Cyclohexylene-bridged porphyrin quinones with variable acceptor strength as biomimetic models for photosynthesis: Evidence for twist-boat conformation

被引:16
作者
Dieks, H [1 ]
Senge, MO [1 ]
Kirste, B [1 ]
Kurreck, H [1 ]
机构
[1] FREE UNIV BERLIN, INST ORGAN CHEM WE 02, D-14195 BERLIN, GERMANY
关键词
D O I
10.1021/jo970752k
中图分类号
O62 [有机化学];
学科分类号
070303 ; 081704 ;
摘要
Rigidly and covalently linked porphyrin quinones are well-suited as biomimetic model compounds for studying the photoinduced electron transfer (PET reaction occurring in primary processes of photosynthesis. In this context, the synthesis of new porphyrin quinones with a cis-or trans-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, four quinones with different structures and therefore redox potentials were used as electron acceptor components. As a whole. two series of each four new cis-and trans-1,4-cyclohexylene-bridged porphyrin quinones with variable acceptor strength were synthesized. The most important synthetic steps comprised the free radical addition of the ester functionalized 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 aldehydes with pyrrole and 4-methylbenzaldehyde under equilibrium conditions. Analysis of the H-1 NMR spectra unambiguously indicated the chair conformation for the cyclohexane ring of all porphyrin precursors and trans-cyclohexane-bridged porphyrin quinones, whereas the cis-cyclohexane-bridged porphyrin quinones had the cyclohexane ring in the unusual twist-boat conformation. This was additionally confirmed by an X-ray crystal structure of one of the cis-porphyrin quinones and the corresponding trans-porphyrin quinone. NOE experiments gave information about the spatial arrangement of the diastereomeric target compounds in solution.
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页码:8666 / 8680
页数:15
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