Ipso substitution in selected phenolic systems

Abstract

Reaction of 2-chloro- (28a), 2-bromo- (28b) and 2-methyl (28c) 6-nitro-4-methylphenols with fuming nitric acid at 50° gives the 5-chloro (29a), 5-bromo- (29b) and 5-methyl- (29c) 2-methyl-3-nitro-1,4-benzoquinones respectively, each reaction involving methyl migration to the ring carbon adjacent to the nitro substituent. Reaction of 2,6-dibromo- (50) and 2,3,6-tribromo- (31) 4-methylphenols with fuming nitric acid at 50° gives the 2,6-dibromo- (51) and 2,3,5-tribromo- (48) 1,4-benzoquinones respectively, each reaction involving methyl loss. A reaction mechanism is proposed for these transformations. Reaction of tetrabromo- (55) and tribromo-6-nitro- (58) 4-methylphenols with fuming nitric acid at 50° gives the corresponding 4-nitrato-4-methylcyclohexa-2,5-dienones (56) and (59) via the 4-hydroxy-4-methylcyclohexa-2,5-dienones (57) and (60). Similar reactions are observed for 4-ethyl substrates. Treatment of tetrabromo- (63) and tribromo-6-nitro (66) 4-hydroxy-4-ethylcyclohexa-2,5-dienones with concentrated sulphuric acid gives the 3,5,6-tribromo- (71) and 5,6- dibromo-3-nitro- (67) 2-ethyl-1,4-benzoquinones. When the ethyl groups were replaced by methyl groups, reaction proceeds to give diphenylethane products (73) and (75). Treatment of tetrabromo- (57), tetrachloro- (77), 2,3,5-tribromo-6-methyl- (81) and 3,5-dibromo-2,6-dimethyl- (84) 4-hydroxy-4-methylcyclohexa-2,5-dienones with sodium hydroxide yields C3-hydroxy derivatives; the 2,5,6-tribromo 3,4-dihydroxy-4-methylcyclohexa-2,5-dienone (80) was identified by X-ray crystal analysis. Treatment of 2,3,5- tribromo-4-hydroxy-4-methyl-6-nitrocyclohexa-2,5-dienone (60) with sodium hydroxide gives 3,4-dibromo-5-methyl-5-(2’nitroethan-1'-one)-2(5H)-furanone (86), identified by X-ray crystal analysis. A mechanism is proposed for this reaction. Rearrangement of tetrachloro-4-methyl-4-nitrocyclohexa- 2,5-dienone (87) in (D)-chloroform gives tetrachloro-4- hydroxy-4-methylcyclohexa-2,5-dienone (77) and 3,4,6-trichloro- 5-methyl-1,2-benzoquinone (89). The rearrangement is further investigated using several solvents and variously substituted 4-methyl-4-nitrodienones, sometimes in the presence a radical scavanging phenol. A mechanism is postulated for the rearrangement.