Zur Photochemie von 1H‐ und 2H‐Indazolen in saurer Lösung

Abstract

On the Photochemistry of 1H‐ and 2H‐Indazoles in Acidic SolutionIt is shown that 1H‐ and 2H‐indazoles (cf. Scheme 2) on protonation (0, 1N H2SO4 in water or alcoholic solution) give analogous indazolium ions (see Fig. 1 and 2) which on irradiation undergo heterolytic cleavage of the N (1), N (2) bond whereby aromatic nitrenium ions in the singlet ground state are formed (cf. Scheme 13). If the para position of these nitrenium ions is not occupied by a substituent (e.g. a methyl group) they are readily trapped by nucleophiles present (e.g. water, alcohols, chloride ions) to yield the corresponding 5‐substituted 2‐amino‐benzaldehydes or acetophenones (cf. Schemes 4–10). Photolysis of indazole (4) and 3‐methyl‐indazole (5) in 0,75N H2SO4 in alcoholic solutions gives in addition minor amounts of the corresponding 3‐substituted 2‐amino‐benzaldehydes and acetophenones, respectively (cf. Schemes 6 and 8 and Table 2). Phenylnitrenium ions carrying a methyl group in the para position give in aqueous sulfuric acid mainly the reduction products, i.e. 2‐amino‐5‐methyl‐benzaldehydes (cf. Schemes 11 and 12 and Table 3). In methanolic sulfuric acid, in addition to the reduction products, 6‐methoxy substituted benzaldehydes are found (cf. Schemes 11 and 12 and Table 3) which are presumably formed by an addition‐elimination mechanism (cf. Scheme 18). It is assumed that precursors of the reduction products are the corresponding nitrenium ions in the triplet ground state. Singlet‐triplet conversion of the nitrenium ions may become efficient when addition of nucleophiles to the singlet nitrenium ions is reversible (cf. Scheme 22) thus, enhancing the probability of conversion or when conjugation in the singlet nitrenium ions is disturbed by steric effects (cf. Scheme 20) thus, destabilizing the singlet state relative to the triplet state.