Regioselectivity in the reactions of the acetonate ion with electron-deficient arenes

The reactions of the acetonate ion with 1,3-dinitro-5-X-, 1.3-X-2-5-nitro- (X = NO2, CN, COOCH3, CONH2, COO-, and H), and 1,3,5-tricyanobenzenes were studied by H-1 NMR and electronic absorption spectroscopy and by quantum-ch emical methods. Tile kinetic factor is decisive for the initial attack of t he carbanion on the C(2) atom of unsymmetrical arenes. However, a-adducts i n which a nucleophile is added to the C(4) atom are more stable thermodynam ically. In the case of 1,3-X-2-5-nitrobenzenes (X = CN, COOCH3, or CONH2), the sigma -adducts with the acetonate group in para-position to the X group unexpectedly proved to be very stable. The structures of the a-adducts bas ed on trinitro- and 1,3-dinitro-5-cyanobenzenes were determined by X-ray di ffraction analysis. Quantum-chemical calculations (the AM1 and PM3 semiempi rical methods and the density functional method) were used to interpret the reaction regioselectivity and the molecular and electronic structures of t he sigma -adducts.