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DOI: 10.1055/s-0028-1088049
Synthesis of 3,6-Dimethoxybenzene-1,2-diamine and of 4,7-Dimethoxy-2-methyl-1H-benzimidazole
Publikationsverlauf
Publikationsdatum:
14. April 2009 (online)
Abstract
Hydrogenation of a mixture of ortho- and para-dinitro derivatives of 1,4-dimethoxybenzene in ethyl acetate under palladium catalysis, allows 3,6-dimethoxybenzene-1,2-diamine to be isolated as the sole product; this diamine is then converted into 4,7-dimethoxy-2-methyl-1H-benzimidazole, a building block for the preparation of imidazobenzo(hydro)quinones.
Key words
benzimidazoles - diamines - heterocycles - hydroquinones - reduction
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References
Ref. 7 reads that this is a co-crystallization process and that, provided there are no ‘small aggregates’, the separation of ‘rhombic plates’ from ‘long columns’ can be effected by hand.
9The ratio of isomers is determined by relative integration in ¹H NMR spectra, for data see experimental.
10The mp recorded for this material is in the range of literature values (68-70 ˚C,4e 75 ˚C,² 85-87 ˚C4a). Note that this material is labile: darkening with time of this electron-rich diamine has been noted previously and we found experimentally that even after minimal exposure to air, 1 became unreactive.
11A H2-filled balloon is used, monitoring of the reaction can be performed by TLC or NMR (for data see experimental) but, most simply, it can be done by visual inspection as the mixture becomes colorless when reaction is complete.
12Hydrogenation of a 6:4 mixture of 3 and 4 was performed under the same protocol and, here also, only diamine 1 is isolated (51% yield; 85% based on 3).
15Impure 2 could not be freed from byproducts by crystallization or by chromatography, which furthermore led to severe losses of material.