Mechanistic studies on the photochemistry of 2‐alkylindazoles in aqueous solutions.
The photochemistry of 2‐alkylindazoles 1 in aqueous solutions is rather complex, the relative yields of different products being dependent on the pH‐value of the irradiated solution: In neutral or basic solutions (pH > 7) as well as in most of the organic solvents isomerization to 1‐alkyl‐benzimidazoles 2 takes place. In dilute sulfuric acid (pH 2–4) this reaction is suppressed and the dihydro‐azepinones 3 and 4 are formed. Irradiation in strongly acid solutions (pH < 1) yields the o‐amino‐acetophenones 5 (Scheme 1).
The relative quantum yields of the photoproducts 2–5 have been measured as a function of the pH‐value of the irradiated solution (Fig. 1). A comparison of these yields with the protonation equilibrium of the indazole in its first excited singlet state (p__K__ = 2.8) suggests that 2 and 3 are both photoproducts of the neutral indazole molecule, whereas 4 as well as 5 are formed from the protonated indazole.
The rearrangement of the indazole 1 to the benzimidazole 2 proceeds via an intermediate 6, which can be produced in high concentrations by monochromatic irradiation of 1 at low temperatures. The thermal reactivity of this intermediate in dilute sulfuric acid could be investigated: At pH 8 the only product is the benzimidazole 2. With decreasing pH‐value increasing amounts of 3 are formed and at pH < 4 the formation of 2 is completely suppressed, the only product being the azepinone 3. Thus, 3 is a solvolysis product of the intermediate 6 (Scheme 2).
The most probable primary product of singlet indazolium is the nitrenium ion 7. From this intermediate the formation of 5 can proceed in well‐known thermal reactions. The formation of 4 is possibly due to a further protonation equilibrium nitrenium‐nitrene. The nitrene 7 can be converted into the azepinone 4 via the azirine 8 (Scheme 3). The p__K__‐values of different indazoles and intermediates are listed in the Table.