Synthesis of thiophene-based ligands in micellar solution
P. Quagliotto, C. Barolo, N. Barbero, R. Buscaino, E. Chiavazza, G. Viscardi
Dipartimento di Chimica and Interdepartmental “Nanostructured Surface and Interfaces” NIS Centre, Università di Torino, via P. Giuria 7, 10125 - Torino.
e-mail: pierluigi.quagliotto@unito.it
Thienylpyridines and thienyloligopyridines can be used for several applications,1,2
including conductive polymers.3 The inclusion of thiophene-based N-ligands into
polymers allows to obtain materials that can be responsive to the coordination of metals or alkylating reagents.4 The urge for synthetic procedures complying with Green
Chemistry pushed us to explore the Suzuki reaction in a micellar environment, by combining thiopheneboronic acids with bromopyridines, with excellent yields.5 Now,
we propose the preparation of thienylpyridines and thienylbispyridines with the Suzuki reaction using less reactive and cheaper chloropyridines in a system consisting of Pd(OAc)2 (2%) or PdCl2 (1-2%), an electron releasing phosphine, XPhos (1.25 eq.
based on Pd), thiopheneboronic acids, CTAB and water.
Scheme 1
The reactions were explored mainly at room temperature, obtaining good yields (54 to 80%) as a function of boronic acids and chloropyridines structure. The 4-chloropyridine reacted at higher temperature with respect to 2-chloropyridine, and gave a 54 % yield with 2-thiopheneboronic acid at 80°C. The 3-thiopheneboronic acid reactivity was lower than its 2- isomer and required longer times (24h) to give good yields.
A strong base such as NaOH performed better than a weak base, K2CO3. A four-fold
excess of base over the limiting reagent was found optimal.
Using PdCl2 as catalyst, instead of Pd(OAc)2, slightly lower yields were obtained with
chloropyridines. PdCl2 performed better with bromopyridines5 at 80°C. Its ability to
give surfactant-stabilized Pd nanoparticles was demonstrated by TEM imaging. A trial reaction with the classical Pd(PPh3)4 catalyst was performed with 3-thiopheneboronic
acid and 2-chloropyridine at 80°C for 30 min with the help of microwaves, giving substantially similar yields as those obtained with Pd(OAc)2 and XPhos at 25°C reacting
for 24h. The Pd(OAc)2 / XPhos system can thus be exploited, using lower temperature
and lower loading of phosphine compounds.
A copolymer containing 3-hexylthiophene and a thiophene-based bispyridine ligand was also prepared and studied.
1) O’Regan, B.; Grätzel, M. Nature 1991, 353, 737-740.
2) Mishra, A.; Ma, C.; Bauerle, P. Chem.Rev. 2009, 109, 1141-1276.
3) Manca, P. Pilo, M.I.; Casu, G.; Gladiali, S.; Sanna, G.; Scanu, R.; Spano, N.; Zucca, A.; Zanardi, C.; Bagnis, D. Valentini, L. J. Polym Sci. 2011, 49, 3513-3523.
4) Gannot, Y.; Hertzog-Ronen, C.; Tessler, N.; Eichen, Y. Adv. Funct. Mater. 2010, 20, 105-110. 5) Quagliotto, P.; Barolo, C.; Carfora, P.; Prosperini, S., Viscardi G. submitted.
