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Axial, Helical, and Planar Chirality in Directly Linked Basket-Handle Porphyrin Arrays

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posted on 04.02.2016, 15:55 by Andreas C. Gehrold, Torsten Bruhn, Gerhard Bringmann
The single-electron oxidative dimerization of basket-handle porphyrins (BHPs) with different coordinated metal ions [Cu­(II), Ni­(II), Pd­(II), Zn­(II)] yielded directly meso-meso linked dimers in excellent yields. The synthetic protocol is suited for coupling substrates with different meso-substituents (H, Br, aryl, alkyl) opposite to the coupling site. Experimental findings concerning reactivities and selectivities were in good agreement with theoretical investigations using ALIE calculations. The dimers, which all are axially chiral, were resolved into their enantiomers by HPLC on a chiral phase. ECD spectra were measured in the stopped-flow mode and compared with results from quantum-chemical ECD calculations to assign the absolute configuration. One directly linked dimer was further oxidized to a fused system, which possessed a stable helical chirality. Its absolute configuration was again assigned by ECD investigations. Furthermore, functionalized BHPs and tetraarylporphyrins were coupled under Suzuki conditions to give dimers and trimers with either β-meso or β-β linkages. Because of the steric shielding of one of the BHP hemispheres, the products were formed with full diastereoselectivity regarding all porphyrin-porphyrin axes. The stereostructures of these arrays were investigated by quantum-chemical calculations (DFT-D3, TD DFT, and sTD DFT), and the absolute configurations were assigned for all chiral representatives.