Synthesis and structure of annulated dithieno[2,3-b;3ʹ,2ʹ-d]thienyl- and ring-opened 3,3ʹ-bithienyl Fischer carbene complexes

Zandria Lamprecht,Frederick P Malan,David C Liles,Simon Lotz,Daniela I Bezuidenhout

doi:10.1016/j.jorganchem.2020.121466

Zandria Lamprecht, Frederick P Malan + Show 3 more

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https://doi.org/10.1016/j.jorganchem.2020.121466

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Abstract

Nucleophilic attack on the central sulphur of dithieno[2,3-b;3ʹ,2ʹ-d]thiophene (DTT) by n-BuLi opens the central thiophene ring and afforded, after subsequent reaction with Cr(CO)6 and alkylation with [Et3O][BF4], a series of mono- and biscarbene complexes containing a 3,3ʹ-dithienyl backbone with a SBu substituent. Repeating the reaction with diisopropylamine as the nucleophile, leads to a dihydrodesulphurization reaction with ring-opening of the central thiophene ring of DTT and elimination of the sulphur atom. Subsequent reaction with n-BuLi or LDA, Cr(CO)6 and [Et3O][BF4] afforded 3,3ʹ-dithienyl mono- and biscarbene complexes. In both instances α,αʹ-dithienothiophene biscarbene complex was observed spectroscopically but not isolated. By using α,αʹ-dibromodithieno[2,3-b;3ʹ,2ʹ]thiophene as substrate, improved yields of the targeted mono- and biscarbene complexes of [2,3-b;3ʹ,2ʹ-d]-DTT (M=Cr, W) could be obtained. The biscarbene complexes were unstable in the reaction mixture but in the case of tungsten could be isolated after in situ aminolysis with dimethylamine. The use of KHMDS as base converted Cr(CO)6 to K[Cr(CO)5(CN)] and after reaction with DTT and subsequent alkylation with [Et3O][BF4], afforded the chromium tetracarbonyl carbene complex of DTT.

Highlights

Compared to chain structures of α,α-oligo- and polythiophenes, planar condensed thiophenes display more efficient and extended π -conjugation, better p-orbital overlap, improved charge transfer properties and higher chemical stability [1]. α- and βannulated oligothiophenes, dithienothiophene (DTT) derivatives have already been developed as promising semiconducting materials, organic conductors, photosensitive materials and light emitting devices [2,3,4,5,6,7].The preparation of annulated thiophenes is challenging and examples in organometallic chemistry are limited
Carbene complexes were prepared by making use of P1 in three different methods by using a range of strong bases: n-BuLi, a mixture of nucleophilic diisopropylamine (HDA) and n-BuLi (with subsequent formation of lithium diisopropylamide (LDA)), and potassium bis(trimethylsilyl)amide (KHMDS)
Reactions were performed at low temperatures (–78°C) to create the opportunity to discriminate between reaction sites

Summary

Introduction

Compared to chain structures of α,α-oligo- and polythiophenes, planar condensed thiophenes display more efficient and extended π -conjugation, better p-orbital overlap, improved charge transfer properties and higher chemical stability [1]. α- and βannulated oligothiophenes, dithienothiophene (DTT) derivatives have already been developed as promising semiconducting materials, organic conductors, photosensitive materials and light emitting devices [2,3,4,5,6,7].The preparation of annulated thiophenes is challenging and examples in organometallic chemistry are limited. Compared to chain structures of α,α-oligo- and polythiophenes, planar condensed thiophenes display more efficient and extended π -conjugation, better p-orbital overlap, improved charge transfer properties and higher chemical stability [1]. When considering annulated thiophenes as precursors for organometallic compounds, stability factors need to be taken into account [8,9]. Derivatives of the six possible constitutional isomers of DTT are planar, electron excessive, π -conjugated and aromatic. D]-DTT with the sulphur atoms in relative orientations up-down-up, displays a linear conjugated pathway throughout the molecule, allowing communication between the α-positions without involving the sulphur atoms. A linear conjugated pathway through [2,3-b;3ʹ,2ʹ-d]-DTT, another isomer, is made impossible by the central cross-conjugated double bonds (sulphur atoms orientated on the same side of the ring, up-up-up).

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