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

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.