pH through their conjug ate acid form, which in one resonance form, can be regarded as benzylic-typ e cation.