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 phenazines, in
some cases, 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 does not follow 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.