Meso‐Trialkyl‐Substituted Subporphyrins

Abstract

In recent years, subporphyrins, which are ring-contracted porphyrins, have emerged as promising functional molecules because of their attractive features, including bowl-shaped structures, 14p-electronic aromatic systems, porphyrinlike spectral characteristics, intense fluorescence, and supramolecular chemistry based on axial chelation of the boron atom. [1][2][3] Notably, the free rotation of meso-aryl substituents of subporphyrins leads to the large electronic effects that give rise to highly perturbed optical properties, as demonstrated by oligo-1,4-phenyleneethynylene and 4-aminophenyl substituted subporphyrins. [4] Despite this progress, the chemistry of subporphyrins still remains in its infancy and an effective synthetic entry into novel subporphyrins is highly desirable. Considering the important roles that meso-alkyl-substituted porphyrins have played in the development of porphyrin chemistry since the first porphyrin synthesis in 1935, [5] it is desirable to study the chemistry of meso-trialkyl-substituted subporphyrins, but the synthesis of this class of molecules has not been reported to date. Herein, we report the first synthesis of meso-trialkyl-substituted subporphyrins.We initially applied our synthetic protocol for mesotriaryl-substituted subporphyrin [1c] to the synthesis of mesotrialkyl-substituted subporphyrins. Pyridine-tri-N-pyrrolylborane was condensed with 1-pentanal under various reaction conditions by changing reaction parameters such as solvent, temperature, molar ratio, and any possible additives. Unfortunately, the desired meso-tributyl subporphyrin was not detected. These negative results led us to explore an indirect route that involved the synthesis of meso-thienyl subporphyrins, and subsequent reductive desulfurization with Raney nickel. Thus, subporphyrins 1 a and 1 b were prepared by using our protocol in 1.7 and 3.7 % yield, respectively (Scheme 1).The higher yield of 1 b may be ascribed to the absence of the free a-thienyl position, since free a-thiophenes are prone to oxidative degradation. It is noteworthy that these compounds are the first examples of subporphyrins that bear fivemembered aromatic heterocycles. Single-crystal X-ray diffraction analysis revealed the bowl-shaped structure of 1 b (Figure 1), in which the dihedral angles [6] of the meso-thienyl substituents are all small (31.7, 35.6, and 48.28), thus allowing the strong electronic conjugation of these substituents with the subporphyrin core. These dihedral angles are slightly smaller than those of meso-triphenyl subporphyrin 3 (38.3, 45.7, and 48.18), which reflect the compact size of the mesothienyl substituents. Scheme 1. Synthesis of meso-trialkylsubporphyrins 2 a-c.Figure 1. X-ray crystal structure of subporphyrin 1 b (thermal ellipsoids are set at the 50 % probability level).

BIII 5‐Arylsubporphyrins and BIII Subporphine

“…Similar to porphyrins, B III subporphyrins are 14π aromatic compounds and exhibit Soret‐like bands and Q‐like bands. As unique characteristics, B III subporphyrins take bowl‐shaped conformations due to the sp 3 ‐hybridized boron center and their meso ‐aryl substituents can rotate rather freely to cause large influences on the electronic and optical properties . B III 5,10‐diphenylsubporphyrin 2 was synthesized in 2012 and has been shown as a useful precursor for a wide range of functionalized B III subporphyrins .…”

“…As unique characteristics,B III subporphyrins take bowl-shaped conformationsd ue to the sp 3 -hybridized boron center and their meso-aryl substituents can rotate rather freely to cause large influences on the electronic and optical properties. [2][3][4][5] B III 5,10-diphenylsubporphyrin 2 was synthesized in 2012 [6] and has been shown as au seful precursor for aw ide range of functionalizedB III subporphyrins. [7][8][9][10] As ar ational extension,w ed isclose the synthesis of B III 5-arylsubporphyrins and B III subporphine as promising precursors for differently meso-substituted B III subporphyrins (Scheme 1).…”

meso‐Functionalization of Boron(III) Subporphyrin with Boron(III) meso‐Lithiosubporphyrin

“…Since the first report in 2006, [1] B III subporphyrins have emerged as an ew class of ring-contracted porphyrinoids with attractive attributes such as 14 p-electronic aromatic character, bowl-shapedc urved structures, and variable electronic properties depending upon the peripheral substituents. [2,3] Recent progress in the chemistry of B III subporphyrins has enabledt he synthesis of variousB III subporphyrins including meso-aryl-substituted, [4] meso-alkyl-substituted, [5] b-alkyl-substituted, [6] mesounsubstituted, [7] pyrrole-reduced, [8] and pyrrole-modified [9] derivatives. However,i ti ss till desirable to develop new methods for the effective functionalization of B III subporphyrins.…”

Diarylamine‐Fused Subporphyrins: Proof of Twisted Intramolecular Charge Transfer (TICT) Mechanism

“…[1] Amongt hese, nitrogen-embedded porphyrins [2] are interesting molecules, which undergof acile oxidation to give stable cation radicals. [2g] Subporphyrinatoboron(III)s [3] (hereafterc alled subporphyrins) have emerged as an ew class of functional molecules that display various absorption spectra depending upon meso-substituents, [4] self-assemblies involving axial substituents, [5] and high performance (> 10 %) in dye-sensitized solar cell. [2g] Subporphyrinatoboron(III)s [3] (hereafterc alled subporphyrins) have emerged as an ew class of functional molecules that display various absorption spectra depending upon meso-substituents, [4] self-assemblies involving axial substituents, [5] and high performance (> 10 %) in dye-sensitized solar cell.…”

“…[2f,g] Actually,s uitable substituted cation radicals of diarylamine-fused porphyrins are remarkably stable,a llowing their manipulations under ambient conditions similar to usual closed-shell organic molecules. [6] Charac-teristically, meso-aryl substituents of subporphyrins can rotate freely to cause large influences on the optical and electrochemicalp roperties, [4,7] which is not observedi np orphyrins. [6] Charac-teristically, meso-aryl substituents of subporphyrins can rotate freely to cause large influences on the optical and electrochemicalp roperties, [4,7] which is not observedi np orphyrins.…”