2,6-Difunctionalization of N-Substituted Dithienothiazines via Dilithiation

Abstract

The regioselective lithiation of dithienothiazines followed by electrophilic trapping in a one-pot fashion is an efficient route to 2-mono-and 2,6-difunctionalized dithienothiazines. A pseudo five-component dilithiation-diformylation-double-Wittig olefination sequence gives a dithienothiazine symmetrically functionalized with α,β-unsaturated ester side chains in excellent yield.In recent years, interest in electroactive organic molecules has increased enormously because of their important technological applications, ranging from organic light-emitting diodes 1 through organic photovoltaic devices 2 to organic field-effect transistors. 3 The main advantages of using organic materials are their low production costs; their favorable properties, such as flexibility, transparency, and light weight; and their good processability.We recently described dithienothiazines as a new class of electron-rich heterocycles. 4 As a consequence of their unique electronic properties, which show two reversible oxidations with Nernstian behavior at low oxidation potentials, dithienothiazines are well suited, in principle, for use as hole conductors or as donor components in donoracceptor compounds. Functionalization of the heterocyclic core of dithienothiazines represents a key step to achieving potential applications of these compounds, and is a major challenge. Most interestingly, annelation of thiophene offers an easy entry to typical thiophene transformations, such as lithiation in the α-position with respect to the sulfur atom; 5 furthermore, it is also amenable to sequential one-pot processing. 6 Here, we report a practical scale-up of the synthesis of three selected N-substituted dithienothiazines, together with the functionalization of this new class of electron-rich heterocycles through dilithiation and electrophilic trapping.We examined the scale-up of the intermolecular/intramolecular Buchwald-Hartwig synthesis of 4-phenyl-4H-dithieno[2,3-b:3′,2′-e][1,4]thiazine (3a) from bis(3-bromo-2-thienyl) sulfide (1) and aniline (2a). Linear scale-up from 0.5 to 3 mmol was uneventful (Scheme 1). Furthermore, we were able to reduce the catalyst loading to 5 mol% and the ligand loading to 10 mol% without any decrease in yield. Besides aniline (2a), hexan-1-amine (2b) was successfully used as an amine component, albeit with a lower yield of the corresponding dithienothiazine 3b. Nevertheless, the introduction of a solubilizing hexyl group is very attractive. Therefore, by combining a hexyl substituent with the higher yield of aniline derivatives, we successfully introduced a 4-hexylphenyl substituent in the dithienothiazine 3c.Scheme 1 Synthesis of selected dithienothiazines 3 by twofold Buchwald-Hartwig coupling Next we tested several methods for functionalizing dithienothiazines by exploiting the inherent reactivity of thiophenes. However, attempted bromination with bromine or N-bromosuccinimide as brominating reagent 5a to give 2,6-dibromodithienothiazines failed in a range of solvents and at various reaction temperatures. As a last r...

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

“…Scheme 4. Diversity-oriented synthesis of syn-syn dithieno [1,4]thiazines 2a via inter-intra-molecular Buchwald-Hartwig coupling; a: Dielectric heating at 160 °C [35][36][37]. In the case of syn-syn isomers 2a the rather time-consuming synthesis according to Grol (Scheme 2) was cut short by an alternative strategy for generating the 1,4-thiazine ring.…”

“…Important dithieno [1,4]thiazine building blocks 5, for example, aldehydes, iodides, or alcohols, are accessible with mostly good yields using dimethylformamide (DMF), iodine, or acetone as electrophiles, respectively [36,37].…”

“…α-Lithiation of thiophenes using n-butyllithium sets the stage for various 2,6-functionalization. Thereby, di-as well as monofunctionalizations are readily realized [36,44,45].…”

“…Moreover, the Wittig reaction could be implemented in a one-pot sequence starting from 2,6-unsubstituted dithieno [1,4]thiazine 2a-f, circumventing the isolation of the intermediary aldehyde. This pseudo-five-component reaction is clearly superior to the stepwise synthesis due to the shortened preparation time and increased yield (Scheme 10) [36]. These building blocks set the stage for a diverse range of follow-up chemistry.…”

“…Scheme 10. Synthesis of the π-expanded dithieno [1,4]thiazine 5f in the sense of a pseudo five-component formylation-Wittig sequence via post-functionalization of syn-syn 2a-f; a: 58% overall-yield in a two-step synthesis isolating the aldehyde intermediate [36].…”