Gold‐Catalyzed 1,2‐/1,2‐Bis‐acetoxy Migration of 1,4‐Bis‐propargyl Acetates: A Mechanistic Study

The late transition metal catalyzed rearrangement of propargyl acetates offers an interesting platform for the development of synthetically useful transformations. We have recently shown that gold complexes can catalyze a highly selective tandem 1,2‐/1,2‐bis‐acetoxy migration in 1,4‐bis‐propargyl acetates to form 2,3‐bis‐acetoxy‐1,3‐dienes. In this way, (1Z,3Z)‐ or (1Z,3E)‐ and (1E,3Z)‐1,3‐dienes could be obtained in a stereocontrolled manner depending on the electronic and steric features of the ancillary ligand bound to gold and the substituents at the propargylic positions. In this work, we report an experimental study on the scope of this transformation, plus a detailed theoretical examination of the reaction mechanism, which has revealed the key features responsible for the reaction stereoselectivity. Synthetic applications towards the one‐pot synthesis of quinoxaline heterocycles and tandem Diels–Alder processes have also been devised.     

Two C3‐Symmetric Dy3III Complexes with Triple Di‐μ‐methoxo‐μ‐phenoxo Bridges,Magnetic Ground State,and Single‐Molecule Magnetic Behavior

Two series of isostructural C₃‐symmetric Ln₃ complexes Ln₃ ? [BPh₄] and Ln₃ ? 0.33[Ln(NO₃)₆] (in which Ln^III=Gd and Dy) have been prepared from an amino‐bis(phenol) ligand. X‐ray studies reveal that Ln^III ions are connected by one μ₂‐phenoxo and two μ₃‐methoxo bridges, thus leading to a hexagonal bipyramidal Ln₃O₅ bridging core in which Ln^III ions exhibit a biaugmented trigonal‐prismatic geometry. Magnetic susceptibility studies and ab initio complete active space self‐consistent field (CASSCF) calculations indicate that the magnetic coupling between the Dy^III ions, which possess a high axial anisotropy in the ground state, is very weakly antiferromagnetic and mainly dipolar in nature. To reduce the electronic repulsion from the coordinating oxygen atom with the shortest Dy?O distance, the local magnetic moments are oriented almost perpendicular to the Dy₃ plane, thus leading to a paramagnetic ground state. CASSCF plus restricted active space state interaction (RASSI) calculations also show that the ground and first excited state of the Dy^III ions are separated by approximately 150 and 177 cm^?1, for Dy₃ ? [BPh₄] and Dy₃ ? 0.33[Dy(NO₃)₆], respectively. As expected for these large energy gaps, Dy₃ ? [BPh₄] and Dy₃ ? 0.33[Dy(NO₃)₆] exhibit, under zero direct‐current (dc) field, thermally activated slow relaxation of the magnetization, which overlap with a quantum tunneling relaxation process. Under an applied H_dc field of 1000 Oe, Dy₃ ? [BPh₄] exhibits two thermally activated processes with U_eff values of 34.7 and 19.5 cm^?1, whereas Dy₃ ? 0.33[Dy(NO₃)₆] shows only one activated process with U_eff=19.5 cm^?1.     

Reductive Dimerization of Triruthenium Clusters Containing Cationic Aromatic N‐Heterocyclic Ligands

The cationic cluster complexes [Ru₃(μ‐H)(μ‐κ²N,C‐L¹ Me)(CO)₁₀]⁺ ( 1 ⁺; HL¹ Me=N‐methylpyrazinium), [Ru₃(μ‐H)(μ‐κ²N,C‐L² Me)(CO)₁₀]⁺ ( 2 ⁺; HL² Me=N‐methylquinoxalinium), and [Ru₃(μ‐H)(μ‐κ²‐N,C‐L³ Me)(CO)₁₀]⁺ ( 3 ⁺; HL³ Me=N‐methyl‐1,5‐naphthyridinium), which contain cationic N‐heterocyclic ligands, undergo one‐electron reduction processes to become short lived, ligand‐centered, trinuclear, radical species ( 1 – 3 ) that end in the formation of an intermolecular C? C bond between the ligands of two such radicals, thus leading to neutral hexanuclear derivatives. These dimerization processes are selective, in the sense that they only occur through the exo face of the bridging ligands of trinuclear enantiomers of the same configuration, as they only afford hexanuclear dimers with rac structures (C₂ symmetry). The following are the dimeric products that have been isolated by using cobaltocene as reducing agent: [Ru₆(μ‐H)₂{μ₆‐κ⁴N₂,C₂‐(L¹ Me)₂}(CO)₁₈] ( 5 ; from 1 ⁺), [Ru₆(μ‐H)₂{μ₆‐κ⁴N₂,C₂‐(L² Me)₂}(CO)₁₈] ( 6 ; from 2 ⁺), and [Ru₆(μ‐H)₂{μ₄‐κ⁸N₂,C₆‐(L³ Me)₂}(CO)₁₈] ( 7 ; from 3 ⁺). The structures of the final hexanuclear products depend on the N‐heterocyclic ligand attached to the starting materials. Thus, although both trinuclear subunits of 5 and 6 are face‐capped by their bridging ligands, the coordination mode of the ligand of 5 is different from that of the ligand of 6 . The trinuclear subunits of 7 are edge‐bridged by its bridging ligand. In the presence of moisture, the reduction of 3 ⁺ with cobaltocene also affords a trinuclear derivative, [Ru₃(μ‐H)(μ‐κ²N,C‐L^3′ Me)(CO)₁₀] ( 8 ), whose bridging ligand (L^3′ Me) results from the formal substitution of an oxygen atom for the hydrogen atom (as a proton) that in 3 ⁺ is attached to the C⁶ carbon atom of its heterocyclic ligand. The results have been rationalized with the help of electrochemical measurements and DFT calculations, which have also shed light on the nature of the odd‐electron species, 1 – 3 , and on the regioselectivity of their dimerization processes. It seems that the sort of coupling reactions described herein requires cationic complexes with ligand‐based LUMOs.     

Experimental and DFT study of the aza-Diels-Alder reaction between cyclopentadiene and protonated benzylimine derivated from glyoxylates

The Diels-Alder reaction of protonated N-benzyl imine of methyl glyoxylate with cyclopentadiene in different solvents gave mixtures of exo/endo adducts. The exo/endo selectivity of the reaction was elucidated by NMR experiments. Theoretical calculations by means of density functional theory (DFT) at the B3LYP/6-31G(d) level have also been performed to elucidate the molecular mechanism of this reaction. The DFT results suggest a highly asynchronous concerted mechanism, which in turn can explain the preferred exo stereoselectivity of the reaction. Inclusion of solvent effects enhances the exo selectivity, and this effect increases with the polarity of the solvent, in good agreement with the experimental findings.     

Spectroscopic characterization of halogen- and cyano-substituted pyridinevinylenes synthesized without catalyst or solvent

An efficient Knoevenagel route using green chemistry conditions was applied for the synthesis of halogen- and cyano- substituted pyridinevinylene compounds. Absorption and fluorescence emission spectra of these conjugated compounds were recorded and compared in order to evaluate the effect of substituents on the electronic properties of pyridinevinylene compounds. The substituents studied were terminal Cl and F, two or three aromatic rings, as well as a cyano group attached to a C=C double bond. The compounds synthesized are: (E)-2-(4-fluorostyryl)pyridine, (E)-2-(4-chlorostyryl)pyridine, (E)-4-(4-chlorostyryl)pyridine, 2,3-diphenylacrylonitrile, 3-phenyl-2-(pyridin-2-yl)acrylonitrile, 3-phenyl-2-(pyridin-3-yl)acrylonitrile, 2-phenyl-3-(pyridin-2-yl)acrylonitrile, 3,3′-(1,4-phenylene)bis(2-phenylacrylonitrile), 3,3′-(1,4-phenylene)bis(2-(pyridin-2-yl)acrylonitrile), and 3,3′-(1,4-phenylene)bis(2-(pyridin-3-yl)acrylonitrile). The solvent-free method used in this work allows obtaining each compound by controlling the reaction temperature. The compounds were characterized by infrared spectroscopy and 1H-NMR spectroscopy.     

The Stereochemistry of 1,2,3‐Triols Revealed by 1H NMR Spectroscopy: Principles and Applications

The conformational compositions of the tris(α‐methoxy‐α‐phenylacetic acid) ester derivatives of 1,2,3‐prim,sec,sec‐triols are presented. These conformations have been determined by theoretical and experimental data (i.e., energy‐ and chemical‐shift calculations, circular dichroism (CD) experiments, coupling‐constant analysis, enantioselective deuteration experiments, and low‐temperature NMR spectroscopic studies). A detailed analysis of the anisotropic effects due to the most significant conformers in the ¹H NMR spectra supported the correlation between the ¹H NMR spectra (Δδ^RS value of H(3′) and |Δ(Δδ^RS)| parameters) and the absolute configuration of the substrate. The study also allows the identification of the pro‐R and pro‐S methylene protons from their vicinal coupling constants and relative chemical shifts.     

Dual Nucleophilic Substitution Reactions of O,O‐Diethyl 2,4‐dinitrophenyl Phosphate and Thionophosphate Triesters

The reactions of the title compounds with phenoxides, secondary alicyclic (SA) amines, and pyridines, in 44 wt% ethanol–water, at 25°C and an ionic strength of 0.2 M, were subjected to kinetic and product studies. From analytical techniques (HPLC and NMR), two pathways were detected (nucleophilic attack at the phosphoryl center and at the C‐1 aromatic carbon) for the reactions of all the nucleophiles with the phosphate ( 2 ) and for the pyridinolysis of the thionophosphate ( 1 ). Only aromatic nucleophilic substitution was found for the reactions of 1 with phenoxides and SA amines. For the dual reactions, the nucleophilic rate constants (k_N) were separated in two terms: $k_{\rm N}^{\rm P}$ and $k_{\rm N}^{{\rm Ar}}$, which are the rate constants for the corresponding electrophilic centers. The absence of a break in the Brønsted‐type plots for the attack at P is consistent with concerted mechanisms. The Brønsted slopes, β_Ar 0.32–0.71, for the attack at the aromatic C‐1, are in agreement with stepwise mechanisms where formation of a Meisenheimer complex is the rate‐determining step. © 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 202–211, 2013     

A meshless finite point method for three‐dimensional analysis of compressible flow problems involving moving boundaries and adaptivity

A finite point method for solving compressible flow problems involving moving boundaries and adaptivity is presented. The numerical methodology is based on an upwind‐biased discretization of the Euler equations, written in arbitrary Lagrangian–Eulerian form and integrated in time by means of a dual‐time steeping technique. In order to exploit the meshless potential of the method, a domain deformation approach based on the spring network analogy is implemented, and h‐adaptivity is also employed in the computations. Typical movable boundary problems in transonic flow regime are solved to assess the performance of the proposed technique. In addition, an application to a fluid–structure interaction problem involving static aeroelasticity illustrates the capability of the method to deal with practical engineering analyses. The computational cost and multi‐core performance of the proposed technique is also discussed through the examples provided. Copyright © 2013 John Wiley & Sons, Ltd.     

Phenol Nitration from A 2-(Nitrooxy)Ethyl Side Chain

A novel nitration of phenols is described on 2-(3-hydroxy-4-methoxyphenyl)ethyl nitrate (2), which is synthesized by three alternative routes.