Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by the [α-SiW₉O₃₄]^10– Polyoxometalate

The reaction of Na₁₀[α-SiW₉O₃₄] with tetravalent metallic cations such as 4f ((NH₄)₂Ce­(NO₃)₆), 5d (HfCl₄), or 5f (UCl₄ and Th­(NO₃)₄) in a pH 4.7 sodium acetate buffer solution leads to the formation of four sandwich-type polyoxometalates [Ce₄(μ³-O)₂(SiW₉O₃₄)₂(CH₃COO)₂]^10– (<b>1</b>), [U₄(μ³-O)₂(SiW₉O₃₄)₂(CH₃COO)₂]^10– (<b>2</b>), [Th₃(μ³-O)­(μ²-OH)₃(SiW₉O₃₄)₂]^13– (<b>3</b>), and [Hf₃(μ²-OH)₃(SiW₉O₃₄)₂]^11– (<b>4</b>). All four compounds consist of a polynuclear cluster fragment stabilized by two [α-SiW₉O₃₄]^10– polyanions. Compounds <b>1</b> and <b>2</b> are isostructural with a tetranuclear core (Ce₄, U₄), while compound <b>3</b> presents a trinuclear Th₃ core bearing a μ³-O-centered bridge. It is an unprecedented configuration in the case of the thorium­(IV) cluster. Compound <b>4</b> also possesses a trinuclear Hf₃ core but with the absence of the μ³-O bridge. The molecules have been characterized by single-crystal X-ray diffraction, ¹⁸³W and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis

Deciphering the Mechanism of the Nickel-Catalyzed Hydroalkoxylation Reaction: A Combined Experimental and Computational Study

The [Ni­(0)­(cod)₂]/P^∩P-catalyzed hydroalkoxylation of butadiene to form butenyl ethers is studied mechanistically, where P^∩P = 1,4-bis­(diphenylphosphino)­butane (dppb) and 1,2-bis­(diphenylphosphinomethyl)­benzene (dppmb). Experimental studies suggest the intermediacy of [(P^∩P)­Ni­(0)­(butadiene)] and [(P^∩P)­Ni­(II)­(allyl)] intermediates and rule out the involvement of Ni–H species. The related species [(dppb)­Ni(0)­(1,4-diphenylbutadiene)], <b>1</b>, and [(P^∩P)­Ni­(II)­(crotyl)­(Cl)] complexes <b>2</b> (P^∩P = dppmb) and <b>3</b> (P^∩P = dppb) have been synthesized and characterized on the basis of VT NMR spectroscopy and X-ray crystallographic studies. Compounds <b>2</b> and <b>3</b> are shown to be catalytically competent for the hydroalkoxylation reaction. Computational studies on [(dppmb)­Ni(0)­(butadiene)] indicate a facile protonation that forms a cationic allylic intermediate [(dppmb)­Ni­(II)­(η-C₄H₇)]­OMe. C–O bond formation then occurs via external attack by the solvent-stabilized methoxide nucleophile. Hydroalkoxylation proceeds with modest computed barriers of ca. 18 kcal/mol, and the butenyl ether product formation is only marginally exergonic. Overall, the results are consistent with initial kinetic control leading to the major branched isomer followed by a reversible isomerization process operating under thermodynamic control

Deeper Mechanistic Insight into Ru Pincer-Mediated Acceptorless Dehydrogenative Coupling of Alcohols: Exchanges, Intermediates, and Deactivation Species

The mechanism of acceptorless dehydrogenative coupling reaction (ADC) of alcohols to esters catalyzed by aliphatic pincer P<i>H</i>NP ruthenium complexes was experimentally studied. Relevant intermediate species involved in the catalytic cycle were isolated and structurally characterized by single-crystal X-ray diffraction studies, and their reactivity (including toward substrates related to the catalytic process) was probed. VT NMR studies unveiled several chemical exchanges connecting the Ru amido hydride, the Ru alkoxide, and the alcohol substrate. Under catalytic conditions, in situ IR spectroscopy monitoring demonstrated the production of ester via aldehyde as intermediate. A Tishchenko-like pathway is proposed as the main path for the production of ester from aldehyde, involving alkoxide and hemiacetaloxide Ru species (the latter being identified in the reaction mixture by NMR). Catalytic system deactivation under base-free conditions was found to be related to water traces in the reaction medium (either as impurity or derived from aldol reactions) that lead to the formation of catalytically inactive acetato Ru complexes. These react with alkali metal alkoxides to afford catalytically active Ru species. In line with this observation, running the ADC reaction in the presence of water scavengers or alkoxides allows maintaining sustained catalytic activity

Structural determination of lipids X and Y.

<p>Lipids X and Y were purified from cell wall extract of <i>M. bovis</i> BCG Pasteur culture, following treatment with SRI-224 (5 µg/ml) for 24 h. (A) Conventional TLC showing purity of the samples containing lipids X and Y, that were used for structural analyses, as seen by staining with phosphomolybdic acid and charring. (B) m/z values from MALDI-TOF-MS spectra correspond to [M+Na]⁺ adducts of a family of methylated keto-mycolates and α-mycolates for purified lipids X and Y, respectively. (C) For ¹H-NMR analysis, protons are labelled (a to h) according to their respective positions in functional groups. Relative integrations of protons have been normalized according to the number of ethylenic protons (2 for X and 4 for Y) and are indicated in brackets. *stands for proton ¹H signals of contaminant ethanol present in the NMR tubes.</p

<i>In vivo</i> identification and relative quantification of <i>cis</i>-cyclopropanes by ¹H HR-MAS NMR.

