PHOTOCHEMICAL NITRATION BY TETRANITROMETHANE .40. REGIOCHEMISTRY OF TRINITROMETHYL ATTACHMENT IN THE PHOTOLYSIS OF BENZOFURAN WITH TETRANITROMETHANE

The photolysis of the charge-transfer (CT) complex of benzofuran (3) a nd tetranitromethane in dichloromethane by light exciting only the CT complex (lambda>435 nm) gives the epimeric pairs of adducts, 3-nitro-2 -trinitromethyl-2,3-dihydrobenzofurans 10 and 11, 2-nitro-3-trinitrome thyl-2,3-dihydrobenzofurans 12 and 13, 3-hydroxy-2-trinitromethyl-2,3- dihydrobenzofurans 15 and 16, 7-nitro-4-trinitromethyl-4,7-dihydrobenz ofurans 17 and 18, nitronic ester 14, dinitro dimers 19 and (21), 6-ni tro- and 4-nitro-benzofurans 9 and 22, and 4-trinitromethylbenzofuran 21. In acetonitrile solution similar photolysis gives the same group o f products and in addition the epimeric pairs, 2,3-dinitro-2,3-dihydro benzofurans 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 at tack of trinitromethanide ion at C2 in the benzofuran radical cation 3 (.+), adducts 12, 13 and 14 by similar attack al C3, and adducts 17 an d 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 o f benzofuran (3) and tetranitromethane gives substantially the dinitro dimers 19 and 20, with small amounts of 3-nitro- (8), 6-nitro-(9), an d 4-nitro-benzofuran (22). Reaction of benzofuran (3) with nitrogen di oxide in dichloromethane in the dark gives mainly the 2,3-dinitro addu cts 4 and 5, and the 3-hydroxy-2-nitro adducts 6 and 7. High-level qua ntum chemical calculations (CASSCF/CASPT2), in combination with the va lence bond configuration mixing (VBCM) model of the transition state o f the radical cation-nucleophile reaction, were performed in order to obtain reactivity indices expressing the reactivity of the various sit es of 3(.+) toward a nucleophile for comparison with the experimental results. X-ray crystal structures are reported for compounds 5, 8, 14 and 21.