A ‘Click’ Chemistry Route to ‘Capped’ Porphyrins

Abstract

Synthesis of a new ‘capped’ porphyrin 3 with rigid 1,2,3-triazole linkers between the porphyrin and ‘capping’ group has been achieved. A ‘click’ reaction, the copper(I)-catalysed variant of the Huisgen 1,3-dipolar cycloaddition, is used to create four linkages concomitantly under anhydrous conditions in N,N-dimethylformamide.

Key words

porphyrin - alkyne - azide - 1,3-dipolar cycloaddition - 1,2,3-triazoles

References and Notes

• 1a Almog J. Baldwin JE. Dyer RL. Peters M. J. Am. Chem. Soc.  1975,  97:  226 
• 1b Almog J. Baldwin JE. Crossley MJ. Debernadis JF. Dyer RL. Huff JR. Peters MK. Tetrahedron  1981,  37:  3589 
• 2a Almog J. Baldwin JE. Huff JR. J. Am. Chem. Soc.  1975,  97:  227 
• 2b Hashimoto T. Dyer RL. Crossley MJ. Baldwin JE. Basolo F. J. Am. Chem. Soc.  1982,  104:  2101 
• 3 Baldwin JE. Cameron JH. Crossley MJ. Dagley IJ. Hall SR. Klose T. J. Chem. Soc., Dalton Trans.  1984,  1739 
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9

Selected data for compound 4: mp 119-120 ˚C. ^¹H NMR (200 MHz, CDCl₃): δ = 8.15 (s, 2 H), 4.95 (d, J = 2.45 Hz, 8 H), 2.57 (t, J = 2.45 Hz, 4 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 164.9, 133.9, 130.1, 76.7, 76.0, 53.8.

10

Tetrahydrofuran-H ₂ O Method for the Cycloaddition: To a stirred mixture of porphyrin 5 (65 mg) and Et₃N (40 µL) in THF (100 mL) were added CuSO₄˙5H₂O (4 mg) and ascorbic acid (5 mg) dissolved in H₂O (15 mL). Then a solution of tetraalkyne 4 in THF (35 mL) was added and the mixture was heated at reflux for 24 h. An additional portion of CuSO₄˙5H₂O (4 mg) and l-ascorbic acid (5 mg) in H₂O
(5 mL) was added and refluxed for a further 6 h. The reaction was then filtered, evaporated to approximately 20 mL with CH₂Cl₂ (100 mL) and H₂O (100 mL) added. The organic phase was collected and the aqueous phase was extracted with a mixture of MeOH and H₂O (1:1, 2 × 100 mL). This was then dried over Na₂SO₄, filtered and evaporated. Purification on silica with 4% MeOH in H₂O yielded capped porphyrin 3 in 19% yield.

12

Selected data for compound 3: mp >300 ˚C. ^¹H NMR (400 MHz, CDCl₃): δ = 8.72 (s, 4 H), 8.68 (s, 4 H), 8.41 (br d, J = 7.6 Hz, 4 H, meso-aryl H6), 7.85 (t, J = 7.6 Hz, 4 H, meso-aryl H3), 7.77 (br d, J = 7.6 Hz, 4 H, meso-aryl H4), 7.64 (br s, 4 H), 7.32 (s, 2 H), 7.08 (br s, 4 H, meso-aryl H2), 5.93 (A of ABq, J = 16.1 Hz, 4 H), 5.79 (B of ABq, J = 16.1 Hz,
4 H), 5.13 (A of ABq, J = 13.0 Hz, 4 H), 5.03 (B of ABq, J = 13.0 Hz, 4 H). MS (ESI): m/z [M + Na] calcd for C₇₀H₄₆N₁₆O₈Zn: 1325.3; found: 1325.4.

13

Dimethylformamide Method for the Cycloaddition: To a stirred solution of 5 (45 mg), (NCCH₃)₄CuPF₆ (4 mg) and Et₃N (28 µL) in DMF (100 mL) was added 4 (20 mg). This was then put under nitrogen and stirred at 70 ˚C for 16 h. This was then added to a mixture of CH₂Cl₂ and EtOAc (1:1, 200 mL), washed with H₂O (2 × 100 mL), and the organic phase was filtered and washed again with H₂O (2 × 100 mL). The organic phase was then dried over Na₂SO₄, filtered and evaporated with the residue subjected to purification on silica, eluting capped porphyrin 3 in 14% yield.