Bromophenols, isomerization

Hydrobromic acid is used in the preparation of inorganic bromide salts. The acid also is used for many organic syntheses, including alkyl bromides from alcohols or olefins, and bromophenols from phenols. The compound also is used as an acid catalyst in many alkylation, selective oxidation, isomerization, and dehydrogenation reactions. Other apphcations are in extraction of minerals and use as a reducing agent. 

Photolytic cyclization of the phenolic bromo compounds 278 and 279 led to ( + )-oxocrinine (275) (methylenedioxy instead of methoxy-hydroxy) and ( )-oxomaritidine (275), known intermediates to crinine and maritidine (277) (59, 60). Oxocrinine was also obtained by photolysis of the isomeric bromophenol 280 (61). 

The mechanism of the aqueous photochemistry of 4-chlorophenol has been reviewed earlier [5,8]. Its basic features are the same as those of the carbene pathway described above for 2-bromophenol. The main differences are the fact that this is the only photolytic reaction of 4-chlorophenol and that its quantum yield is considerably higher than that of the 2-sub-stituted analogues

triplet carbene, 4-oxocyclohexa-2,5-dienylidene (A.max = 384 and 370 nm) from aqueous 4-chlorophenol (see Fig. 1) [20]. Photoproduct analysis yielded p-benzo-quinone (in the presence of O2), phenol (in the presence of an alcohol), hydroquinone and isomeric chlorodihydroxybiphenyls, which could all be accounted for by carbene reactions [20]. 

In HF-SbFs solution 4-aIkyl- and 2,6-dialkylphenols and their methyl ethers (anisoles) are converted to the O-protonated forms, which react with Br2 to afford m-bro-mophenol derivatives selectively (Scheme 14.35) [83, 84]. In HF-SbFs o- and p-bro-mophenols isomerize to m-bromophenol by a 1,2-Br shift in fhe protonated forms [85], When NaBr or KBr is used instead of Br2, the meta selectivity is reduced [86]. ElectrophiUc halogenation of arenes can also be performed with Ar SeCl [87]. 

Two syntheses of racemic and/or enantiomeric mixtures of pakistanamine are now available. In the first synthesis, ferricyanide oxidation of racemic berbamunine [ -(53)] resulted in formation of an isomeric mixture (54), which upon treatment with diazomethane provided an oil. Chromatography of this oil yielded racemic material corresponding to pakistanamine (55). More recently, irradiation of the dimeric bromophenol (58) in refluxing liquid ammonia containing potassium t-butoxide gave rise to pakistanamine in 25—30% yield. ... 

---