Strategic Synthesis of 2,6‐Pyridylene‐Bridged β‐to‐β Porphyrin Nanorings through Cross‐Coupling

Abstract

Since the crystal structure of LH2 was elucidated to be circularly arranged chromophoric assemblies, [1–3] much effort has been devoted towards the synthesis of cyclic porphyrin arrays to study the excitation energy transfer (EET) and electronic coupling along the wheel. [4] These cyclic porphyrin arrays are also interesting in host–guest chemistry, [5] single-molecule photochemistry, [6] nonlinear optical (NLO) materials [7] and so on. [8–12] Cyclic porphyrin arrays are constructed by means of covalent bonds, noncovalent bonds, or metal coordination bonds. [8–12] Although the last two are beneficial to construct cyclic porphyrin arrays, those are often unstable toward solvent change or addition of other competing species. The covalently bonded arrays are structurally robust, but often difficult to synthesize. In addition, the final macrocyclization steps are the most tedious and generally need assistance from a suitable template. [5] Although there are some reports on covalently bonded cyclic porphyrin arrays, most of them were constructed through meso-to-meso bridging methods. As rare examples, we recently reported two b,b’-bridged cyclic porphyrin arrays with a 1,3-butadiyne or a 2,5-thienyl spacer. [13] Herein, we wish to report the efficient synthesis of 2,6-pyridylene-bridged b-tob porphyrin nanorings by Suzuki–Miyaura coupling, which is particularly effective for medium to large porphyrin rings. We have achieved the synthesis of 2,5-thienylene-bridged cyclic porphyrin dimers and trimers, but it was difficult to expand the size of the macrocycle beyond trimers. We then examined the Suzuki–Miyaura coupling of b,b’-diborylated Ni II porphyrin 1 with 2,6-dibromopyridine, which provided linear oligomers as the main products. However, we noticed that bromopyridyl-terminated oligomers were selectively obtained with the use of an excess amount of 2,6-dibromopyridine. For instance, b,b’-diborylporphyrin 1–Ni II[14] was crosscoupled with 2,6-dibromopyridine (5 equiv) under standard conditions to give 1–Br in 80 % yield, along with 2–Br (10 %) and 3–Br (2 %) (Scheme 1). It is worth noting that deborylated products were hardly detected in this reaction. Compound 2–Br was also obtained in 42 % yield through the three-component coupling of 1 (1 equiv), 1–Br (2.5 equiv), and 2,6-dibromopyridine (2.5 equiv). Further cross-coupling of 1 (1 equiv) with 1–Br (5 equiv) or 2–Br (5 equiv) afforded 3–Br or 5–Br in 50 or 45 % yields, respectively. Compound 7–Br was prepared in 50 % yield from 1 and 3–Br. With the linear precursors 2–Br, 3–Br, 5–Br, and 7–Br in hand, we examined the cyclization with 1 through a 1:1 Suzuki–Miyaura coupling reaction, which worked very effectively to give porphyrin rings 3–Ni, 4–Ni, 6–Ni, and 8–Ni in 60, 58, 55, and 55 % yields, respectively, even without any template. Treatment of 3–Ni, 4–Ni, 6–Ni, and 8–Ni with sulfuric acid in chloroform at room temperature induced Ni II demetalation to provide 3–H, 4–H, 6–H, and 8–H quantitatively, which were all converted into 3–Zn, 4–Zn, 6–Zn, and 8–Zn upon treatment with ZnA2 in quantitative yields. These newly synthesized porphyrin rings were fully characterized by high-resolution mass spectrometry and