One-Pot Reductive Methylation of Nitro- and Amino-Substituted (Hetero)Aromatics with DMSO/HCOOH: Concise Synthesis of Fluorescent Dimethylamino-Functionalized Bibenzothiazole Ligands with Tunable Emission Color upon Complexation

One-pot reductive N,N-dimethylation
of suitable nitro- and amino-substituted (hetero)­arenes can be achieved
using a DMSO/HCOOH/Et3N system acting as a low-cost but
efficient reducing and methylating agent. The transformation of heteroaryl-amines
can be accelerated by using dimethyl sulfoxide/oxalyl chloride or
chloromethyl methyl sulfide as the source of active CH3SCH2+ species, while the exclusion of HCOOH
in the initial stage of the reaction allows avoiding N-formamides as resting intermediates. The developed procedures are
applicable in multigram-scale synthesis, and because of the lower
electrophilicity of CH3SCH2+, they
also work in pathological cases, where common methylating agents provide N,N-dimethylated products in no yield or
inferior yields due to concomitant side reactions. The method is particularly
useful in one-pot reductive transformation of 2-H-nitrobenzazoles to corresponding N,N-dimethylamino-substituted heteroarenes. These, upon Cu­(II)-catalyzed
oxidative homocoupling, afford 2,2′-bibenzazoles substituted
with dimethylamino groups as charge-transfer N^N ligands with intensive
absorption/emission in the visible region. The fluorescence of NMe2-functionalized bibenzothiazoles remains intensive even upon
complexation with ZnCl2, while emission maxima are bathochromically
shifted from the green/yellow to orange/red spectral region, making
these small-molecule fluorophores, exhibiting large emission quantum
yields and Stokes shifts, an attractive platform for the construction
of various functional dyes and light-harvesting materials with tunable
emission color upon complexation.