CYCLOHEXYLENE-BRIDGED PORPHYRIN QUINONES WITH VARIABLE ACCEPTOR STRENGTH AS BIOMIMETIC MODELS FOR PHOTOSYNTHESIS - EVIDENCE FOR TWIST-BOAT CONFORMATION

Rigidly and covalently linked porphyrin quinones are well-suited as bi omimetic model compounds for studying the photoinduced electron transf er (PET reaction occurring in primary processes of photosynthesis. In this context, the synthesis of new porphyrin quinones with a cis-or tr ans-1,4-disubstituted cyclohexylene bridge linking the electron donor and the electron acceptor is reported. To study the dependence of the PET rate of the difference of the free enthalpy of the PET reaction, f our quinones with different structures and therefore redox potentials were used as electron acceptor components. As a whole. two series of e ach four new cis-and trans-1,4-cyclohexylene-bridged porphyrin quinone s with variable acceptor strength were synthesized. The most important synthetic steps comprised the free radical addition of the ester func tionalized cyclohexylene bridge to the quinone, reduction of the ester to the alcohol group with lithium borohydride or DIBALH, oxidation of the alcohol to the corresponding aldehyde with PCC or TEMPO (2,2,6,6- tetramethylpiperidine-1-oxyl)/NaOCl, and condensation of these aldehyd es with pyrrole and 4-methylbenzaldehyde under equilibrium conditions. Analysis of the H-1 NMR spectra unambiguously indicated the chair con formation for the cyclohexane ring of all porphyrin precursors and tra ns-cyclohexane-bridged porphyrin quinones, whereas the cis-cyclohexane -bridged porphyrin quinones had the cyclohexane ring in the unusual tw ist-boat conformation. This was additionally confirmed by an X-ray cry stal structure of one of the cis-porphyrin quinones and the correspond ing trans-porphyrin quinone. NOE experiments gave information about th e spatial arrangement of the diastereomeric target compounds in soluti on.