Zur Regioselektivitat von Cycloadditionen photochemisch erzeugter Benzonitril-isopropylide

This paper deals with the physical and chemical differences of zwitterionic benzonitrile-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 2H-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 alpha-methacrylate even though F and OCH3 have quite different sigma-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-isopropyiide (4). All three dipolar species 4-6 react regjospecifically according to modus A with methyl trifluoroacetate (Scheme 3). 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