All reagents and solvents were purchased from Aldrich Chemical Co. and used as received. Benzoxazine 1 was prepared according to the reported procedure, 5c and the purity is >95% as determined by 1 H NMR.Measurements. 1 H NMR spectra were taken on a Bruker AM-360 (360 MHz). Chemical shifts were reported in parts per million relative to TMS as an internal standard (δ TMS = 0) for 1 H NMR spectra. The solvent for NMR measurement was deuterated DMSO or deuterated chloroform. DSC studies were done on a DSC-Q20 thermal analyzer from TA Instruments with N 2 as a purge gas at a scanning rate of 10 °C/min. About 5 mg of samples was tested in high-pressure closed aluminum pans.General Procedures for Ring-Opening Polymerization of Benzoxazine 1. In a test tube, benzoxazine 1 (113 mg, 0.5 mmol) and a catalyst (1 or 5 mol %) were dissolved in acetone (∼0.2 mL). The mixture was dried at room temperature under high vacuum for ∼1 h and heated under the conditions showed above. Then the resulting mixture was subjected to routine analysis.

No abstract

Porphyrin and their analogues usually have a stable aromatic (4n + 2) π-electron system. Porphyrin itself shows aromatic behavior with an 18 π-electron main conjugation pathway in the molecule and has been intensively investigated. 1 However, porphyrin derivatives with 4n π-electron delocation pathways have been little explored owing to their synthetic difficulty, 2,3 and the majority of them are expanded porphyrins 3 because the difference between resonance stabilization energies of 4n and (4n + 2) π-electron systems decreases for larger values of n. 4 To the best of our knowledge, the synthesis of 20 π-electron N,N′-dihydroporphryin (isophlorin) B, referred to for the first time by Woodward during the synthesis of chlorophyll about half a century ago, 5 has represented a challenge for porphyrin chemistry and remained elusive thus far because of its intensive tendency to undergo a twoelectron oxidation and deprotonation to form its corresponding 18 π-electron aromatic porphyrin A.(Scheme 1) Herein, we report the isolation and structural characterization of the 20 π-electron nonaromatic isophlorin 2 and its N-methylation reactions.During our ongoing efforts in synthesis and application of perfluoroalkylated porphyrins, 6 a facile and effective synthesis of the isophlorin 2 was achieved with the accidental discovery of a ready reduction of Cu(II) -tetrakis(trifluoromethyl)-meso-tetraphenylporphyrin (Cu1). 7 When adding activated zinc powder into a DMSO solution of Cu1 at room temperature under nitrogen, the color of the contents changed form green to brown after 1 h. TLC indicated almost complete consumption of the starting porphyrin and formation of a main orange product 2. (Scheme 2) It was relatively air stable 8 to be successfully isolated and thoroughly characterized, although being easily reoxidated to free-base -tetrakis(trifluoromethyl)-meso-tetraphenylporphyrin (H 2 1) with 2,6-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).X-ray crystal diffraction analysis of 2 ( Figure 1) elucidated that it was a 20 π-electron isophlorin with a highly distorted skeleton that displays a nonplanar saddle conformation. 9 The average displacement for the -carbons from the least-squares plane of the meso-carbon atoms, which can be used to evaluate the magnitude of distortion in the macrocycle, was 1.36 Å for 2 in contrast with 0.89 Å for Cu1. 8 It is also a manifestation of serious distortion of the macrocycle that the pyrrole rings with trifluoromethyl groups and pyrroline rings without trifluoromethyl groups are titled on average by 69.7°and 33.5°, respectively, out of the least-squares plane. The remarkably strong twisting of the pyrrole units might be attributed to considerable steric congestion present between meso and substituents, as well as NH-hydrogen atoms. In addition, an expected feature exhibited in the structure of 2 is that clear C-C bond-length alternation is observed around the macrocycle periphery, which dramatically contrasts with its corresponding 18 π-electron aromatic porphyrin Cu1. 8 Spectroscopic data ...

A benzimidazole-based nonheme manganese complex efficiently catalyzes benzylic, aliphatic C-H as well as tertiary C-H oxidation with hydrogen peroxide as the oxidant in the presence of acetic acid as additive. (18)O labeling experiments suggest the reaction may proceed via a high-valent manganese-oxo intermediate.