Photochemical nitration by tetranitromethane. Part XL. Regiochemistry of trinitromethyl attachment in the photolysis of benzofuran with tetranitromethane

The photolysis of the charge-transfer (CT) complex of benzofuran (3) and tetranitromethane in dichloromethane by light exciting only the CT complex (λ>435nm) gives the epimeric pairs of adducts, 3-nitro-2-trinitromethyl-2,3-dihydrobenzofurans 10 and 11, 2-nitro-3-trinitromethyl-2,3-dihydrobenzofurans 12 and 13, 3-hydroxy-2-trinitromethyl-2,3-dihydrobenzofurans 15 and 16, 7-nitro-4-trinitromethyl-4,7-dihydrobenzofurans 17 and 18, nitronic ester 14, dinitro dimers 19 and (21), 6-nitro- and 4-nitro-benzofurans 9 and 22, and 4-trinitromethylbenzofuran 21. In acetonitrile solution similar photolysis gives the same group of products and in addition the epimeric pairs, 2,3-dinitro-2,3-dihydrobenzofurans 4 and 5, 3-hydroxy-2-nitro-2,3-dihydrobenzofurans 6 and 7, and 3-nitrobenzofuran (8). Adducts 10, 11, 15 and 16 are formed by attack of trinitromethanide ion at C2 in the benzofuran radical cation 3^·+, adducts 12, 13 and 14 by similar attack at C3, and adducts 17 and 18, and 4-trinitromethylbenzofuran (21) by attack at C4. In 1,1,1,3,3,3-hexafluoropropan-2-ol the analogous photolysis of the CT complex of benzofuran (3) and tetranitromethane gives substantially the dinitro dimers 19 and 20, with small amounts of 3-nitro- (8), 6-nitro- (9), and 4-nitro-benzofuran (22). Reaction of benzofuran (3) with nitrogen dioxide in dichloromethane in the dark gives mainly the 2,3-dinitro adducts 4 and 5, and the 3-hydroxy-2-nitro adducts 6 and 7. High-level quantum chemical calculations (CASSCF/CASPT2), in combination with the valence bond configuration mixing (VBCM) model of the transition state of the radical cation-nucleophile reaction, were performed in order to obtain reactivity indices expressing the reactivity of the various sites of 3^·+ toward a nucleophile for comparison with the experimental results. X-ray crystal structures are reported for compounds 5, 8, 14 and 21.