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).
Development of a bio-based wood adhesive is a significant goal for several wood-based material industries. In this study, a novel adhesive based upon sucrose and ammonium dihydrogen phosphate (ADP) was formulated in hopes of furthering this industrial goal through realization of a sustainable adhesive with mechanical properties and water resistance comparable to the synthetic resins used today. Finished particleboards exhibited excellent mechanical properties and water resistance at the revealed optimal adhesive conditions. In fact, the board properties fulfilled in principle the requirements of JIS A 5908 18 type standard, however this occured at production conditions for the actual state of development as reported here, which are still different to usual industrial conditions. Thermal analysis revealed addition of ADP resulted in decreases to the thermal thresholds associated with degradation and curing of sucrose. Spectral results of FT-IR elucidated that furanic ring chemistry was involved during adhesive curing. A possible polycondensation reaction pathway was proposed from this data in an attempt to explain why the adhesive exhibited such favorable bonding properties.
Axial fabrications of subporphyrins have been conveniently accomplished by the reaction of B(methoxo)triphenylsubporphyrin with Grignard reagents such as aryl-, heteroaryl-, ferrocenyl-, β-styryl-, phenylethynyl-, and ethylmagnesium bromides. The axial groups thus introduced are not conjugated with the subporphyrin core. This situation leads to effective fluorescence quenching of subporphyrins when the axial group is strongly electron donating such as 4-dimethylaminophenyl and ferrocenyl groups.
Subbacteriochlorins, which were prepared via hydrogenation of subporphyrins with Raney nickel, are modestly aromatic due to 14π-diazaannulenic circuit and exhibit characteristic blue-shifted Soret-like bands, intensified fluorescence spectra, and high oxidation potentials.
A subporphyrin is a genuine ring-contracted porphyrin that has a 14p electron aromatic system and a bowl-shaped structure. The chemistry of subporphyrins began with the synthesis of tribenzosubporphine in 2006, [1] and the synthesis of meso-aryl-substituted subporphyrins was achieved later by two groups independently.[2] Subporphyrins are characterized by distinct aromaticity, a porphyrin-like intense absorption, and green fluorescence. Large effects of meso-aryl substituents on the electronic properties of subporphyrin owing to their free rotation have been demonstrated for meso-4-aminophenyl-substituted subporphyrins, [3a] meso-oligo-1,4-phenyleneethynylene-substituted subporphyrins, [3b] and meso-oligo-2,5-thienylene-substituted subporphyrins.[3c]Other chemical modifications, such as peripheral modifications [4a,b] and meso-alkyl substitution, [4c] have been also developed. Despite these efforts, the chemistry of subporphyrins still remains at its infant stage, and exploration of novel subporphyrins is highly desirable to expand their chemistry.meso-Alkenylidenyl-substituted porphyrins hold a unique position in porphyrin chemistry in view of extended conjugation, perturbed absorption spectra, [5a] O 2 reduction systems, [5b,c] solvatochromism, [5d] and anion binding.[5e] In many cases, their structures in solution are uncertain because they exist as a variety of tautomers.[6] For example, 3,5-di-tertbutyl-4-hydroxyphenyl-substituted porphyrin is readily oxidized to oxocyclohexadienylidene (OCH)-substituted porphyrin, which has an extended quinonoid conjugated structure.[6] To the best of our knowledge, however, none of mesoalkenylidene-substituted subporphyrins has been reported to date. Herein, we present OCH-substituted subporphyrin as the first example of meso-alkenylidene-substituted subporphyrin.First, we synthesized meso-tris(3,5-di-tert-butyl-4-hydroxyphenyl)subporphyrin 1 based on the condensation of pyridine-tri-N-pyrrolylborane [4b] and 3,5-di-tert-butyl-4-hydroxybenzaldehyde with the yield of isolated product being 1.9 %. A high-resolution electrospray ionization (HR-ESI) mass measurement revealed an intense borenium cation peak at m/z 855.5446 (calcd for C 57 H 69 B 1 N 3 O 3 = 855.5436 [MÀOMe] + ). The 1 H NMR spectrum had a singlet at d = 8.16 ppm for the six b-pyrrolic protons and a single set of signals that are due to the meso-aryl substituents, and a singlet at d = 0.90 ppm for the B-axial methoxy protons. The bowlshaped structure of 1 was unambiguously confirmed by singlecrystal X-ray diffraction analysis (Figure 1). [7] The dihedral angles of the meso-aryl substituents towards the subporphyrin core are 34.18, 48.08, and 57.68, respectively, and the bowl depth, defined as the distance from the central boron atom to the mean plane of peripheral six b-carbon atoms, is 1.37 . These structural features are common to those of usual mesoaryl-substituted subporphyrins.[2a]Subporphyrin 1 was readily oxidized by MnO 2 to OCHsubstituted subporphyrin 2 in 50 % yield. It is worth noting that 2...
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