Photochemische Cyclisierung von o‐, m‐, P‐Allylanisolen und o‐Allylanilinen. 52. Mitteilung über Photoreaktionen

Abstract

Photochemical Cyclization of o‐, m‐, p‐Allylanisoles and o‐AllylanilinesThe compounds irradiated are summarized in Scheme 1. 2‐Allylaniline and N‐Alkyl Derivatives. Irradiation (ca. 3 h) of compounds 1–3 with a high pressure mercury lamp in benzene solution under argon (quartz vessel) gave in 40–80% yield the corresponding 2‐methylindolines 20, 22 and 25, respectively (Scheme 3). Tetrahydroquinolines (23, 26) were formed only in minor amounts (0,5%). Irradiation in methanol solution yielded in addition to the indolines the 2‐(2′‐methoxypropyl)‐anilines 21,24 and 27, respectively, in a ratio of ca. 0.3, 1.5 and 1.0 with respect to the indolines (Scheme 3). Similar results were obtained in ethanol solution. The observation that the photoreactions in benzene or methanol are not quenched by (E)‐piperylene or sensitized by acetone suggests that the transformation starts from the singlet manifold of the aniline chromophor. As outlined in Scheme 11 it is proposed that the excited molecules undergo an intramolecular electron transfer to give an acceptor(olefinic side chain)/donor(aniline part) complex (EDA complex; see [28]) of type {a,b} in which the positive charge is mainly located at the nitrogen atom and the negative charge at C(3′) of the allyl substituent. That the negative charge resides predominantly at C (3′) ‐ independently of alkyl substitution at C (3′) (see experiments cited in [26]) ‐ may be due to electrostatic attraction of the charges. Thus, the following H‐transfer occurs almost regiospecifically to give the singlet diradical c which cyclizes directly or via the spirocyclopropane derivative d to the indoline derivative 22. Intermediate d is also responsible for the formation of the 2′‐methoxypropyl compounds: It is suggested that in the polar solvent methanol d is partially converted to the zwitterion e, the immediate precursor of 24. Experiments with the deuteriated reactants N‐d‐2 and 2′‐d1‐2 (Scheme 3) are in agreement with the proposed mechanism.N, N‐Dialkyl‐2‐allylanilines and Allylanisoles. Upon irradiation in methanol or benzene, these aniline derivatives undergo cyclization to give as the only products the corresponding 2‐cyclopropylanilines in 50‐70% yield (Scheme 4). 2‐, 3‐ and 4‐ allylated anisoles behave in the same way on irradiation (Schemes 6‐8) as long as the allyl group carries no substituent (CH3, Cl) at the double bond (Schemes 9, 10). No photolytic cyclopropane ring formation is observed with the naphthalene derivatives 7, 8, 17 or 18 (Scheme 1). Experiments with the deuteriated compounds 2′‐d1‐4 and 1′, 1′‐d2‐11 ‐ synthesized according to Scheme 2 ‐ indicate that in all cases the cyclopropane formation occurs with concomitant 1,2 aryl migration (Schemes 5, 6) which characterizes the reaction as an aromatic di‐π‐methane rearrangement (Scheme 14). In contrast to the photoreactions described above the cyclopropane ring formation can be sensitized by acetone or quenched totally by (E)‐piperylene. A comparison of the triplet energies (ET) of the aromatic and olefinic chromophor of the reactants (cf. Table 4) shows that the di‐π‐methane rearrangement is only effective when ET of the aryl part is lower than that of the olefinic part, but not by more than 20 kcal/mol. In substrates carrying substituents at the olefinic double bond the energy of T1 of the allyl group drops beneath that of the aryl part. In these cases no di‐π‐methane rearrangement is observed because an effective deactivation of T1 of the aryl part occurs by (E)/(Z) isomerization of the side chain as is demonstrated by the photochemical behaviour of compound 10 (Scheme 10). This concept seems to be of general significance for related di‐π‐methane rearrangements in cyclic systems (cf. Scheme 16; further examples: Scheme 15).