Quantum chemical screening reveals that 4H-dithieno[2,3-b:3',2'-e][1,4]thiazines possess the highest HOMO among four constitutional isomers, even 0.27 eV higher in energy than the well established 10H-phenothiazine. N-Substituted 4H-dithieno[2,3-b:3',2'-e][1,4]thiazines are readily accessible by twofold Pd-catalyzed amination. According to cyclic voltammetry dithienothiazines are reversibly oxidized and can be considered as new donors for functional π-systems.
2,6-Di(hetero)aryl substituted dithienothiazines with fine-tunable electronic properties are efficiently accessible by lithiation–lithium–zinc exchange–Negishi cross-coupling in a one-pot fashion.
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...
Based upon a twofold bromine-lithium exchange with 2,5-dibromo thiophene and sequential trapping of the dilithio intermediate, organo zinc halides were generated in situ and subsequently transformed by Negishi cross-coupling to unsymmetrically substituted thiophenes in a one-pot fashion. Application of this novel sequence to diiodo(hetero)arenes quickly furnishes highly interesting building blocks for materials science applications.Halogen-metal exchange offers a versatile transformation of organic halides into carbon nucleophiles which can be subsequently trapped by addition of a broad variety of electrophiles in a one-pot fashion. 1 In case of dihalogenated aryl compounds, dual halogen-metal exchange and dual electrophilic trapping furnishes symmetrical products. However, desymmetrizing symmetrical dilithio(hetero)arenes to unsymmetrical target molecules represents an attractive synthetic manipulation. Usually, this transformation has been carried out by repetitive brominelithium exchange followed by electrophilic trapping in a stepwise fashion with intermediate workup and purification after each bromine-lithium exchange-trapping sequence. 2 However, direct sequential electrophilic trapping of dilithio species in a one-pot fashion appears to be more efficient and economical. Just recently, we reported first examples of this type of one-pot methodology disclosing a straightforward and effective way to desymmetrized products. Upon variation of the electrophilic species, the sequential trapping methodology has become an entry to new consecutive multicomponent processes in a one-pot fashion (Scheme 1). 3Interestingly, the order of electrophile addition can even open access to highly reactive organometallic species setting the stage for a subsequent cross-coupling reaction in a consecutive one-pot fashion. Therefore, we set out to introduce zinc bromide as a second electrophile for generating unsymmetrical organozinc halides, suitable nucleophiles for envisioned Negishi coupling. 4 Here we communicate a facile one-pot, three-component brominelithium exchange-sequential electrophilic trappingNegishi coupling sequence that rapidly leads to unsymmetrically substituted thiophenes.Upon double bromo-lithium exchange of 2,5-dibromothiophene (1) with n-butyllithium in the presence of TMEDA at low temperatures, the corresponding dilithio species is generated in situ. 3 To ensure quantitative conversion, reverse addition was chosen, that is, 2,5-dibromothiophene was added dropwise to a precooled solution of n-butyllithium. 5 Then, sequential transmetalation with trimethylsilylchloride and anhydrous zinc bromide furnishes an in situ generated organozinc derivative that is immediately submitted to Negishi coupling in the same pot, simply by addition of catalytic amounts of tetrakis(triphenylphosphine)palladium(0) and a slight excess of various iodo(hetero)arenes 2, to give unsymmetrically substituted thiophenes 3 in moderate to good yields (Scheme 2, Table 1). 6 The structures of all new compounds have been unambiguously assign...
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