Reaktionen von 3‐Dimethylamino‐2,2‐dimethyl‐2H‐azirin mit Phenolen und Halogenaromaten

Reactions of 3‐dimethylamino‐2,2‐dimethyl‐2H‐azirine with phenols and aryl halides The reactions of 3‐dimethylamino‐2,2‐dimethyl‐2H‐azirine (1) with phenols are described in chap. 1. The azirine 1 reacts with the 2‐formyl‐ and 2‐acetylphenols 5–8 to yield the N′‐methylidene derivatives of 2‐amino‐N,N‐dimethyl‐isobutyramide 9‐12 (Scheme 2, tautomeric form b). These products are in equilibrium with the tautomeric quinoide forms 9a‐12a. Under similar conditions 4‐hydroxybenzaldehyde did not react with 1. Reaction of 1 with 4‐hydroxycoumarine (13) gives the 4‐amino‐coumarine 14 (Scheme 2). The mechanism of these reactions is analogous to the previously reported one for the reaction of 1 with cyclic enolisable 1,3‐diketones [2] [3]. Activated phenols with pKa‐values < 8, e.g. 2‐ and 4‐nitrophenol, 2,4‐dinitrophenol and pentachlorophenol, undergo addition reactions with 1 in boiling benzene solution to give the aniline derivatives 15‐18 (Scheme 3). A reaction mechanism is given in Scheme 3: after protonation of the azirine 1 followed by attack of the phenolate ion at the amidinium‐C‐atom, the intermediate of type e undergoes a rearrangement to the spiro‐Meisenheimer complexes of type f. Ring opening leads to 15–18. A similar reaction is observed for 2,4‐dinitro‐thiophenol and 1, giving 2‐(N′‐(2,4‐dinitrophenyl)amino)‐N,N‐dimethyl‐isobutyrothioamide (19). The azirine 1 reacts with the more acidic 2,4,6‐trinitrophenol (picric acid) to yield 3,3,6,6‐tetramethylpiperazine‐2,5‐bis(N,N‐dimethyliminium) dipicrate (21, Scheme 4). The methacrylamidinium salt 22 is the only product (97% yield) in the reaction of 8‐hydroxy‐5,7‐dinitroquinoline and 1 in acetonitrile solution. The reaction of 1 with picric acid can be explained in a similar way as the previously reported one with strong acids (cf. Scheme 1, [1] [3] [5]). An alternative mechanism without formation of the 1‐aza‐allylcation c is postulated in Scheme 5, together with a mechanism which could explain the exclusive formation of 22 in the reaction of 1 with 8‐hydroxy‐5,7‐dinitroquinoline. In chap. 2 a few reactions of the azirine 1 with aryl halides are reported. In the reaction with 2,4‐dinitrofluorobenzene it is shown by UV. and NMR., that m,n and o are intermediates (Scheme 6). Working up the reaction mixture with water, hydrogen sulfide or benzylamine leads to the aniline derivatives 17, 19 and 26, respectively. With picryl chloride and 8‐hydroxy‐5,7‐dinitroquinoline the azirine 1 undergoes a nucleophilic aromatic substitution to afford the intermediates p and q, which via deprotonation and ring opening give acrylamidine derivatives (27 and 29, Scheme 7 and 8). The steric hindrance in p and q between the aziridine ring and the two groups in o‐position could be the reason for the different behaviour of the intermediates n and p or q (cf. Schemes 6 and 8).