Zur Regioselektivität von Cycloadditionen photochemisch erzeugter Benzonitril-isopropylide. 51. Mitteilung über Photoreaktionen

On the regioselectivity of cycloaddition reactions of photochemically generated benzonitrile-isopropylides This paper deals with the physical and chemical differences of zwitterionic benzo-nitrile-isopropylides, which differ by a p-substituent in the phenyl ring (H, F, OCH₃; Scheme 2). These dipolar species (4–6) are produced by irradiation of the corresponding 2 H-azirines ( 1–3 ; Scheme 1) in a 2-methylpentane glass at ?185°. Their UV. spectra are reproduced in the Figure. The spectra of 4 and 5 are characterized by an ‘aromatic band’ at short wavelength, and a longer wavelength band at approximately 275 nm, which is considered to be characteristic of the nitrile-ylide system. The UV. spectrum of the methoxy derivative 6 , which shows a broad absorption at 260 nm, arises by an addition of the ‘nitrile-ylide band’ and the anisole band. The three dipolar species 4–6 do not show any significant differences in the regioselectivity of the cycloaddition with methyl α-methacrylate even though F and OCH₃ have quite different σ-constants (Scheme 1). The addition according to modus A is very much preferred (B/A = 0,076). – It seems, that the substituents F and OCH₃ do not affect the physical and chemical behaviour of the parent benzonitrile-isopropylide (4) . All three dipolar species 4–6 react regiospecifically according to modus A with methyl trifluoroacetate (Scheme 1). The regioselectivity is reduced in the cycloaddition of 4 with methyl propiolate and ethyl phenylpropiolate (B/A = 0,04 and 0,28, respectively). The reduced regioselectivity in the latter case may be attributed to a reduced polarity of the triple bond in the dipolarophile.