Efficient Preparation of Photoswitchable Dithienylethene-Linker-Conjugates by Palladium-Catalyzed Coupling Reactions of Terminal Alkynes with Thienyl Chlorides and Other Aryl Halides
M. Zastrow , S. Thyagarajan, S. Ahmed, P. Haase, S. Seedorff, D. Gelman, J. Wachtveitl, E. Galoppini, K. Ruck-Braun
Chemistry, an Asian journal (2010), 5(5), 1202-12
Three photochromic dithienylethene-linker-conjugates with an adamantane core containing different spacer lengths and footprint areas with carboxylic anchoring groups are synthesized. The synthetic routes start either from the ethynylene-linker 5 or the iodo-substituted linker 8. Reaction conditions for the final Sonogashira coupling step between ethynylene-linker 5 with the chloro-substituted dithienylethene 4 in the presence of [PdCl2(CH3CN)(2)]/X-Phos and Cs2CO3 or K3PO4 are optimized using 2-chloro-5-methylthiophene (9) and triethylsilylacetylene or triisopropylsilylacetylene (10a,b) as model compounds. Experimental conditions are found to suppress the activation of the C(sp)-Si bond in TIPS-acetylene 10b, a reaction leading to a subsequent cross-coupling reaction to form by-product 12. Furthermore, activation of the C(sp)-Si bond in the presence of the fluorinated backbone of the chloro-substituted dithienylethene 4 can also be prevented. The photochromic properties of the conjugate 3 and its precursor dithienylethene 7b are also investigated.