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

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.