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

Irradiation of 4-Allylated 2,6-Dimethylanilines in Methanol 4-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 spiro2.5]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.