Cyclohexylene-Bridged Porphyrin Quinones with Variable Acceptor Strength as Biomimetic Models for Photosynthesis:  Evidence for Twist-Boat Conformation

Abstract

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 1H 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.

EPR and ENDOR of radicals of chlorin- and bacteriochlorin-quinone model compounds for electron transfer in photosynthesis

“…In these studies, spin-polarized transient triplet states of the (bacterio)chlorins and the charge-separated biradicals could be detected after laser excitation. 10 The previous crystallographic studies of porphyrin-quinone model compounds 11 are currently being extended to the (bacterio)chlorins.…”

Synthesis and reactivity of aliphatic sulfur pentafluorides from substituted (pentafluorosulfanyl)benzenes

“…The presumed intermediate 2-(pentafluorosulfanyl)-1,4-benzoquinone ( 12 ) in the pathway toward 4 was prepared independently ( Scheme 4 ). Azo coupling [ 20 ] of phenol 11 with a diazonium salt prepared from sulfanilic acid afforded red azo product 13 . The regioselectivity is governed by the strong para -directing hydroxy group despite the presence of the bulky SF 5 group.…”

“…Compounds with substituents ortho to the SF 5 group are relatively uncommon [ 21 ]. Reduction with sodium dithionite [ 20 ] provided aminophenol 14 in good overall yield and oxidation with MnO 2 [ 22 ] gave benzoquinone 12 also in a good yield. Further oxidation with H 2 O 2 /H 2 SO 4 resulted in a mixture of four SF 5 -containing products.…”

Rotaxanation as a sequestering template to preclude incidental metal insertion in complex oligochromophores

“…multisite catalysts, mimics of biochemically important species (e.g., cytochrome c oxidase), sensors, molecular electronics and/or solar-energy conversion. [1][2][3][4][5][6][7][8][9][10][11][12][13] An intensively studied area of application involves the use of porphyrinoids as components of artificial photosynthetic system models [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] with corroles making up a significant class of compounds used for this purpose. [30][31][32][33][34][35] We have previously contributed to this field with our work on corrole-porphyrin dendritic conjugates synthesized by using copper-mediated click reaction.…”