Efficient Synthesis of meso-meso-Linked Diporphyrins by Nickel(0)-Mediated Ullmann Homocoupling

Abstract

meso-meso-Linked diporphyrins have been prepared by nickel(0)-mediated Ullmann homocoupling under mild conditions.

Key words

diporphyrins - bromoporphyrins - dimerization - Ullmann homocoupling - debromination

References and Notes

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8

General Procedure for Preparation of meso - meso -Linked Ni(II) Porphyrin Dimers A mixture of powdered NiCl₂(PPh₃)₂ (6.5 mg, 10 µmol), zinc powder (6.5 mg, 0.1 mmol), TBAI (3.6 mg, 10 µmol), and 1,5-COD (0.1 mL, 0.82 mmol) was added to a solution of meso-bromoporphyrin (Ni1a, 34 mg, 50 µmol) in N,N-dimethylacetamide (2 mL) at r.t. under nitrogen atmosphere. The reaction mixture was stirred for 1 h. The reaction was quenched immediately with H₂O (10 mL) and was extracted with CH₂Cl₂ (40 mL). The organic layer was separated, washed with H₂O (3 × 20 mL) and brine, and dried over anhyd Na₂SO₄. After removal of the solvent in vacuo, the residue was purified by silica column chromatography (eluent: PE-CH₂Cl₂, 3:1 v/v) to afford Ni3a (8 mg) in a 26% yield and the desired compound Ni ₂ 4a (21 mg) in a 71% yield.
Porphyrin Ni3a: ^¹H NMR (300 MHz, CDCl₃): δ = 9.85 (s, 1 H, meso-H), 9.15 (d, J = 4.8 Hz, 2 H, β-H), 8.90 (d, J = 5.1 Hz, 2 H, β-H), 8.79 (s, 4 H, β-H), 8.05 (m, 6 H, PhH), 7.70 (m, 9 H, PhH). MS (MALDI): m/z = 594.1 [M⁺]. HRMS (MALDI): m/z calcd for C₃₈H₂₄N₄Ni [M⁺]: 594.1349; found: 594.1354.
Diporphyrin Ni ₂ 4a: ^¹H NMR (300 MHz, CDCl₃): δ = 8.83 (dd, J = 9.9, 5.1 Hz, 8 H, β), 8.57 (d, J = 4.8 Hz, 4 H, β), 8.12 (m, 8 H, Ph + β, overlapped), 8.04 (m, 8 H, Ph), 7.73 (m, 6 H, Ph), 7.63 (m, 12 H, Ph). UV/vis (CH₂Cl₂): λ_max (relative intensity): 536 nm (1.0), 445 (4.3), 413 (3.6). MS (MALDI): m/z = 1186.3 [M⁺]. HRMS (MALDI): m/z calcd for C₇₆H₄₆N₈Ni₂ [M⁺]: 1186.2547; found: 1186.2555.

9

General Procedure for the Bromination Reaction meso-Free Ni(II) porphyrin (Ni3c, 26 mg, 0.05 mmol) was dissolved in CH₂Cl₂-MeOH (5:1 v/v, 3 mL). N-Bromo-succinimide (NBS, 20 mg, 0.11 mmol, 2.2 equiv) was added. The reaction mixture was stirred under air at r.t. for 15 min and quenched with acetone (1 mL). The resulting red solution was evaporated to dryness, the residue was purified by silica gel column chromatography using PE and CH₂Cl₂ (PE-CH₂Cl₂, 2:1 v/v) as eluant to provide the pure desired compound Ni1f in a 94% yield.

10

General Procedure for the Debromination Reaction To a solution of meso-dibromodiporphyrins (Ni1f, 34 mg, 50 µmol) in DMF (50 mL), NiCl₂(PPh₃)₂ (163 mg, 0.25 mmol), and zinc powder (16.3 mg, 0.25 mmol) were added sequentially under nitrogen atmosphere, and the resulting mixture was heated to reflux for 2 h. The reaction mixture was cooled down to r.t., extracted with CH₂Cl₂ (200 mL). The organic layer was separated, washed with H₂O (3 × 
200 mL) and brine, and dried over anhyd Na₂SO₄. After concentration in vacuo, the residue was purified by silica column chromatography (eluent: PE-CH₂Cl₂, 2:1 v/v) to obtain the desired compound Ni3c (25 mg) in a 97% yield.
Porphyrin Ni3c: ^¹H NMR (300 MHz, CDCl₃): δ = 9.94 (s, 2 H, meso-H), 9.19 (d, J = 4.5 Hz, 4 H, β-H), 8.94 (d, J = 4.5 Hz, 4 H, β-H), 8.07 (m, 4 H, PhH), 7.71 (m, 6 H, PhH). MS (MALDI): m/z = 518.1 [M⁺]. HRMS (MALDI): m/z calcd for C₃₂H₂₀N₄Ni [M⁺]: 518.1036; found: 594.1037.