Improved Synthesis of 5,5′′-Dibromo-2,2′:6′,2′′-terpyridine

Abstract

5,5′′-dibromo-2,2′:6′,2′′-terpyridine has been synthesized in very good yield and on a multigram scale modifying the Stille cross-coupling strategy published by Schlüter and coworkers in a previous work.

5,5′′-dibromo-2,2′:6′,2′′-terpyridine - Stille cross-coupling - terpyridine derivatives

References

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5-Bromo-2-iodopyridine (5.85 g, 20.5 mmol) was dissolved in 200 mL of toluene. 16.2 mL BuLi (1.4 M in hexane, 22.5 mmol) were added dropwise onto the solution maintained at -20 °C during the beginning of the addition (after the first drops of BuLi, the solution became red) and progressively the temperature was lowered to -76 °C. The solution was then stirred at -76 °C for two hours after which a solution of ClSnMe₃ (4.92 g, 24.6 mmol) in toluene was added carefully. At the end of the addition the yellow solution was stirred for one additional hour at -76 °C and then allowed to warm up to r.t. Toluene was evaporated and the crude product taken up in CH₂Cl₂. Insoluble salts were filtered off on a sintered glass. The filtrate was evaporated to dryness and the resulting yellow oil was submitted to a very fast chromatography (15 min, over 100 g of oven dried alumina,

eluted with ether). This short purification afforded 3 in 97% yield (6.39 g, yellow oil). ¹H NMR (200 MHz, CDCl₃): δ = 8.82 (d, 1 H, H₆, ⁴ J = 2.5 Hz); 7.68 (dd, 1 H, H₄, ³ J = 8.1 Hz, ⁴ J = 2.5 Hz); 7.35 (d, 1 H, H₃, ³ J = 8.1 Hz); 0.32 (s with Sn satellites, 9 H, CH_3,
J[1H-119Sn] = 54.4 Hz, J[1H-117Sn] = 51.2 Hz).
MS (EI): m/z = 321.0 ([M⁺] calcd 320.8).

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<A NAME="RG32101ST-12">12</A>

5-Bromo-2-trimethylstannylpyridine 3 (6.4 g, 19.9 mmol) and 2,6-diiodopyridine 5 (3.17 g, 9.5 mmol) were dissolved in toluene (200 mL). The solution was degassed three times, Pd(PPh₃)₄ (660 mg, 0.6 mmol) added, and the mixture was degassed three times again. The solution was heated at reflux for 18 h. Pd⁰ was filtered off and the solvent evaporated. Filtration of the resulting solid on alumina with CH₂Cl₂ as eluent yielded 6 in 90% yield (3.32 g, white solid, according to ¹H NMR, purity is ≥ 95%). All spectroscopic data are in good agreement with literature. [5]