BrBrII3aBr4aTMSBrSSTMSTMS5aTMSAPd(PPh3)2Cl2/CuI(i-Pr)2NHtoluene1. 4eq t-BuLi2. S83. EtOHTMS
Scheme 1. Synthesis of the anti-BDT precursor 5a.15a,19
TMSSHHSTMS4bTMSBrBrSSTMSTMS5b1. 4eq t-BuLi2. S83. EtOHTMS
Scheme 2. Synthesis of the syn-BDT precursor 5b.
Modifying the latter literature procedure towards the anti-derivative 5a,15a we were able to obtain the syn-derivative 5b by iodination of 1,3-dibromo-benzene followed by Pd-catalyzed Cassar-Heck-Sonogashira coupling of TMS-acetylene;19 subsequent treatment of 4b with four equivalents of tert.-butyllithium provided the dilithiated intermediate that was quenched subsequently with sulfur and ethanol (Scheme 2). The resulting dithiol is not stable towards rearrangement and the trimethylsilyl functionalized syn-BDT 5b is formed in situ through an intramolecular cyclization reaction, similar to the formation of the anti-isomer 5a.
Notably, our first attempts to further functionalize this synthon for subsequent reactions, involved the bromination of the 2,6-positions. This functionalization was then to be utilized in a cross-coupling protocol to extend the degree of p-conjugation within our materials by introduction of various electron–donating or –withdrawing aromatic groups. However, bromination of 5b with two equivalents of NBS in analogy to reported procedures for somewhat related dithienothiophenes21 gave only an inseparable mixture of several brominated products. For that reason we shifted our focus towards an inversely functionalized building block suitable for cross-coupling reactions. Introduction of suitable functional groups, such as boryl (Suzuki- Miyaura) or stannyl (Stille), can usually be achieved by lithiation of the aryl and treating it with an alkoxy/hydroxy boronate or a chloro stannane.22 To provide the necessary reactivity at our building block, desilylation with tetrabutylammonium fluoride (TBAF)15b was performed to provide 2 selectively. The parent syn-BDT compound 2 was then functionalized with two equivalents of tributyltin chloride after lithiation with n-butyllithium at -78ºC in THF to give 6 as light orange oil. This difunctionalization with tributyltin groups subsequently allowed for Stille coupling of 6 with halogenated arenes, as expected (Scheme 3).
