Synlett 2009(17): 2867-2871  
DOI: 10.1055/s-0029-1217969
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Efficient One-Pot Regioselective Synthesis of 2,3-Dibromo-5,10,15,20-tetra­arylporphyrins from 5,10,15,20-Tetraarylchlorins

Ke-Lai Lia, Can-Cheng Guo*a, Qing-Yun Chen*a,b
a College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. of China
e-Mail: ccguo@hnu.cn;
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. of China
Fax: +86(21)64166128; e-Mail: chenqy@mail.sioc.ac.cn;
Further Information

Publication History

Received 22 June 2009
Publication Date:
09 September 2009 (online)

Abstract

2,3-Dibromo-5,10,15,20-tetraarylporphyrins were prepared from easily available 5,10,15,20-tetraarylchlorins in one flask and two steps, namely regioselective dibromination and subsequent oxidation. An improvement in the synthesis of 2,3,12,13-tetrabromo-5,10,15,20-tetraarylporphyrins has also been achieved.

    References and Notes

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10

Typical Procedure for the Synthesis of 2,3-Dibromo-5,10,15,20-tetraarylporphyrins 4
5,10,15,20-Tetraarylchlorin 2 (0.2 mmol) and NBS (80 mg, 0.44 mmol) were added to a Schlenk flask (50 mL). The flask was then evacuated and backfilled with nitrogen (three cycles). Then dry CHCl3 (EtOH free, 20 mL) was charged with a syringe. The reaction mixture was stirred and heated under reflux for 4 h. After being slightly cooled, a solution of DDQ (184 mg, 0.8 mmol) in toluene (2 mL) was added, and the mixture was refluxed for further 1 h. After being cooled to r.t., Et3N (1 mL) was added to neutralize the acids produced in the reaction. Then the reaction mixture was filtered through a short silica plug (300-400 mesh, eluting with CH2Cl2). The filtrate was evaporated to dryness, and the resulting solid was purified by flash chromatography (silica gel, 300-400 mesh, PE-CH2Cl2 as eluent) to yield the products 4.

11

Synthesis of 12,13-Dibromo-2,3-dihydro-5,10,15,20-tetraphenylchlorin 3a
5,10,15,20-Tetraphenylchlorin (2a, 124 mg, 0.2 mmol) and NBS (80 mg, 0.44 mmol) were added to a Schlenk flask (50 mL). The flask was then evacuated and backfilled with nitrogen (three cycles). Then dry CHCl3 (EtOH free, 20 mL) was charged with a syringe. The reaction mixture was stirred and heated under reflux for 4 h. After being cooled to r.t., Et3N (1 mL) was added to neutralize the acids produced in the reaction. Then the reaction mixture was quickly filtered through a short silica plug (300-400 mesh, eluting with CH2Cl2). The filtrate was concentrated and recrystallized from CH2Cl2-MeOH to give pure products 3a.
12,13-Dibromo-2,3-dihydro-5,10,15,20-tetraphenylchlorin (3a)
Purple crystals, 151 mg, yield 98%. ¹H NMR (300 MHz, CDCl3): δ = 8.51 (d, J = 2.7 Hz, 2 H, β-H), 8.08 (d, J = 2.7 Hz, 2 H, β-H), 8.02 (d, J = 6.0 Hz, 4 H, o-PhH), 7.82 (d, J = 6.0 Hz, 4 H, o-PhH), 7.67-7.70 (m, 12 H, m-PhH and p-PhH), 4.07 (s, 4 H, CH2), -1.40 (s, 2 H, NH). UV/vis (CH2Cl2): λmax (rel. int.) = 428 (1.00), 529 (0.08), 598 (0.05), 650 (0.10) nm. MS (MALDI): m/z = 774.1 [M+]. Anal. Calcd for C44H30Br2N4×2H2O (from CH2Cl2-wet MeOH): C, 65.20; H, 4.23; N, 6.91. Found: C, 65.00; H, 4.01; N, 6.90.

12

Synthesis of Zinc 12,13-Dibromo-2,3-dihydro-5,10,15,20-tetraphenylchlorin Zn3a
Zn3a was synthesized according to the literature method. [¹8] 12,13-Dibromo-2,3-dihydro-5,10,15,20-tetraphenylchlorin (3a, 77 mg, 0.1 mmol) and zinc acetate dihydrate (66 mg, 0.3 mmol) were added to a Schlenk flask (20 mL). The flask was then evacuated and backfilled with nitrogen (three cycles). Then pyridine (10 mL) was charged with a syringe. The reaction mixture was stirred and heated at 100 ˚C under nitrogen for 1 h. To the cooled reaction mixture, benzene (20 mL) and distilled H2O (20 mL) were added. The organic layer was quickly washed with distilled H2O and brine and then filtered through a short silica plug (300-400 mesh, eluting with CH2Cl2). The filtrate was concentrated and recrystallized from CH2Cl2-MeOH to give monopyridinate complex of Zn3a.
Zinc 12,13-Dibromo-2,3-dihydro-5,10,15,20-tetraphenylchlorin (Zn3a)
Purple crystals, 66 mg, yield 72%. ¹H NMR (300 MHz, CDCl3): δ = 8.36 (d, J = 4.2 Hz, 2 H, β-H), 7.96 (d, J = 4.2 Hz, 2 H, β-H), 7.86 (d, J = 7.2 Hz, 4 H, o-PhH), 7.77 (d, J = 7.2 Hz, 4 H, o-PhH), 7.59-7.61 (m, 12 H, m-PhH and p-PhH), 3.97 (s, 4 H, CH2). UV/vis (CH2Cl2): λmax (rel. int.) = 424 (1.00), 553 (0.05), 619 (0.08) nm. MS (MALDI): m/z = 836.0 [M+]. Anal. Calcd for C44H28Br2N4Zn˙C5C5N×4H2O (from CH2Cl2-wet MeOH-pyridine): C, 59.50; H, 4.18; N, 7.08. Found: C, 59.68; H, 3.86; N, 7.08.

