Ipso-nitration studies

Nitration of 2,6-dimethyl-4-nitrophenol (40a) with fuming nitric acid gives the pair of C2-epimeric cyclohex-3-enones, (41) and (42), the dihydroxy cyclohex-3-enone and the 2,6-dimethyl-3,4-dinitrophenol (43). Reaction of the nitro phenol (40a) with nitrogen dioxide also gives compounds (41), (42), (43) and (44). The nitration of 2,6-dimethyl-4-bromophenol (40c) with fuming nitric acid (addition of the phenol to the acid) yields both possible C2-epimeric cyclohex-3-enones, (53) and (54), the trinitro cyclohex-3-enone (55) which decomposes to give the dinitro phenol (43), nitro phenol (58) and the 1,4-benzoquinone derivative (59). Nitration of the bromo phenol (40c) in fuming nitric acid (addition of the acid to the phenol) and reaction of the bromo phenol (40c) with nitrogen dioxide both lead to extensive nitrodebromination. The possible reaction pathways for phenols (40a) and (40c) are discussed. Nitration of 1,2,3,5-tetramethylbenzene (66a) with fuming nitric acid gives the tetramethylnitrobenzene (85), products of side-chain modification (86)-(90), the rearranged 6,6-dimethylcyclohexenones (91), (92), (93) and (94), and 2,3,4,6-tetramethyl ketone derivatives (73)-(76), (95) and (96). Reaction of 2,3,4,6-tetramethylphenol (71) with nitrogen dioxide gives the hydroxy dinitro ketone (72) in addition to the trinitrocyclohexenones (74)-(77) and (82). The possible modes of formation of these compounds are discussed. Nitration of 1,2,3-trimethyl-4,6-dinitrobenzene (103) with fuming nitric acid gives dimethylpropanedioic acid (108) (72%), hydroxy dienone (110) (8%) and the substituted benzoic acid (109) (9%). Corresponding nitration of 1,2,4,5-tetramethyl- 3,6-dinitrobenzene (117) gives the nitro dicarboxylic acid (119) (33%), dimethylpropanedioic acid (108) (11%) and the substituted benzoic acid (118) (49%). Compounds (108) and (119) are products of reaction pathways involving ipsosubstitution, followed by methyl migration. Nitration of 2,4-dimethyl-6-nitrophenol (128) with fuming nitric acid gives two 1,4-benzoquinone derivatives (129) and (130), in addition to the two C4-epimeric cyclohex-2-enones (131) and (132), and a single cyclohex-3-enone (133). In addition, reaction of 2,4-dimethyl-6-nitrophenol (128) with nitrogen dioxide also gives (129), (130), (131), (132) and (133). Comment is made on the reaction mechanism and on the probable mode of conversion of the cyclohex-3-enone (133) to give the C4-epimeric cyclohex-2-enones (131) and (132). The structures of nine compounds (42), (44), (54), (74), (82), (92), (93), (94) and (96), have been determined unambiguously by single crystal X-ray structure analyses