<p>(A) Detail of ¹H HR-MAS spectrum of control whole cells of <i>M. bovis</i> BCG. (B) Unidimensional selective COSY spectrum after irradiation of Ha signal showing ³<i>J</i> and ²<i>J</i> connectivities of <i>cis</i>-cyclopropyl ring Hb and Hc protons to Ha as depicted in (C). (D) Relative quantification by ¹H HR-MAS NMR of <i>cis</i>-cyclopropanes based on differential integration of the Ha signals in control untreated cells (c) or cells treated with TAC-treated (1 µg/ml) (TAC) or SRI-224-treated (1 µg/ml) (224). Results are representative of two independent experiments.</p

Partial recovery of mycolic acid synthesis in the presence of TAC or SRI-224 in strains overexpressing the <i>CMAS</i> genes.

<p>Autoradiogram of FAMEs and MAMEs extracted from exponentially growing cells that were radiolabeled <i>in vivo</i> with ¹⁴C-acetate. (A) <i>M. bovis</i> BCG or (B) <i>M. marinum</i> containing the plasmid vector pMV261 or the same vector carrying <i>cmaA2, mmaA2</i> or <i>pcaA</i> of <i>M. tb</i> H37Rv. Extracts were obtained from untreated control cells (c) or cells treated with 5 µg/ml of either TAC or SRI-224 as indicated. All other details are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0001343#pone-0001343-g003" target="_blank">Figure 3</a>.</p

Inhibition of mycolic acid biosynthesis in <i>M. bovis</i> BCG by treatment with TAC or its analogue SRI-224.

<p>Exponentially-growing cultures were treated with the drugs for 18 h and labeled by adding ¹⁴C-acetate for another 8 h. Fatty acid methyl esters (FAMEs) and mycolic acid methyl esters (MAMEs) were then extracted and separated by TLC on 10% silver nitrate-impregnated plates prior to exposure to a film overnight. All extracts were loaded equally for 100,000 cpm on silica plates impregnated with 10% silver nitrate. The autoradiographs show FAMEs, MAMES, oleic acid methyl esters (OAMEs), α- and keto-mycolates (k) and the lipids X and Y as indicated by arrowheads. (A) 1D TLC analysis using petroleum ether and diethyl ether (17∶3, v/v) as solvents. Drug concentrations employed are indicated in µg/ml. (B) 1D TLC profile of MAMEs extracted from cells treated with low concentrations of SRI-224 for either 1 day or over a period of 5 days, as indicated. (C) Extracts prepared after delipidation of the cells to remove the free and loosely bound lipids, while retaining the covalently bound mycolates. Extract from cells treated with SRI-224 but not subjected to delipidation is included to identify the lipids X and Y by comparison with extracts from delipidated cells that were either untreated (c) or treated with 5 µg/ml of the indicated drug for 24 h. (D) 2D TLC analysis on silica plates impregnated with 10% silver nitrate. Extracts were separated in the first direction by using two developments with hexane/ethyl acetate (19∶1, v/v) and in the second direction by using a triple development with petroleum ether/diethylether (17∶3, v/v). (E) Extracts from cells radiolabeled with [<i>methyl</i>-¹⁴C]-methionine are compared with those from cells radiolabeled with ¹⁴C-acetate.</p

Proposed mechanism for generation of mycolic acid sub-types by the action of CMAS enzymes (A) and inhibition by TAC/SRI-224 (B).

<p>The generation of α- and the oxygenated mycolic acids is considered to follow to two independent pathways. A common, di-unsaturated precursor, Y, is envisaged for the two pathways. Y is subsequently transformed into α-mycolic acids by the action of the MmaA2 and PcaA that modify the distal or proximal double bond, respectively. Action of MmaA4 commits Y to the pathway for the oxygenated mycolic acids, by producing the precursor X. MmaA3, which is required for generation of methoxy-mycolic acids in <i>M. tb</i> is inactive in <i>M. bovis</i> BCG Pasteur due to the presence of a point mutation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0001343#pone.0001343-Behr1" target="_blank">[50]</a>. The proximal double bond is modified by the CmaA2 (and MmaA2) or PcaA to generate <i>trans</i>- or <i>cis</i>-cyclopropanated derivatives, respectively. In the presence of TAC, all the CMASs mentioned above are inhibited, except for MmaA4. Due to inhibition of MmaA2, excess of Y is diverted to MmaA4 leading to generation of X, which accumulates due to lack of activities of CmaA2 and MmaA2. SRI-224 appears to affect MmaA4 to a certain degree, leading to accumulation Y in addition to X. (For simplicity, only the meromycolyl moiety of mycolates has been depicted).</p

Structures and occurrence of mycolic acid sub-types in mycobacterial species presented in this study.

<p>* proximal position of oxygenated mycolates is unsaturated <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0001343#pone.0001343-Daffe2" target="_blank">[37]</a>.</p

Inhibition of synthesis of cell wall mycolic acids in different mycobacterial species by treatment with TAC or its analogue SRI-224.

<p>Autoradiogram of FAMEs and MAMEs extracted from exponentially growing cells that were radiolabeled <i>in vivo</i> with ¹⁴C-acetate. Drug concentrations are indicated in µg/ml. The different mycolates are indicated by arrows. Mycolic acid profiles from <i>M. marinum</i> (A) and from <i>M. chelonae</i> (B). All other details are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0001343#pone-0001343-g003" target="_blank">Figure 3</a>.</p