Use of the extended one-pot (EOP) procedure for the preparation of ethynylated thiophene derivatives and related palladium-ethynylthiophene organometallic oligomers

The palladium-catalyzed coupling (Stille coupling) of 2,5-diiodothiophene (1) with tributyl(ethynyl)tin forms the 2,5-bis(ethynyl)thiophene (3) and tributyltin iodide as side product (step 1). Addition of lithium diisopropylamide (LDA) to this mixture causes deprotonation of the bis-alkyne and its reaction with the tin halide present in the medium to form the 2,5-bis[(tributyltin)ethynyl]thiophene (4) (step 2). To this mixture was subsequently added trans-dichlorobis(tri-n-butylphosphine)palladium (5), and the corresponding trans-bis(tri-n-butylphosphine)-mu- [2,5-bis(ethynyl)thiophene]palladium oligomer (6) was obtained (step 3). Alternatively, the same route can be directed toward the formation of ethynylated thiophene oligomers: after formation of the 2,5-bis[(tributyltin)ethynyl]thiophene (4) (step 2), addition of 2-iodothiophene (8) or 2-iodo-5-(trimethylsilyl)thiophene (10) led to the formation of 2,5-bis(2-thienylethynyl)thiophene (9) (step 3) and [2-trimethylsilyl(ethynyl)thiophene]-2,5-bisethynylthiophene (11) (step 3') respectively. The latter can be easily desilylated to obtain the [2-(ethynyl)thiophene]-2,5-bisethynylthiophene (13), while treatment of 9 with sec-BuLi/I-2 formed the 2,5-[2,2'-(5,5'-diiodo)bisthienyl]bisethynylthiophene (12). Through a sequence of transformations similar to steps 1-3, the oligo(iodo)ethynylthiophene 12 has been connected to the bis(tri-n-butylphosphine)palladium moiety to form the trans-bis(tri-n-butylphosphine)-,u-[2,2'-bis(ethynyl)thiophenel-2,5-bisethynylthiophene]palladium polymer (15). To compare the advantages of the above extended one-pot (EOP) procedures over classical routes, polymers 6 and 15 were also prepared by the copper-catalyzed reaction of trans-dichlorobis(tri-n-butylphosphine)palladium (5) with 2,5-bis(ethynyl)thiophene (3) and [2-(ethynyl)thiophene]-2,5-bisethynylthiophene (13).