Regioselective addition/annulation of ferrocenyl thioamides with 1,3-diynes via a sulfur-transfer rearrangement to construct extended π-conjugated ferrocenes with luminescent properties

Herein a regioselective addition/annulation strategy of ferrocenyl (Fc) thioamides with alkynes to construct thienylferrocene (ThienylFc) structures, involving a rhodium-catalyzed C–H activation, an unusual C2-selective addition of 1,3-diyne, and an unexpected intramolecular sulfur-transfer rearrangement process is described. In this protocol, thioamide not only serves as a directing group to activate the ortho-C–H bond of the ferrocene, but also as a sulfur source to form the thiophene ring. The resulting carboxylic ester group after sulfur transfer can act as a linkage to construct extended π-conjugated ferrocenes (OCTFc) with luminescent properties. ThienylFc displays effective fluorescence quenching due to the photoinduced electron transfer (PET) from the Fc unit to the excited luminophore, which turns out to be a promising type of redox molecular switch. OCTFc exhibit relatively strong emission owing to their intramolecular charge transfer (ICT) characteristics. The ring-fused strategy is herein employed for the first time to construct luminescent materials based on ferrocenes, which provides inspiration for the development of novel organic optoelectronic materials, such as electroluminescent materials based on ferrocenes.


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network working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode (0.52 V, vs Fc/Fc + ).
Then, the mixture was warmed to room temperature and stirred overnight. The reaction was quenched with ethanol (10 mL) carefully and poured into ice-water. The resulting mixture was extracted with dichloromethane twice. The combined organic layer was washed with brine, dried with anhydrous Na2SO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (petroleum ether/ethyl acetate = 20/1, v/v) to give 5-bromo-1-octyl-1H-indole as a yellow oil in 87% yield (1.34 g).

Synthesis of 1,3-diyne 2t
The reaction mixture was stirred at 0 °C for 15 min and then warmed to room temperature for 2 h. The mixture was poured to 20% of NaOAc solution (100 mL) under N2 and stirred for 1.5 h. The mixture was extracted with dichloromethane and washed with saline. The organic phase was dried over MgSO4, filtered and the filtrate was concentrated under reduced pressure to give the crude product. To the solution of the crude product in 1,4-dioxane (8 mL) under N2, 0.5 M of NaOH solution (5 mL) was added. Then the reaction mixture was refluxed for 5 min and poured into ice-water (50 mL). The mixture was subsequently neutralized by 1 N of HCl solution and extracted with ethyl acetate. The organic layer was washed with saline, dried over MgSO4, filtered and the filtrate was concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (petroleum ether/ethyl acetate = 20/1, v/v) to give ethynyl ferrocene in a yield of 61% (0.64 g).

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CuI (38.1 mg, 10 mol%) and TMEDA (0.3 mL, 2 mmol) was added to the solution of ethynyl ferrocene (0.42 g, 2 mmol) in DMF (5 mL). The resulting mixture was then stirred at room temperature overnight. Then, the mixture was diluted in ethyl acetate (50 mL). The organic layer was washed with saline, dried over MgSO4, filtered and the filtrate was concentrated under reduced pressure. The residue was then purified by column chromatography on silica gel (petroleum ether/ethyl acetate = 20/1, v/v) to give 1,4-diferrocenylbuta-1,3-diyne (2t) as a red solid in a yield of 67% (0.28 g).

Synthesis of 1,3-diyne 2u and 2v 5
A three-necked flask with a magnetic stir bar was charged with the brominated arenes (5 mmol), trimethylsilylacetylene (10 mmol), Pd(PPh3)2Cl2 (5 mol%), CuI (10 mol%), triethylamine (3 mL) and THF (9 mL) under an N2 atmosphere. After refluxed at 80 °C for 24 h, the resulting mixture was cooled to room temperature. Then, KF (10 mmol) was added along with CH3OH (10 mL), and the mixture was stirred under air for another 4 h. The mixture was filtered through a celite pad and washed with 20 mL of dichloromethane. Then the filtrate was concentrated under reduced pressure and purified by column chromatography on silica gel to provide the corresponding terminal alkynes.
CuI (10 mol%) and TMEDA (2 mmol) was added to the solution of the acquired terminal alkyne (2 mmol) in DMF (5 mL). The resulting mixture was then stirred at room temperature overnight. Then, the mixture was diluted in ethyl acetate (50 mL).
The organic layer was washed with saline, dried over MgSO4, filtered and the filtrate was concentrated under reduced pressure. The residue was then purified by column chromatography on silica gel to give the final 1,3-diyne products.

IV. Optimization of regioselective addition/annulation of ferrocenyl thioamide 5 with 1,3-diyne 2a
A 25 mL Schlenk sealed tube with a magnetic stir bar was charged with ferrocenyl

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The residue was dissolved in CDCl3 and analyzed by 1 H NMR with CH2Br2 (3.5 µL, 0.05 mmol) as the internal standard. The NMR yields determined by the characteristic proton signal of 6a at 4.88 ppm relative to the internal standard signal of CH2Br2 at 4.93 ppm. If necessary, the residue would be purified by column chromatography on silica gel (petroleum ether/ethyl acetate = 15/1, v/v) to give the isolated yield of 6a.

VI. Gram-scale synthesis of 6a
A 100 mL three-necked flask with a magnetic stir bar was charged with ferrocenyl (1.20 g, 6.0 mmol, 2.0 equiv.), Cl3CCH2OH (7.5 mL) and CH3OH (22.5 mL) under an N2 atmosphere. The resulting mixture was stirred at 80 °C for 24 h and then diluted with 20 mL of dichloromethane. The mixture was filtered through a celite pad and washed with 30 mL of dichloromethane. Then the filtrate was concentrated under reduced S10 pressure and purified by column chromatography on silica gel (petroleum ether/ethyl acetate = 15:1, v/v) to provide the desired product 6a (1.03 g, 72% yield).

VII. Mechanistic studies
A 25 mL Schlenk sealed tube with a magnetic stir bar was charged with ferrocenyl    The resulting mixture was stirred at 80 °C for 24 h and then diluted with 10 mL of dichloromethane. The mixture was filtered through a celite pad and washed with 20 mL of dichloromethane. The filtrate was concentrated under reduced pressure, followed by dissolved in CDCl3. In-situ 1 H NMR spectra of the reaction mixture showed that there was no product 11 obtained.