Highly Chemoselective Catalytic Photooxidations by Using Solvent as a Sacrificial Electron Acceptor

Catalyst recovery is an integral part of photoredox catalysis. It is often solved by adding another component-a sacrificial agent-whose role is to convert the catalyst back into its original oxidation state. However, an additive may cause a side reaction thus decreasing the selectivity and overall efficiency. Herein, we present a novel approach towards chemoselective photooxidation reactions based on suitable solvent-acetonitrile acting simultaneously as an electron acceptor for catalyst recovery, and on anaerobic conditions. This is allowed by the unique properties of the catalyst, 7,8-dimethoxy-3-methyl-5-phenyl-5-deazaflavinium chloride existing in both strongly oxidizing and reducing forms, whose strength is increased by excitation with visible light. Usefulness of this system is demonstrated in chemoselective dehydrogenations of 4-methoxy- and 4-chlorobenzyl alcohols to aldehydes without over-oxidation to benzoic acids achieving yields up to 70 %. 4-Substituted 1-phenylethanols were oxidized to ketones with yields 80–100 % and, moreover, with yields 31-98 % in the presence of benzylic methyl group, diphenylmethane or thioanisole which are readily oxidized in the presence of oxygen but these were untouched with our system. Mechanistic studies based on UV-Vis spectro-electrochemistry, EPR and time-resolved spectroscopy measurements showed that the process involving an electron release from an excited deazaflavin radical to acetonitrile under formation of solvated electron is crucial for the catalyst recovery

Sulfur- and Nitrogen-Containing Porous Donor-Acceptor Polymers as Real-Time Optical and Chemical Sensors

Fully aromatic, organic polymers have the advantage of being composed from light, abundant elements, and are hailed as candidates in electronic and optical devices “beyond silicon”, yet, applications that make use of their π-conjugated backbone and optical bandgap are lacking outside of heterogeneous catalysis. Herein, we use a series of sulfur- and nitrogen-containing porous polymers (SNPs) as real-time optical and electronic sensors reversibly triggered and re-set by acid and ammonia vapors. Our SNPs incorporate donor-acceptor and donor-donor motifs in extended networks and enable us to study the changes in bulk conductivity, optical bandgap, and fluorescence life-times as a function of π-electron de-/localization in the pristine and protonated states. Interestingly, we find that protonated donor-acceptor polymers show a decrease of the optical bandgap by 0.42 eV to 0.76 eV and longer fluorescence life-times. In contrast, protonation of a donor-donor polymer does not affect its bandgap; however, it leads to an increase of electrical conductivity by up to 25-fold and shorter fluorescence life-times. The design strategies highlighted in this study open new avenues towards useful chemical switches and sensors based on modular purely organic materials

Determination of the oligomeric state of Mtb Pck by analytical ultracentrifugation.

<p>(A) Sedimentation velocity experiment. Fitted data (upper panel) with residual plots (middle panel) representing the accuracy of the fit are shown together with the calculated continuous size distribution(s) of the sedimenting species (lower panel). (B) Equilibrium sedimentation distribution of Pck at 10–12–14–16–18–20–22,000 rpm. The upper panel shows absorbance data with fitted curves (non-interacting discrete species model, lines); the lower panel shows residuals derived from the fitted data.</p

The crystal structure of Mtb Pck.

<p>(A) Overall three-dimensional structure and secondary structural elements of Pck-GDP-Mn²⁺ complex. The protein is shown in cartoon representation. The N-terminal domain (residues 1–241), C-terminal domain (residues 242–309 and 407–604) are colored green and blue, respectively), and small N-terminal subdomain (residues 310–406) is red. GDP is shown as yellow sticks. (B) Detail of the guanine base of GDP, which interacts in the Pck active site with aromatic residues Phe502, Phe510, and Phe515. The GDP F_o—F_c electron density map rendered at 3σ prior to the inclusion of the ligands into the model is shown as a blue mesh. The carbon atoms of GDP are shown in green, oxygen and nitrogen are colored red and blue, respectively. Phosphate is orange. The residues Phe502, Phe510 and Phe515 are depicted in blue sticks and their solvent accessible surface are colored by electrostatic potential (red for negative, blue for positive). (C) Interactions of Mn²⁺ (purple sphere) in the active site of Pck-GDP-Mn²⁺ complex. Mn²⁺ is octahedrally coordinated by the side chains oxygen of Thr276, O3B atom of GDP and four water molecules (shown as red spheres). The 2F_o—F_c electron density map rendered at 1.5σ p is shown as blue mesh. Coordinate bonds are shown as dashed lines. The carbon atoms of GDP and Thr276 are shown in green, oxygen, nitrogen and phosphate are colored red, blue and orange, respectively.</p

Activities of Pck mutants.

<p>(A) Dependence of the gluconeogenic reaction velocities of Pck mutants on GTP concentration. (B) Dependence of the anaplerotic reaction velocities of Pck mutants on GDP. The assays were performed as described in Materials and Methods. The concentrations of individual components were as follows: 2 mM PEP and 2 U/ml MDH for the anaplerotic reaction; 0.3 mM OAA, 10 U/ml LDH, and 3 U/ml PK for the gluconeogenic reaction.</p

Data collection statistics.

<p>Rmerge = ΣhklΣi<i>I</i>i(hkl)—⟨<i>I</i>(hkl)⟩|/ ΣhklΣ i <i>I</i>i(hkl), where <i>I</i>i(hkl) is the individual intensity of the ith observation of reflection hkl and ⟨<i>I</i>(hkl)⟩ is the average intensity of reflection hkl with summation over all data.</p><p>^b Wilson B calculated with the Sfcheck program from the CCP4 suite (CCP4, 1994).</p><p>^c R-value = ||<i>F</i>_o|—|<i>F</i>_c||/|<i>F</i>_o|, where <i>F</i>_o and <i>F</i>_c are the observed and calculated structure factors, respectively.</p><p>^d R_free is equivalent to R value but is calculated for 5% of the reflections chosen at random and omitted from the refinement process [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120682#pone.0120682.ref025" target="_blank">25</a>].</p><p>^e as determined by MolProbity [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120682#pone.0120682.ref026" target="_blank">26</a>].</p><p>Data collection statistics.</p

Kinetic constants for wt Mtb Pck in the presence of different divalent cations.

<p>^<i>a</i>For gluconeogenic reaction 100 mM HEPES-NaOH, pH 7.2, 0.3 mM OAA, 0.2 mM GTP, 2 mM MgCl₂, 0.2 mM MnCl₂, 10 mM DTT, 10 U/ml LDH, 3 U/ml PK and 0.2 mM NADH was used.</p><p>^<i>b</i>For anaplerotic reaction 100 mM HEPES-NaOH, pH 7.2, 100 mM KHCO₃, 37 mM DTT, 2 mM PEP, 1 mM GDP, 2 mM MgCl₂, 0.1 mM MnCl₂, 2 U/ml MDH and 0.25 mM NADH was used.</p><p>n.d.: Kinetic constants cannot be calculated.</p><p>Kinetic constants for wt Mtb Pck in the presence of different divalent cations.</p

Mtb Pck Amino acid (AA) residues forming hydrogen bonds and close contacts (˂ 3.7Å) with guanosine-5'-diphosphate (GDP) in Mtb Pck-GDP-Mn²⁺ complex.

<p>Mtb Pck Amino acid (AA) residues forming hydrogen bonds and close contacts (˂ 3.7Å) with guanosine-5'-diphosphate (GDP) in Mtb Pck-GDP-Mn²⁺ complex.</p