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.