Mechanismus der photochemischen Methanol‐Addition an 2‐allylierte Aniline
Authors: Jolidon Synèse , Hansen Hans‐Jürgen 
Published: December 2004

Mechanism of the Photochemical Addition of Methanol to 2‐Allylated Anilines

We studied in methanol the photoreaction of the 2‐allylated anilines, given in Scheme 3 (cf. also [1]). Irradiation of N‐methyl‐2‐(1′‐methylallyl)aniline (15) with a high pressure mercury lamp yielded trans‐ and cis‐1,2,3‐trimethylindoline (trans‐ and (cis‐34) as well as erythro‐ and threo‐2‐(2′‐methoxy‐1′‐methylpropyl)‐N‐methylaniline (erythro‐ and threo‐35; Scheme 7). When the corresponding aniline d3‐15, specifically deuterated in the 1′‐methyl group, was irradiated in methanol, a mixture of trans‐ and cis‐d3‐34, and of erythro‐ and threo‐d3‐35 was obtained. Successive dehydrogenation of the mixture of cis/trans‐d3‐34 by Pd/C in boiling xylene and by MnO2 in boiling benzene lead to the corresponding indole d3‐36 (cf. Scheme 9), the 1H‐ and 2H‐NMR. spectra of which showed that both cis‐d3‐ and trans‐d3‐34 had bound the deuterium labeled methyl group exclusively at C(3). The 1H‐ and 2H‐NMR. analyses of the separated methanol addition products revealed that erythro‐d3‐35 contained the deuterium label to at least 95% in the methyl group at C(1′), and threo‐d3‐35 to 50% in CH3C(1′) and to 50% in CH3C(2′) (cf. Scheme 9). To confirm these results 2‐(1′‐ethylallyl)aniline (16) was irradiated in methanol, whereby a complex mixture of at least 6 products was obtained (cf. Scheme 11). Two products were identified as trans‐ and cis‐3‐ethyl‐2‐methylindoline (trans‐ and cis‐37). The four other products represented erythro‐ and threo‐2‐(1′‐ethyl‐2′‐methoxypropyl)aniline (erythro‐ and threo‐39) as major components, and erythro‐ and threo‐2‐(2′‐methoxy‐1′‐methylbutyl)aniline (erythro‐ and threo‐40). These results clearly demonstrate that the methanol addition products must arise from spirodienimine intermediates of the type of trans‐9 and cis‐11 (R1 = CD3 or C2H5, R2 = CH3 or H; Scheme 2) which are opened solvolytically with inversion of configuration by methanol. Thus, cis‐11 (R1 = CD3, R2 = CH3) must lead to a 1:1 mixture of threo‐13 and threo‐14 (i.e.) a 1:1 distribution of the deuterium labelled methyl group between C(1′) and C(2′) in threo‐35) The formation of erythro‐d3‐35 with at least 95% of the deuterium label in the methyl group at C(1′) indicates that trans‐9 (R1 = CD3, R2 = CH3) reacts with methanol regioselectively (> 95%) at the C(2), C(3) bond. Similarly, the formation of the methanol addition products in the photoreaction of 16 (Scheme 11) can be explained. Since the indolines, formed in both photoreactions, show no alteration in the position of the subsituent at C(1′) with respect to the starting material we suppose that the diradical 7 (R1 = CD3 or C2H5, R2 = CH3 or H; Scheme 2) is a common intermediate which undergoes competetive 1.3 and 1.5 ring closure yielding the spirodienimines and the indolines.

This conception is supported by irradiation experiments with N, 3,5‐trimethyl‐2‐(1′‐methylally)aniline (17) and 2‐(2′‐cyclohexenyl)‐N‐methylaniline (18) in methanol. In the former case the formation of spirodienimines is hindered by the methyl group at C(3) for steric reasons, thus leading to a ratio of the indoline to the methoxy compounds of about 6.3 as compared with ca. 1.0 for 15 (cf. Scheme 12). On the other hand, no methoxy compounds could be detected in the reaction mixture of 18 (cf. Scheme 13) which indicates that in this case the 1.3 ring closure cannot compete with the 1.5 cyclization in the corresponding cyclic diradical of the type 7 (R1–C(1′)–C(2′) is part of a six‐membered ring; Scheme 2).

We suppose that the diradicals of type 7 are formed by proton transfer in an intramolecular electron‐donor‐acceptor (EDA) complex arising from the excited single state of the aniline chromophor and the allylic side chain. This idea is supported by the fluorescence specta of 2‐allylated N‐methylanilines (cf. Fig.1‐4) which show pronounced differences with respect to the corresponding 2‐alkylated anilines. Furthermore, the anilines 18 and 20 when irradiated in methanol in the presence of an excess of trans‐1,3‐pentadiene undergo preferentially an intermolecular addition to the diene, thus yielding the N‐(1′‐methyl‐2′‐butenyl)anilines 52 and 51, respectively (Scheme 15), i.e. as one would expect the diene with its low lying LUMO is a better partner for an EDA complex than the double bond of the allylic side chain.