Bestrahlung von 4‐allylierten 2,6‐Dimethylanilinen in Methanol

Abstract

Irradiation of 4‐Allylated 2,6‐Dimethylanilines in Methanol4‐Allyl‐, 4‐(1′‐methylallyl)‐, 4‐(2′‐butenyl)‐, and 4‐(1′,1′‐dimethylallyl)‐2,6‐dimethylaniline (14–17; cf. Scheme 3) were obtained by the acid catalysed, thermal rearrangement of the corresponding N‐allylated anilines in good yields. Aniline 14, when irradiated with a high pressure mercury lamp through quartz in methanol, yielded as main product 4‐(2′‐methoxypropyl)‐2,6‐dimethylaniline (22; cf. Scheme 4) and, in addition, 2,6‐dimethyl‐4‐propylaniline (18) and 4‐cyclopropyl‐2,6‐dimethylaniline (23). The analogous products, namely erythro‐ and threo‐4‐(2′‐methoxy‐1′‐methylpropyl)‐2,6‐dimethylaniline (erythro‐ and threo‐24), 2,6‐dimethyl‐4‐(1′‐methylpropyl)aniline (19), trans‐ and cis‐2,6‐dimethyl‐4‐(2′‐methylcyclopropyl)aniline (trans‐ and cis‐25), as well as small amounts of 4‐ethyl‐2,6‐dimethylaniline (26), were formed by irradiation of 15 in methanol (cf. Scheme 5). When this photoreaction was carried out in O‐deuteriomethanol, erythro‐ and threo‐24 showed an up‐take of one deuterium atom in the side chain. The mass spectra of erythro‐ and threo‐24 revealed that in 50% of the molecules the deuterium was located at the methyl group at C(1′) and in the other 50% at the methyl group at C(2′) (cf. Scheme 6). This is a good indication that the methanol addition products arise from methanolysis of intermediate spiro[2.5]octa‐4,7‐dien‐6‐imines (cf. Scheme 7). This assumption is further supported by the photoreaction of 17 in methanol (cf. Scheme 8) which led to the formation of 4‐(2′‐methoxy‐1′,2′‐dimethylpropyl)‐2,6‐dimethylaniline (28) as main product. The occurrence of a rearranged side chain in 28 can again be explained by the intervention of a spirodienimine 31 (cf. Scheme 9). In comparison with 14, 15 and 17, the 2′‐butenylaniline 16 reacted only sluggishly on irradiation in methanol (cf. Scheme 10). It is suggested that all photoproducts ‐ except for the cyclopropyl derivatives which are formed presumably via a triplet di‐π‐methane rearrangement ‐ arise from an intramolecular singlet electron‐donor‐acceptor complex between the aniline and ethylene chromophor of the side chain. Protonation of this complex at C(3′) or C(2′) will lead to diradicals (e.g. 33 and 34, respectively, in Scheme 11). The diradicals of type 33 undergo ring closure to the corresponding spirodienimine intermediates (e.g. 31) whereas the diradicals of type 34 take up two hydrogen atoms to yield the photo‐hydrogenated compounds (e.g. 21) or undergo to a minor extent fragmentation to side chain degraded products (e.g. 30; see also footnote 7).–Irradiation of 4‐ally‐2,6‐dimethylaniline (14) in benzene or cyclohexane yielded the corresponding azo compound 38 (cf. Scheme 12), whereas its N,N‐dimethyl derivative 41 was transformed into the cyclopropyl derivative 42. The allyl moiety in 14 is not necessary for the formation of azo compounds since 2,4,6‐trimethylaniline (39) exhibited the same type of photoreaction in benzene solution.