Efficient Preparation of Photoswitchable Dithienylethene‐Linker‐Conjugates by Palladium‐Catalyzed Coupling Reactions of Terminal Alkynes with Thienyl Chlorides and Other Aryl Halides

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 or the iodo-substituted linker . Reaction conditions for the final Sonogashira coupling step between ethynylene-linker with the chloro-substituted dithienylethene in the presence of [PdCl2(CH3CN)2]/X-Phos and Cs2CO3 or K3PO4 are optimized using 2-chloro-5-methylthiophene () and triethylsilylacetylene or triisopropylsilylacetylene (,) as model compounds. Experimental conditions are found to suppress the activation of the C(sp)Si bond in TIPS-acetylene , a reaction leading to a subsequent cross-coupling reaction to form by-product . Furthermore, activation of the C(sp)Si bond in the presence of the fluorinated backbone of the chloro-substituted dithienylethene can also be prevented. The photochromic properties of the conjugate and its precursor dithienylethene are also investigated.