Porphyrins in 1,3-Dipolar Cycloaddition Reactions: Synthesis of a Novel Pyrazoline-fused Chlorin and a Pyrazole-fused Porphyrin

Abstract

β-Nitro-meso-tetraphenylporphyrin reacts with diazomethane to give a pyrazoline-fused chlorin as the main product. This compound was converted into the corresponding pyrazole-fused porphyrin, by treatment with DBU, and into a methanochlorin by refluxing in toluene.

Key words

cycloadditions - diazo compounds - porphyrins - chlorins - pyrazolines

References

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Reaction of β-nitro-meso-tetraphenylporphyrin with diazomethane: a solution of 1 (50.6 mg, 0.08 mmol) in diethyl ether (200 mL) was saturated twice with diazomethane and allowed to stand at r.t. during 15 d. The reaction was monitored by TLC and UV/Vis spectra. Since after this period a significant quantity of porphyrin 1 remained, the reaction mixture was saturated again with diazomethane and left at r.t. for another period of 15 d. The solvent was then evaporated to dryness and the residue was purified by column chromatography in silica gel using a mixture of petroleum ether-dichloromethane (1:1) as eluent. The first fraction corresponded to a small amount of the methanochlorin 4 (1.7 mg, 3%) and the second one to the unchanged starting porphyrin 1 (23.0 mg, 45%). The third fraction was the pyrazoline-fused chlorin 2 (22.2 mg, 41%), which was followed by a small amount of the pyrazole-fused porphyrin 3 (3.3 mg, 7%).

15

Spectroscopic data for chlorin 2: ¹H NMR (300 MHz, CDCl₃, J in Hz): δ = -2.15 and -2.12 (2 s, 2 H, NH); 4.75 (dd, 1 H, J = 19.0 and J = 4.7, H-2^{3 trans} ); 5.08 (dd, 1 H, J = 19.0 and J = 9.2, H-2^{3 cis} ); 5.45 (dd, 1 H, J = 9.2 and J = 4.7, H-3); 7.56-7.62, 7.70-7.78, 7.87-7.90 and 7.99-8.24 (4 m, 20 H, H-Ph); 8.29 and 8.38 (2 dd, 2 H, J = 4.9 and J = 1.8, H-β); 8.51 and 8.54 (2 d, 2 H, J = 4.9, H-12,13); 8.67 and 8.72 (2 dd, 2 H, J = 4.9 and J = 1.8, H-β). MS (LSIMS): 702 [M + H]⁺, 674 [(M - N₂) + H]⁺, 655 [(M - HNO₂) + H]⁺, 627 {[(M - HNO₂) - N₂] + H}⁺. UV/Vis (CH₂Cl₂): λ_max/nm (log ε): 639 (4.47), 588 (3.98), 546 (4.29), 516 (4.24), 410 (5.39). Anal. Calcd for C₄₅H₃₁N₇O₂.H₂O: C, 75.09; H, 4.62; N, 13.62. Found: C, 74.86; H, 4.19; N, 14.10.

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Spectroscopic data for porphyrin 3: ¹H NMR (300 MHz, CDCl₃, J in Hz): δ = -2.86 (s, 2 H, NH); 7.04 (s, 1 H, H-2³); 7.73-7.91 (m, 12 H, H _meta+para -Ph); 8.17-8.22 (m, 8 H, H _ortho -Ph); 8.75 (s, 2 H, H-12,13); 8.84-8.96 (m, 4 H, H-β). UV/Vis (CH₂Cl₂): λ_max/nm (log ε): 639 (3.18), 585 (3.90), 546 (3.88), 513 (4.34), 417 (5.67). MS (LSIMS): 655 [M + H]⁺, 654 [M⁺]. MS-HRFAB exact mass m/z for C₄₅H₃₀N₆ [M + H]⁺: calcd 655.2610, found 655.2628.

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Spectroscopic data for methanochlorin 4: ¹H NMR (300 MHz, CDCl₃, J in Hz): δ = -2.08 (s, 2 H, NH), 1.95 (dd, 1 H, J = 5.4 and J = 5.2, H-2^{1 trans} ); 3.36 (dd, 1 H, J = 10.0 and J = 5.2, H-2^{1 cis} ); 4.59 (dd, 1 H, J = 10.0 and J = 5.4, H-3); 7.61-7.78 (m, 12 H, H _meta+para -Ph); 7.98-8.23 (m, 8 H, H _ortho -Ph); 8.46 (dd, 1 H, J = 4.9 and J = 1.5, H-β), 8.52 (s, 2 H, H-12,13); 8.56 (dd, 1 H, J = 4.9 and J = 1.5, H-β), 8.70-8.73 (m, 2 H, H-β); UV/Vis (CH₂Cl₂): λ_max/nm (rel. int.): 646 (12%), 592 (7%), 548 (10%), 519 (11%), 415 (100%). MS (LSIMS): 674 [M + H]⁺, 673 [M⁺]. MS-HRFAB exact mass m/z for C₄₅H₃₁N₅O₂ [M + H]⁺: calcd 674.2556, found 674.2542.