Photochemie von 1,1-Dimethyl-4-phenyl- und 1-Methyl-1-phenyl-1,2-dihydronaphthalin; Nachweis einer photochemischen, sigmatropischen [1,7]H-Verschiebung

Abstract

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 omega-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 warming-up, 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 alpha-methyl-omega-vinyl-o-quinodimethanes 12 and 9 undergo upon irradiation with light of lambda > 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 omega- or alpha-position may enter into conjugation with the pi-system in the helical transition state of the [1,7]-hydrogen shift, but not in the reactants 12 and 9.