Synthesis of Ruthenium Bipyridyl Linked with Steroidal Oxidative Quencher for Photo Redox Studies

: Synthesis of ruthenium bipyridyl linked with steroid having oxidative quencher, viologens for study of photo redox properties were described.

It is most attractive feature if ruthenium connected to through a non-conjugate bridge biomolecules having oxidative quencher, such as Viologens 15 . because ruthenium and oxidative quencher viologens have their spectroscopic versatility. Both the donor (Ru(bpy)3 2+*) and acceptor groups can be probed by a variety of different spectroscopic methods. Cholic acid 16 is one of the primary steroids, it has three hydroxyls groups oriented at positions of C3, C7 and C12 on the polar concave side of the molecule, which is defined as hydrophilic face. Its hydrophobic steroidal backbone is defined as -face. The facial amphiphiles of cholic acid can lead to self-aggregation in solution or aggregation with other specific supramolecular arrangement. Therefore, cholic acid has been widely used as the building block in supramolecular chemistry to transport ions and polar molecules through various membranes. In addition, cholic acid can be potentially used as an adjuvant of liver-specific drugs and absorption enhancers. Similarly, viologens are a well-studied species exhibiting three reversible redox states, possessing valuable electrochromic and electron-accepting properties. because of its properties, viologens have become of great interest as functional materials in a wide array of applications; a few to name include electrochromic devices, molecular machines, and organic batteries 17 . In the combined molecules, if reduction of the biomolecule is desired, a directly photoinduced electron transfer from ruthenium polypyridyl complexes through the bridging ligand, Mebpy-COOH, can be performed. 18 If oxidation of the biomolecule is desired, a flashquench method can be employed 19,20 . In the flash-quench method, the excited ruthenium polypyridyl complexes have to be quenched from the non-bridged ligands first, followed by the ET through a bridging ligand 21 .
Present work: In our lab, after synthesizing the Fluorescence Probe 22 to investigate cytochrome c folding kinetics, herein we disclosed the synthesis of a complex molecules having photoredox ruthenium bipyridyl were linked with biomolecules having oxidative quencher, viologen. Herein we used lithocholic acid instead of cholic acid to simplify the synthetic route.
Synthesis Part: Commercially available lithocholic acid was esterified in presence of PTSA to obtained ester 1, in 90% yield, which on reduction by using LAH to get the diol, 2 in 85% yield.
Selective mono tosylation by using tosyl chloride in presence of base, TEA for 72 h at 4 o C, isolated the mono-tosylated product 3, which successfully converted to iodo product 23 yield, under reflux condition with sodium Iodide in acetone. The mono viologen 24 5, is obtained 88 yield by following the modified procedure. Treating the mono viologen 5 with iodo compound 4 in presence of DCC reagent 25 to obtain the salt, which was characterized by NMR and Mass. (Scheme-1).
Ruthenium carbonate is prepared in two steps by treating 2,2-bipyridyl with ruthenium trichloride to obtained intermediate product 30 in 40% yield, which on treated with sodium carbonate under Argon atmosphere gives ruthenium carbonate 8 in 67% yield. (scheme-4) Ruthenium carbonate on treated with compounds 10 under heating condition. The product was separated by using the solubility differences of substrates to get 120 mg of pure product 31,32 in 59% yield. (scheme-5).

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Conclusion: A photoredox complex molecule with lithocholic acid groups as Lineker between Ruthenium bipyridyl and mono viologen was successfully synthesized by coupling reactions for the first time to study its photo redox, fluorescence, and electrochemical properties.
Experimental Part: All reagents were purchased from commercial sources and used without any further purification. Technical solvents were used unless otherwise stated. Anhydrous solvents were obtained by passing solvent through columns of molecular sieves in a solvent purification system. Flash chromatography employed 230-400 mesh silica gel. Solvents used for chromatography are quoted as volume/volume ratios. Analytical thin layer chromatography was performed using silica gel plates precoated with silica gel 60 F254 (0.2 mm) using UV light and 10% ethanolic solution phosphomolybdic acid dip to visualize the products. 1 H NMR spectra were recorded at 298 K unless otherwise stated using Varian VXR (200 MHz) spectrometers. Data is expressed in parts per million (ppm) downfield shift from tetramethyl silane with residual solvent as an internal reference ( δ 7.26 ppm for chloroform) and is reported as position ( δ in ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet = multiplet), coupling constant (J in Hz) and integration (number of protons). 13 C NMR spectra were recorded at 298 K unless otherwise stated using Bruker Avance III 100 MHz spectrometers with complete proton decoupling. Data is expressed in parts per million (ppm) downfield shifts relative to the internal reference ( δ 77.2 ppm for the central peak of deuterated chloroform) and is reported as position ( δ in ppm). IR spectra were recorded on a Perkin Elmer 688 spectrometer. Mass spectra were obtained on a Shimadzu QP 1000 spectrometer. High resolution mass spectra (HRMS) were recorded using electrospray ionization on a Time of Flight (TOF) mass spectrometer at National Taiwan University.