1.4- dibromo-2,5-dimethoxybenzene

The reactions in this group comprise ring substitution and dealkylation of benzenoid compounds and syntheses with 1,3-dimethoxy compounds in the bicyclic and polycyclic series. The monosubstitution of polyalkoxy benzenes is generally not facile. Monobromination of 1,3-dimethoxybenzene in carbon tetrachloride was effected at 30°C during 2 hours with N-bromosuccinimide in the presence of acidic silica gel (type microbead 3A) in 99% yield without formation of dibromo compounds (ref.141). The evolution of HBr is avoided. 

To a solution of thallium(III) trifluoroacetate (5.50 g, 10 mmol) in trifluoroacetic acid (25 ml), 4-bromo-l,2-dimethoxybenzene (422, 4.34 g, 20 mmol) was added and the resulting mixture was stirred until a thallium(III) test was negative (the presence of unreacted Tl causes the appearance of brown-black precipitate of TI2O3 in the reaction of an aliquot with dilute aq. NaOH). Then, the reaction mixture was poured into water (150 ml) and extracted with chloroform (2x100 ml). The extracts were dried (MgS04), and passed through a short column of alumina and 3.50 g (81%) of pure 2,2 -dibromo-4,4 ,5,5 -tetramethoxybiphenyl (421), m.p. 159-160 °C, was obtained after evaporation and crystallization from toluene / petroleum ether (b.p. 100-120 °C). 

Unlike the corresponding oxa-crowns, crown thiothers with fused benzo rings have seen relatively little work. Klar and coworkers [66] prepared the hexa-methoxy derivative of tribenzo-9S3 by a modified Adams-Ferretti reaction between l,2-dibromo-4,5-dimethoxybenzene with l,2-dimercapto-4,5-dimethoxybenzene (Table 4). A variant of this procedure also affords the corresponding 12S4 analogue with four fused benzo units [67]. 

Many monomers for organometallic polycondensation are commercially available, which include p- and nr-dihalobenzenes and diboronic compounds of benzene e.g., / -C6H4(B(OH)2)2 and its bis(pinacol) ester). Various dihalo-benzenes with substituents e.g., l,4-dibromo-2,5-dimethoxybenzene and 1,4-dibromo-2,5-dimethylbenzene) are also commercially available. However, some of the dihalobenzenes with substituents have to be synthesized, for example, using Br2 and V-bromosuccinicimide (NBS). 

Di-Me ether [120231-43-8]. l,2-Dibromo-3,6-diiodo-4,5-dimethoxybenzene. 4,5 Dibromo-3,6-diiodoveratrole CgH BrjIjOj M 547.751 Needles (CHCl3/MeOH). Mp 144-145°. 

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