Porphyrinoids DOI: 10.1002/ange.200906005 Meso-Trialkyl-Substituted Subporphyrins** Shin-ya Hayashi, Yasuhide Inokuma, Shanmugam Easwaramoorthi, Kil Suk Kim, Dongho Kim,* and Atsuhiro Osuka* 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 supramolec- ular chemistry based on axial chelation of the boron atom. [1–3] 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 substi- tuted 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 meso- triaryl-substituted subporphyrin [1c] to the synthesis of meso- trialkyl-substituted subporphyrins. Pyridine-tri-N-pyrrolyl- borane was condensed with 1-pentanal under various reaction conditions by changing reaction parameters such as solvent, temperature, molar ratio, and any possible additives. Unfortu- nately, 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 subporphyr- ins, and subsequent reductive desulfurization with Raney nickel. Thus, subporphyrins 1a and 1b were prepared by using our protocol in 1.7 and 3.7 % yield, respectively (Scheme 1). The higher yield of 1b 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 five- membered aromatic heterocycles. Single-crystal X-ray dif- fraction analysis revealed the bowl-shaped structure of 1b (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 meso- thienyl substituents. Scheme 1. Synthesis of meso-trialkylsubporphyrins 2a–c. Figure 1. X-ray crystal structure of subporphyrin 1b (thermal ellipsoids are set at the 50 % probability level). [*] S. Hayashi, Dr. Y. Inokuma, Prof.Dr. A. Osuka Department of Chemistry Graduate School of Science, Kyoto University Sakyo-ku, Kyoto 606-8502 (Japan) Fax: (+ 81) 75-753-3970 E-mail: osuka@kuchem.kyoto-u.ac.jp Dr. S. Easwaramoorthi, K. S. Kim, Prof. Dr. D. Kim Spectroscopy Laboratory for Functional p-electronic Systems Department of Chemistry, Yonsei University Seoul 120-749 (Korea) Fax: (+ 82) 2-2123-2434 E-mail: dongho@yonsei.ac.kr [**] This work was supported by Grants-in-Aid (no. 19205006 (A), and 20108001 “pi-Space”) from MEXT (Japan). Y.I. thanks the JSPS for a Research Fellowship for Young Scientist. The work at Yonsei University was supported by the Star Faculty and World Class University (2008-1955) We acknowledge programs from MEST. an AFSOR/AOARD Grant (FA4869-08-1-4097), and a grant from the fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. K.S.K. thanks the MEST for a fellowship of the BK21 program. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200906005. Angewandte Chemie 331 Angew. Chem. 2010, 122, 331 –334  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim