Ligand-induced acceleration of the intramolecular [3 + 2] cycloaddition between alkynes and alkylidenecyclopropanes

[chemical reaction: see text]. Bulky phosphite L6 and several sterically robust phosphoramidites are excellent ligands for promoting the Pd-catalyzed [3 + 2] intramolecular cycloaddition between alkylidenecyclopropanes and alkynes. The use of these ligands allows for low catalyst loadings and facilitates the otherwise sluggish cycloaddition of hept-6-ynylidenecyclopropane systems.     

Understanding the mechanism of base-assisted decomposition of (N-halo),N-alkylalcoholamines

The base-assisted decomposition of (N-X),N-methylethanolamine (X = Cl, Br) takes place mainly through two concurrent processes: a fragmentation and an intramolecular elimination. The global process follows second order kinetics, first order relative to both (N-X),N-methylethanolamine and base. Interaction of the base with the ionizable hydroxylic hydrogen triggers the reaction. The intramolecular elimination pathway leads to formaldehyde and 2-aminoethanol as reaction products via base-assisted proton transfer from the methyl to the partially unprotonated hydroxylic oxygen, with loss of halide. Meanwhile, the fragmentation pathway leads to methylamine and two equivalents of formaldehyde via bimolecular base-promoted concerted breakage of the molecule into formaldehyde, halide ion and N-methylmethanimine. Kinetic evidences allow a crude estimation of the concertedness and characterization of the transition structure for both processes, which are slightly asynchronous, the proton transfer to the base taking place ahead of the rest of the molecular events. The degree of asynchroneity increases as the bases become weaker. Electronic structure calculations, at the B3LYP/6-31++G** level, on the fragmentation pathway support the proposed mechanism.     

Layered Monophosphate Tungsten Bronzes [Ba(PO4)2]WmO3m−3: 2D Metals with Locked Charge-Density-Wave Instabilities

Phosphate tungsten and molybenum bronzes represent an outstanding class of materials displaying textbook examples of charge-density-wave (CDW) physics among other fundamental properties. Here we report on the existence of a novel structural branch with the general formula [Ba(PO₄)₂][W_mO_3m−3] (m=3, 4 and 5) denominated ′layered monophosphate tungsten bronzes′ (L-MPTB). It results from thick [Ba(PO₄)₂]⁴⁻ spacer layers disrupting the cationic metal-oxide 2D units and enforcing an overall trigonal structure. Their symmetries are preserved down to 1.8 K and the compounds show metallic behaviour with no clear anomaly as a function of temperature. However, their electronic structure displays the characteristic Fermi surface of previous bronzes derived from 5d W states with hidden nesting properties. By analogy with previous bronzes, such a Fermi surface should result into CDW order. Evidence of CDW order was only indirectly observed in the low-temperature specific heat, giving an exotic context at the crossover between stable 2D metals and CDW order.     

Propanolysis of arenesulfonyl chlorides: Nucleophilic substitution at sulfonyl sulfur

We have studied the mechanism of solvolysis of arenesulfonyl chlorides by propan‐1‐ol and propan‐2‐ol at 303‐323 K. Kinetic profiles were appropriately fit by first‐order kinetics. Reactivity increases with electron‐donating substituents. Ortho‐alkyl substituted derivatives of arenesulfonyl chlorides show increased reactivity, but the origin of this “positive” ortho‐effect remains unclear. Likely, ortho‐methyl groups restrict rotation around the C‐S bond, facilitating the attack of the nucleophile. No relevant reactivity changes have been found with propan‐1‐ol and propan‐2‐ol in terms of nucleophile steric effect. The existence of isokinetic relationships for all substrates suggests a single mechanism for the series. Solvolysis reactions of all substrates in both alcohols show isokinetic temperatures (T_iso) close to the working temperature range, which is an evidence of the process being influenced by secondary reactivity factors, likely of steric nature in the TS. Solvation plays a relevant role in this reaction, modulating the reactivity. In some cases, the presence of t‐Bu instead of Me in para‐ position leads to changes in the first solvation shell, increasing the energy of the reaction (ca. 1 kJ·mol^?1). The obtained results suggest the same kinetic mechanism of solvolysis of arenesulfonyl chlorides for propan‐1‐ol and propan‐2‐ol, as in MeOH and EtOH, where bimolecular nucleophilic substitution (S_N2) takes place with nucleophilic solvent assistance of one alcohol molecule and the participation of the solvent network involving solvent molecules of the first solvation shell.     

