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

Abstract

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.

Exploring Solvent and Substituent Effects on the Excited State Dynamics and Symmetry Breaking of Quadrupolar Triarylamine End-Capped Benzothiazole Chromophores by Femtosecond Spectroscopy

“…In the present work, we apply two fs spectroscopic techniques, namely FU and fs-TA spectroscopy, to unravel the dynamics and symmetry breaking of three quadrupolar molecules of the type D-π-A-π-D comprising triarylamine end-donors and a functionalized benzothiazole acceptor core (Figure , left). The latter S , N -heteroarene unit is, due to its inherent stability and electron-withdrawing nature, recognized as an efficient building block in chromophores attractive for nonlinear optics, dye-sensitized solar cells, or light emitting materials with tunable fluorescence upon metal-ion complexation. − A systematic change of both the electron accepting core and electron donating edge substituents allowed us to examine the effect of molecular and electronic structure on SB and its dynamics. Specifically, the molecule Qbtz-H bears a benzothiazole electron accepting core and triphenylamine edge donors.…”

Microwave-assisted copper catalyzed decarboxylative reductive coupling ofpara-quinone methides with 3-indoleacetic acids: rapid access to polycyclic spiroindolequinone derivatives

“…This process may be a Cu-catalyzed radical Ullmann-type methylation reaction using DMSO as the methyl radical source. 18…”