Photochemie von 1,1‐Dimethyl‐4‐phenyl‐ und 1‐Methyl‐1‐phenyl‐1,2‐dihydronaphthalin; Nachweis einer photochemischen, sigmatropischen [1,7]H‐Verschiebung. 41. Mitteilung über Photoreaktionen

Abstract

Photochemistry of 1,1‐dimethyl‐4‐phenyl‐and 1‐Methyl‐1‐phenyl‐1,2‐dihydronaphthalene; evidence for a photochemical, sigmatropic [1,7]H‐shift.Irradiation of 1,1‐dimethyl‐4‐phenyl‐1,2‐dihydronaphthalene (11) and 1‐methyl‐1‐phenyl‐1,2‐dihydronaphthalene (8) in pentane were investigated at −112° to −118°, using a mercury high pressure lamp. The [1,5]‐hydrogen‐shift products 13 and 17, respectively, the [1,7]‐hydrogen‐shift products 15 and 10, respectively and the photochemical Diels‐Alder products 14 and 18, respectively, were obtained, presumably via the ω‐vinyl‐o‐quinodimethane intermediates 12 and 9 (Schema 3).Irradiation of the 1,2‐dihydronaphthalene 11 at −181° to −183° in a 2,2‐dimethylbutane/pentane matrix, gave rise to an UV.‐maximum at 402 nm which is assigned to the o‐quinodimethane derivative 12. After warming the solution around −130° or to room temperature, a product mixture was obtained, which mainly consist of the [1,7]‐hydrogen‐shift product 15 accompanied by the [1,5]‐hydrogen‐shift products 13 and 16 and the photochemical Diels‐Alder product 14 (Table 1). When the o‐quinodimethane intermediate 12 was irradiated with 406 nm‐light, the longwavelength absorption completely disappeared. This solution, after warmingup, yielded mainly the [1,5]‐hydrogen‐shift products 13 and 16 together with the bicyclic compound 14 and surprisingly a small amount of the [1,7]‐hydrogen‐shift product 15 (Table 1).Similar experiments were carried out with the 1,2‐dihydronaphthalene 8. The results clearly indicate that irradiation of the o‐quinodimethane 9 at −180° to −185° with 406 nm‐light caused [1,5]‐ and [1,7]‐hydrogen shifts in a ratio of approximately 1:1 (Table 2).From the experiments described above it follows, that the phenyl‐substituted α‐methyl‐ω‐vinyl‐o‐quinodimethanes 12 and 9 undergo upon irradiation with light of λ > 400 nm, besides photochemical Diels‐Alder reactions, also [1,5]‐ and [1,7]‐ hydrogen shifts.It is remarkable that the thermal [1,7]‐hydrogen‐shifts of the o‐quinodimethanes 12 and 9 occur readily around −130°, whilst a temperature of −70° is needed to promote [1,7]‐hydrogen‐shifts in the non‐phenylated o‐quinodimethanes of the type 2 (Schema 1). The phenyl group in ω‐ or α‐position may enter into conjugation with the π‐system in the helcal transition state of the [1,7]‐hydrogen shift, but not in the reactants 12 and 9.