Sterically Encumbered Porphyrins by Suzuki Reactions of a 5,15-Bis(4,6-dichloropyrimidin-5-yl) Derivative

Abstract

The high yielding synthesis of novel four picket fence porphyrins via palladium catalyzed Suzuki cross-coupling reactions of 5,15-bis(pyrimidinyl)porphyrin 1 and commercially available arylboronic acids is described.

Key words

porphyrins - Suzuki cross-coupling - palladium - arylations - steric hindrance

References

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Experimental Details, Representative Procedure for Porphyrin 2a: To a mixture of porphyrin 1 (100 mg, 0.12 mmol) and Pd(PPh₃)₄ (4 mg, 3 mol%) in toluene (7.5 mL), phenylboronic acid (87 mg, 0.72 mmol) was added, immediately followed by aq Na₂CO₃ (2 M, 1 mL). The mixture was flushed with N₂ for 5 min and the reaction mixture was then heated under reflux for 24 h. After cooling, the reaction mixture was evaporated under reduced pressure to dryness. To the reaction mixture H₂O (50 mL) was added and the aq layer was extracted with CH₂Cl₂ (3 × 50 mL). The organic layers were collected, dried over MgSO₄ and evaporated in vacuum. Purification by column chromatography (silica gel, CH₂Cl₂/EtOAc: 10/1) furnished the pure compound 2a as a purple solid (83 mg, 70%). R_f = 0.3 (CH₂Cl₂/EtOAc 10/1). ¹H NMR (400 MHz, CDCl₃): δ = -2.58 (s_br, 2 H), 1.61 (s, 12 H), 2.57 (s, 6 H), 6.48 (t, 8 H), 6.60 (t, 4 H), 7.00 (d, 8 H), 7.19 (s, 4 H), 8.32 (d, 4 H), 8.53 (d, 4 H), 9.75 (s, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 21.3, 21.4, 112.4, 118.4, 127.4, 127.7, 127.8, 129.4, 130.2, 131.2, 137.6, 137.9, 138.8, 139.2, 146.5, 158.4, 168.2. MS (electrospray): 1007.5 [(M⁺) + H]. UV/VIS (CH₂Cl₂): λ_max = 432.6 (log ε = 5.586), 528.2 (log ε = 4.271), 564.3 (log ε = 4.025), 603.0 (log ε = 3.829), 662.2 (log ε = 3.673).
The optical spectra are extremely similar to the one of the starting porphyrin 1 but are red-shifted for about 12-16 nm. Suslick et al. [3] reported the same red-shift (ca. around 30 nm) for their porphyrin compared to the tetraphenylporphyrin.

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All arylboronic acids were purchased from Acros.