Covalently linked porphyrin arrays have attracted much attention as biomimetic light harvesting systems for photosynthesis, optoelectronic devices, and molecular wires. [1] The construction of covalent linkages between porphyrin units often employs palladium-catalyzed cross-coupling [2] or oxidative coupling reactions. [3] In particular, the oxidative dimerization of porphyrins offers a versatile and reliable method for the construction of multiporphyrin arrays. Treatment of mesounsubstituted porphyrins with strong oxidants, such as AgPF 6 , [4a,b] 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-Sc(OTf) 3 [4c] (Tf = trifluoromethanesulfonyl), and phenyliodine bis(trifluoroacetate), [4d] yields directly linked porphyrin oligomers with high regioselectivity. These transformations always include bond formation at the meso position because of its high reactivity.Among such oxidative coupling reactions, a fusion reaction allows the connection of two p-electronic systems by two or more bonds in a single step. This strategy is highly useful for constructing rigid and planar structures, which would be effective for p-conjugation and suppression of energetic decay in the photoirradiated excited state. However, such oxidative fusion reactions with porphyrins have been limited to the formation of meso-b doubly linked and meso-meso, bb, b'-b' triply linked diporphyrins. [4c] In the course of our research on b-functionalized porphyrins, we serendipitously found an oxidative dimerization of b-aminoporphyrins that efficiently provides pyrazine-fused diporphyrins. Interestingly, the reaction proceeds smoothly at the b positions with perfect regioselectivity to construct two direct linkages in one step. [5] We examined the oxidation of 3 a, which was prepared through the Pd-catalyzed cross-coupling of porphyrin bistriflate 1 a [7] with benzophenone imine as the key step (Scheme 1). The addition of 4.0 equiv of DDQ into a dilute solution of 3 a in chloroform (1.0 mmol L À1 ) at room temperature afforded 4 a as a single product in 63 % yield. The reaction was sensitive to the reaction media, and no reaction occurred in dichloromethane. The high-resolution mass spectrum of 4 a contained the parent mass-ion peaks at m/ z = 1938.0031 (calcd for C 124 H 142 N 12 Ni 2 Na: 1938.0080 [M+Na + ]), which suggests the formation of a dimeric species of 3 a. The 1 H NMR spectrum of 4 a has four doublet resonances and one singlet resonance for the b protons, which indicates the presence of unsymmetrical porphyrin units. Relative to 3 a, two signals from the aryl protons were shifted to lower field at d = 8.17 and 8.14 ppm, probably as a result of a deshielding effect by the neighboring porphyrin ring current. The NOESY spectrum of 4 a ( Figure S10 in the Supporting Information) showed correlations between these aryl protons and a meso proton of the porphyrin. On the basis of these spectral data, we assigned 4 a as a pyrazine-fused diporphyrin (Scheme 1). [8,9] The oxidation of b-monoaminoporphyrin 3 b also afforded the corresponding pyr...
