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).

Effective meso Fabrications of Subporphyrins

“…An alkyl group bearing an ester group was also installed by a similar coupling reaction with the corresponding functionalized organozinc reagent15 with the aid of an NHC‐Pd complex, Pd‐PEPPSI‐IPent,16 to provide 8 in 50 % yield. These reactions constitute the second synthetic route to meso‐alkyl substituted subporphyrins,2i but are much more useful in terms of rational synthesis and wide functional group compatibility.…”

Subporphyrins: A Legitimate Ring-Contracted Porphyrin with Versatile Electronic and Optical Properties

“…Subporphyrins 42, 49, and 50 were prepared by the conventional method in 1.7%, 3.7%, and 0.9% yields, respectively (Scheme 7). 156 A toluene solution of 49 was treated with W-7 Raney nickel at 40°C for 30 min and TLC analysis confirmed the consumption of 49. The 1 H and 11 B NMR spectra of the reaction mixture indicated the formation of meso-tripentylsubporphyrin 52 along with several over-reduced products.…”

Electron transfer reactions in sub-porphyrin–naphthyldiimide dyads

“…S55 in the ESI †). [35][36][37] Upon conjugation with the NDI acceptor the lifetime becomes about 500 times shorter and thereby impossible to be measured by TCSPC. In Fig.…”

“…Here E ox = 0.71 V and E red = À1.07 V are the oxidation and reduction potentials (vs. Fc/Fc + ) for SubP and NDI in DCM, 37,[46][47][48][49] respectively (Table S3, ESI †). The second term compensates for the difference in solvent polarity between electrochemical and spectroscopic measurements where e S = 2.38 (toluene) and e ref S = 8.93 (DCM) are the relative dielectric constants and r = 5 Å is the estimated average of the radii of the donor and acceptor.…”