Solvent network at the transition state in the solvolysis of hindered sulfonyl compounds

Alcoholysis rates of unhindered benzenesulfonyl chlorides (X‐ArSO₂Cl, X = H‐; 4‐Br‐; 4‐Me‐) are similar in methanol; the same behavior is also observed in ethanol, whereas the reactivity order in iso‐propanol is 4 Me‐ < H‐ < 4‐Br‐. On the other hand, alcoholysis of sterically hindered arenesulfonyl chlorides (X‐ArSO₂Cl) (X = 2,4,6‐Me₃‐3‐NO₂‐; 2,6‐Me₂‐4‐tBu‐; 2,4,6‐Me₃‐; 2,3,5,6‐Me₄‐; 2,4,6‐iPr₃‐; 2,4‐Me₂‐; 2,4,6‐(OMe)₃‐) in all studied alcohols show a significant increase in reactivity, the so‐called positive steric effect. Most of the substrates showed a reaction order b ~ 2 with respect to the nucleophile in methanol and ethanol, and b ~ 3 in iso‐propanol. The correlation between reactivity and the Kirkwood function (1/ξ) gives negative sensitivity (U) for all systems. All substrates showed high sensitivity to media nucleophilicity that depends on Σσ_X. Obtained results suggest the alcoholysis of benzenesulfonyl chlorides proceeds through S_N2 mechanism where the transition state (TS) involves the participation of 2–3 alcohol molecules; such a TS can be cyclic, in the case of unbranched alcohols, or linear, for alcohols with bulkier hydrocarbon groups like iso‐propanol. To include the number of alcohol molecules playing such a role in the TS, the following terminology is proposed: cS_N2s_n for S_N2 reactions involving n solvent molecules in a cyclic (c) TS, where “s” stands for the solvent and “n” is either the closest integer or half‐integer to the reaction order relative to the solvent or, in computational studies, the proposed number of solvent molecules taking part in the TS, whereas S_N2s_n is proposed when the TS is not cyclic. Copyright © 2016 John Wiley & Sons, Ltd.     

Tuning Cellular Biological Functions Through the Controlled Release of NO from a Porous Ti-MOF

Materials for the controlled release of nitric oxide (NO) are of interest for therapeutic applications. However, to date, many suffer from toxicity and stability issues, as well as poor performance. Herein, we propose a new NO adsorption/release mechanism through the formation of nitrites on the skeleton of a titanium-based metal–organic framework (MOF) that we named MIP-177, featuring a suitable set of properties for such an application: (i) high NO storage capacity (3 μmol mg⁻¹_solid), (ii) excellent biocompatibility at therapeutic relevant concentrations (no cytotoxicity at 90 μg mL⁻¹ for wound healing) due to its high stability in biological media (<9 % degradation in 72 hours) and (iii) slow NO release in biological media (≈2 hours for 90 % release). The prospective application of MIP-177 is demonstrated through NO-driven control of mitochondrial respiration in cells and stimulation of cell migration, paving the way for the design of new NO delivery systems for wound healing therapy.     

Influence of the preparation method on the morphology of templated NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel

The synthesis of NiCo₂O₄ spinel by several nanocasting strategies (i.e., multi-step nanocasting, one-step nanocasting and soft-templating), in which nickel and cobalt nitrates are used as precursors and Pluronic P123 as surfactant, is explored. First, in the multi-step nanocasting, the effect of the impregnation method (evaporation, solid–liquid and two-solvent) of the SBA-15 silica template on the morphology of NiCo₂O₄ replica is investigated. The evaporation method seems to be the best choice to obtain mesoporous NiCo₂O₄ powder which, after calcination at 375 °C and subsequent template removal, displays the highest surface area (93.1 m²/g). We have also checked the feasibility of the one-step nanoscating approach for the synthesis of ordered NiCo₂O₄ arrays, though this methodology entails severe difficulties, mainly related to the different decomposition temperature of the nitrate precursors and the P123 surfactant. Finally, randomly oriented, aggregated NiCo₂O₄ nanoparticles are obtained by means of P123 surfactant-assisted soft-templating approach.     

Rhodium‐Catalyzed Intramolecular [3+2+2] Cycloadditions between Alkylidenecyclopropanes,Alkynes, and Alkenes

A Rh‐catalyzed intramolecular [3+2+2] cycloaddition is reported. The cycloaddition affords synthetically relevant 5,7,5‐fused tricyclic systems of type 2 from readily available dienyne precursors. The transformation takes place with moderate or good yields, high diastereoselectivity, and total chemoselectivity.     

First kinetic discrimination between carbon and oxygen reactivity of enols

Nitrosation of enols shows a well-differentiated behavior depending on whether the reaction proceeds through the carbon (nucleophilic catalysis is observed) or the oxygen atom (general acid-base catalysis is observed). This is due to the different operating mechanisms for C- and O-nitrosation. Nitrosation of acetylacetone (AcAc) shows a simultaneous nucleophilic and acid-base catalysis. This simultaneous catalysis constitutes the first kinetic evidence of two independent reactions on the carbon and oxygen atom of an enol. The following kinetic study allows us to determine the rate constants for both reaction pathways. A similar reactivity of the nucleophilic centers with the nitrosonium ion is observed.