Game of Isomers: Bifurcation in the Catalytic Formation of Bis[1]benzothieno[1,4]thiazines with Conformation-Dependent Electronic Properties

Abstract

Two regioisomers of bis[1]­benzothieno­[1,4]­thiazine are unexpectedly obtained by tuning the catalytic conditions of the intermolecular–intramolecular Buchwald–Hartwig amination. Mechanistic insights and evidence of intermediates support a conclusive mechanistic rationale. Furthermore, a computationally based study on the influence of conformational aspects on the HOMO energy level of anellated 1,4-thiazine paves the way to enhance the electronic properties, thus successfully achieving higher luminescent and easier oxidizable syn–syn bis[1]­benzothieno­[1,4]­thiazines.

Dithieno[1,4]thiazines and Bis[1]benzothieno[1,4]thiazines—Organometallic Synthesis and Functionalization of Electron Density Enriched Congeners of Phenothiazine

“…Bis [1]benzothieno [1,4]thiazines 9 not only fluoresce in solution but also in the solid state, depending on their conformation, which, in turn, can be controlled by the anellation mode and the substitution. Ultimately, a rather unique flattened conformation of the 1,4-thiazine ring is adopted in the solid state, for example, for bis [1]benzothieno [1,4]thiazine 9c (R = t-Bu) [49,50]. In addition, the electronic properties in the solid state (redshifted fluorescence) and NICS (nucleus-independent chemical shift) calculation strongly support the antiaromaticity of the fully planarized conformation of anti-anti bis [1]benzo-thieno [1,4]thiazines 9c prevalent in the solid state.…”

“…There are three regioisomers, the syn-syn, the syn-anti and the anti-anti isomer (Scheme 16). Their synthesis is generally in accordance with the synthesis of the dithieno [1,4]thiazines 2 (Scheme 4) [49,50]. A broad spectrum of N-functionalization, including electron-withdrawing and electron-donating N-aryl substituents, is also accessible.…”

“…Scheme 16. Synthesized bis [1]benzothieno [1,4]thiazine regioisomers 9 ( a Additional examples of 9b were obtained as side products in the synthesis of syn-syn isomers 9a) [49,50].…”

“…In some cases, an isomerization occurred during Buchwald-Hartwig coupling using dppf as a ligand. For example, starting from syn-syn bis(3-bromobenzo[b]thiophen-2-yl)sulfane (10a), a 3:1 mixture of the syn-syn and the syn-anti bis [1]benzothieno [1,4]thiazines 9a and 9b was obtained using dppf [49]. Bulkier ferrocene ligands like 1,1 -bis-(dicyclohexylphosphano)ferrocene (dcpf) or 1,1 -bis(di-tert-butylphosphano)ferrocene (dtbpf) decreased the isomerization significantly.…”

Radical cations and dications of bis[1]benzothieno[1,4]thiazine isomers

“…24,25 Recently, we successfully proposed the two-fold thieno expansion of phenothiazine to bis [1]benzothieno [1,4]thiazines (BBTT) that can be considered to be novel electron-enriched phenothiazine congeners (Scheme 1). 26 The resulting compounds likewise possess discretely adjustable properties that depend on the mode of anellation, as for dithienothiazines, 27 as well as on the substitution pattern. Different steric hindrance unequivocally causes the absence of measurable luminescence in syn-syn BBTTs (bis [1]benzothieno[2,3-b:3′,2′-e] [1,4]thiazines), whereas inverse anellation of benzo [b]thiophene units leads to luminescence for syn-anti BBTTs (bis [1]benzothieno[2,3-b:2′,3′-e] [1,4] thiazines) peaking for anti-anti BBTTs (bis [1]benzo-thieno[3,2-b:2',3'e] [1,4]thiazines) with fluorescence quantum yields drastically increased to 45%.…”