Efficient preparation of photoswitchable dithienylethene-linker-conjugates by palladium-catalyzed coupling reactions of terminal alkynes with thienyl chlorides and other aryl halides

Three photochromic dithie-nylethene-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 ethynylenelinker 5 with the chloro-substituted dithienylethene 4 in the presence of [PdCl ₂(CH₃CN)₂]/X-Phos and Cs₂CO ₃ or K3PO4 are optimized using 2-chloro-5-methylthiophene (9) and triethylsily-lacetylene or triisopropylsilylacetylene (10a,b) as model compounds. Experimental conditions are found to suppress the activation of the C(sp)-Si bond in TIPS-acetylene 10 b, 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 di-thienylethene 4 can also be prevented. The photochromic properties of the conjugate 3 and its precursor dithienyl-ethene 7b are also investigated.