Pitfalls in bromination reactions of zinc porphyrins: two-sided ring opening of the porphyrin macrocycle.

Reaction of [Zn(II)(TTP)] (1) (TTP = dianion of 5,10,15,20-meso-tetrakis(p-tolyl)porphyrin) with 16 equiv of N-bromosuccinamide (NBS) in methanol at reflux led to the unexpected two-sidedd open-ring brominated product [Zn(II)(C26H20N2O2Br5)2] (2). Similar observations have been made with other meso-substituted zinc porphyrins as well [Zn(II)(por)] {por = dianion of 5,10,15,20-meso-tetrakis(aryl)porphyrin; aryl = phenyl (TPP), p-(t)Bu-phenyl (TBPP), m-Cl-phenyl (TClPP)}. The respective products [Zn(II)(C24H16N2O2Br5)2] (3), [Zn(II)(C32H32N2O2Br5)2] (4), and [Zn(II)(C24H14N2O2Cl2Br5)2] (5) have been isolated in good to moderate yields and characterized by elemental analysis and UV-vis, (1)H NMR, and mass spectrometry. Additional bromination reaction of 1 with 8 equiv of NBS in a chloroform/methanol mixture led (after the two-sided ring opening) to nonmetalated brominated bi(pyrrole) product, C36H34N2O4Br4 (6). The detailed structures of complexes 1, 2, 3, and 6, available in a single crystal form, have been confirmed by X-ray diffraction analysis.