13

C44H28Br2N4Zn˙C5H5N×0.5CHCl3 (Zn3a) crystallized by diffusion of MeOH into a CHCl3-pyridine (100:1) solution.
Crystal Data
M = 976.68, monoclinic, space group P2(1)/c, a = 13.330 (4), b = 19.918 (6), c = 17.843 (6) Å, α = 90.00, β = 109.285 (4), γ = 90.00˚, V = 4472 (2) ų, T = 293 (2) K, Z = 4, D c = 1.451 g cm, µ (Mo Kα) = 2.466 mm, 21287 reflections measured, 9543 unique which were used in all calculations. R1 (all data) = 0.1106. R1 = 0.0551. CCDC 689519.

14

5,10,15,20-Tetraarylchlorins were synthesized according to the literature method. [9i]
5,10,15,20-Tetra( p -chlorophenyl)chlorin (2b)
Purple crystals, 736 mg, yield 60%. ¹H NMR (300 MHz, CDCl3): δ = 8.56 (d, J = 4.5 Hz, 2 H, β-H), 8.40 (s, 2 H, β-H), 8.18 (d, J = 4.5 Hz, 2 H, β-H), 8.02 (d, J = 7.8 Hz, 4 H, o-PhH), 7.80 (d, J = 7.8 Hz, 4 H, o-PhH), 7.67 (d, J = 7.5 Hz, 8 H, m-PhH), 4.15 (s, 4 H, CH2), -1.52 (s, 2 H, NH). UV/vis (CH2Cl2): λmax (rel. int.) = 419 (1.00), 519 (0.09), 545 (0.06), 599 (0.04), 652 (0.19) nm. MS (MALDI): m/z = 754.1 [M+]. Anal. Calcd for C44H28Cl4N4×0.5H2O (from CH2Cl2-wet MeOH): C, 69.21; H, 3.83; N, 7.34. Found: C, 69.35; H, 3.68; N, 7.34.

15

For the details of the synthesis of 4b and 4c, see ref. 10.
2,3-Dibromo-5,10,15,20-tetrakis( p -chlorophenyl)-porphyrin (4b)
Purple crystals, 172 mg, yield 95%. ¹H NMR (300 MHz, CDCl3): δ = 8.85 (s, 4 H, β-H), 8.68 (s, 2 H, β-H), 8.11 (d, J = 6.0 Hz, 4 H, o-PhH), 8.05 (d, J = 6.6 Hz, 4 H, o-PhH), 7.74-7.76 (m, 8 H, m-PhH), -2.93 (s, 2 H, NH). UV/vis (CH2Cl2): λmax (rel. int.) = 425 (1.00), 522 (0.06), 599 (0.02), 687 (0.05) nm. MS (MALDI): m/z = 909.9 [M+]. Anal. Calcd for C44H24Br2Cl4N4×0.5H2O (from CH2Cl2-wet MeOH): C, 57.49; H, 2.74; N, 6.09. Found: C, 57.39; H, 2.60; N, 6.16.

16

H2(2,3,12,13-Br4TPP) was first synthesized by Callot [5a] in 1974, but its structure was misassigned as 2,7,12,17-tetrabromo-5,10,15,20-tetraphenylporphyrin. In 1991, work by Crossley et al. [6n] revealed the antipodal nature of the bromination reaction of metal-free tetraarylporphyrins, and H2(2,3,12,13-Br4TPP) was prepared by refluxing H2(TPP) with NBS in 80% yield, but no detail operation was reported. In 1994, Xu et al. [6i] obtained H2(2,3,12,13-Br4TPP) using the same method by directly recrystallized from CH2Cl2-MeOH after the reaction mixture was concentrated, but no yield was reported. In 2003, Bhyrappa et al. [6f] reported the synthetic procedure of H2(2,3,12,13-Br4TPP) in detail, in which the crude product was purified by chromatography and the yield was 65%.

17

Typical Procedure for the Synthesis of 2,3,12,13-Tetrabromo-5,10,15,20-tetraarylporphyrins 5
5,10,15,20-Tetraarylporphyrin 1 (0.5 mmol) and NBS (580 mg, 3.25 mmol) were dissolved in CHCl3 (EtOH free, 60 mL). The reaction mixture was stirred and heated under reflux for 4 h. After being cooled to r.t., Et3N (3 mL) was added to neutralize the acids produced in the reaction. Then the reaction mixture was filtered through a short silica plug (300-400 mesh, eluting with CH2Cl2). The filtrate was evaporated to dryness, and the resulting solid was purified by flash chromatography (silica gel, 300-400 mesh, CH2Cl2 as eluent) to yield the products 5. The spectroscopic data were in agreement with literature values. [²g] [6f]