:
The regioselectivity of the reaction of 2,5-dihydroxy-[1,4]-benzoquinone (DHBQ) with diamines could not be
explained satisfactorily so far. In general, the reaction products can be derived from the tautomeric ortho-quinoid structure
of a hypothetical 4,5-dihydroxy-[1,2]-benzoquinone. However, both aromatic and aliphatic 1,2-diamines form in some cases
phenazines, formally by diimine formation on the quinoid carbonyl groups, and in other cases the corresponding 1,2-
diamino-[1,2]-benzoquinones, by nucleophilic substitution of the OH groups, the regioselectivity apparently not following
any discernible pattern. The reactivity was now explained by an adapted theory of strain-induced bond localization (SIBL).
Here, the preservation of the "natural" geometry of the two quinoid C–C double bonds (C3=C4 and C5=C6) as well as the
N–N distance of the co-reacting diamine are crucial. A decrease of the annulation angle sum (N–C4–C5 + C4–C5–N) is tolerated well and the 4,5-diamino-ortho-quinones, having relatively short N–N spacings are formed. An increase in the angular sum is energetically unfavorable, so that diamines with a larger N–N distance afford the corresponding ortho-quinone
imines. Thus, for the reaction of DHBQ with diamines, exact predictions of the regioselectivity, and the resulting product
structure, can be made on the basis of simple computations of bond spacings and product geometries.
Between Aromatic and Quinoid Structure: A Symmetrical UV to Vis/NIR Benzothiadiazole Redox Switch
