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 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 [PdCl 2 (CH 3 CN) 2 ]/X‐Phos and Cs 2 CO 3 or K 3 PO 4 are optimized using 2‐chloro‐5‐methylthiophene ( 9 ) and triethylsilylacetylene or triisopropylsilylacetylene ( 10a , 10b ,) 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.