Photoinduzierte Reaktionen von 3‐Phenyl‐2H‐azirinen mit Carbonsäureestern 40. Mitteilung über Photoreaktionen

Abstract

Irradiation (280–350 nm light) of a benzene solution of 3‐phenyl‐2H‐azirines 1a–e in the presence of carboxylate esters, whose carbonyl groups are activated by electron withdrawing groups situated in the acyl or alkyl moiety, produces 5‐alkoxy‐3‐oxazolines (Tab. 1 and 4, Scheme 2) isolated in 18–82% yield. These heterocycles undoubtedly originate by regiospecific addition of the ester carbonyl group to the azirine‐derived benzonitrile‐methylide ‘dipole’ (Scheme 1). The 5‐(2,′ 2′, 2′‐trifluoroethoxy)‐3‐oxazolines, derived from 2′, 2′, 2′‐trifluoroethyl carboxylic esters, on treatment with methanolic hydrogen chloride at low concentration, are smoothly transformed into the corresponding 5‐methoxy‐3‐oxazolines (e.g. 16 → 17, Tab. 5). Utilizing this process, various hitherto relatively unknown 9. 5‐alkoxy‐3‐oxazolines become accessible.The constitution of the adducts is based essentially on spectral data. The structure of trans‐5‐methoxy‐2,4‐diphenyl‐5‐trifluoromethyl‐3‐oxazoline (trans‐14), the addition product of methyl trifluoroacetate and the benzonitrile‐benzylide from 2,3‐diphenyl‐2H‐azirine (1d), was determined by X‐ray crystallography (Section 5).Benzonitrile‐isopropylide (22), resulting from the photochemical transformation of 2,2‐dimethyl‐3‐phenyl‐2H‐azirine (1a), also reacts with S‐methyl thiobenzoate to give 2,2‐dimethyl‐5‐methylthio‐4,5‐diphenyl‐3‐oxazoline (26). Ethyl cyanoacetate protonates predominantly the dipolar species derived from 1a at the nitrile C‐atom and yields after work‐up ethyl α‐cyano‐cinnamate (29) and ethyl isopropylidene‐cyanoacetate (30) (Scheme 4).The relative rate of addition (k_rel) of benzonitrile‐isopropylide (22) to methyl α‐haloacetates and dimethyl oxalate was determined by competition experiments (Section 6). Log k_rel correlated satisfactorily (r = 0.97) with the pK_a of the acide derived from the ester reactant: log k_rel = − 1.72 pK_a + 2.58 or with Taft's substituent constants σ*: log k_rel = 2.06 σ* − 4.11 [k_rel(methyl dichloroacetate) = 1; Section 7.1].On the basis of the results obtained, the mode of reaction of the so‐called benzonitrile‐methylide ‘dipole’ is discussed and a model for the transition state of addition of ester‐carbonyl groups is proposed that accounts for the observed regiospecifity and steroselectivity.