ISSN:
0018-019X
Schlagwort(e):
Chemistry
;
Organic Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
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 (pK = 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 pK-values of different indazoles and intermediates are listed in the Table.
Zusätzliches Material:
6 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/hlca.19760590709
