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/Et 3 N 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 CH 3 SCH 2 + 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 CH 3 SCH 2 + , 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 NMe 2 -functionalized bibenzothiazoles remains intensive even upon complexation with ZnCl 2 , 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.