Concerted C-N and C-H bond formation in a magnesium-catalyzed hydroamination

Coordinatively saturated To(M)MgMe (1; To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) is an active precatalyst for intramolecular hydroamination/cyclization at 50 °C. The empirical rate law of -d[substrate]/dt = k'(obs)[Mg](1)[substrate](1) and Michaelis-Menten-type kinetics are consistent with a mechanism involving reversible catalyst-substrate association prior to cyclization. The resting state of the catalyst, To(M)MgNHCH(2)CR(2)CH(2)CH═CH(2) [R = Ph, Me, -(CH(2))(5)-], is isolable, but isolated magnesium amidoalkene does not undergo unimolecular cyclization at 50 °C. However, addition of trace amounts of substrate allows cyclization to occur. Therefore, we propose a two-substrate, six-center transition state involving concerted C-N bond formation and N-H bond cleavage as the turnover-limiting step of the catalytic cycle.     

Synthesis and Crystal Structure of the Palladium(IV) Polyoxomolybdate,K0.75Na3.75[PdMo6O24H3.5]·17H2O

The first example of a heteropolyoxomolybdate containing palladium(IV) was isolated and characterized by X‐ray crystallography. The palladium(IV) hexamolybdate, K_0.75Na_3.75[PdMo₆O₂₄H_3.5]·17H₂O, was isolated from an aqueous solution at pH 4.5 in the space group P\bar{1} , a 10.790(2), b 12.244(3), c 14.086(3) Å, α 113.77(1), β 90.41(1),γ 107.86(1)°, and the structure was determined using X‐ray diffraction methods, refining to a residual of 0.0301 for 5334 reflections. A formal “[PdMo₆O₂₄H₃]^5–” subunit exhibits the basic Anderson structure, with two [PdMo₆O₂₄H₃]^5– cluster anions in the structure bridged by a hydrogen atom (formally an H⁺) situated on a center of symmetry to give a “[Pd₂Mo₁₂O₄₈H₇]^9–” dimeric anion. The palladium(IV) atom occupies a slightly distorted octahedral environment, with Pd–O distances ranging from 1.968 to 2.009 Å.     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

High‐temperature superconductivity and long‐range order in strongly correlated electronic systems

A selective review of the question of how repulsive electron correlations might give rise to off‐diagonal long‐range order (ODLRO) in high‐temperature superconductors is presented. The article makes detailed explanations of the relevance to superconductivity of reduced electronic density matrices and how these can be used to understand whether ODLRO might arise from Coulombic repulsions in strongly correlated electronic systems. Time‐reversed electron pairs on alternant Cuprate and the iron‐based pnictide and chalcogenide lattices may have a weak long‐range attractive tail and much stronger short‐range repulsive Coulomb interaction. The long‐range attractive tail may find its origin in one of the many suggested proposals for high‐T_c superconductivity and thus has an uncertain origin. A phenomenological Hamiltonian is invoked whose model parameters are obtained by fitting to experimental data. A detailed summary is given of the arguments that such interacting electrons can cooperate to produce a superconducting state in which time‐reversed pairs of electrons effectively avoid the repulsive hard‐core of the Coulomb interaction but reside on average in the attractive well of the long‐range potential. Thus, the pairing of electrons itself provides an enhanced screening mechanism. The alternant lattice structure is the key to achieving robust high‐temperature superconductivity with dx²‐y² or sign alternating s‐wave or s± condensate symmetries in cuprates and iron‐based compounds. Some attention is also given to the question first raised by Leggett as to where the Coulombic energy is saved in the superconducting transition in cuprates. A mean‐field‐type model in which the condensate density serves as an order parameter is discussed. Many of the observed trends in the thermal properties of cuprate superconductors are reproduced giving strong support for the proposed model for high‐temperature superconductivity in such strongly correlated electronic systems. © 2015 Wiley Periodicals, Inc.     

Palladium catalysed alkyne hydrogenation and oligomerisation: a parahydrogen based NMR investigation

The role phosphine ligands play in the palladium(ii)-bis-phosphine-hydride cation catalysed hydrogenation of diphenylacetylene is explored through a PHIP (parahydrogen induced polarization) NMR study. The precursors Pd(LL')(OTf)(2) () [LL' = dcpe (PCy(2)CH(2)CH(2)PCy(2)), dppe, dppm, dppp, cppe (PCy(2)CH(2)CH(2)PPh(2))] are used. Alkyl palladium intermediates of the type [Pd(LL')(CHPhCH(2)Ph)](OTf) ( and ) are detected and demonstrated to play an active role in hydrogenation catalysis. Magnetization transfer experiments reveal chemical exchange from the alpha-H of the alkyl ligand of (LL' = dcpe) and linkage isomer ' (LL' = cppe) into trans-stilbene on the NMR timescale. Activation parameters (DeltaH( not equal) and DeltaS( not equal)) for this transformation have been determined. These experiments, coupled with GC/MS data, indicate that the catalytic activity is greater in methanol, where it follows the order: dcpe > cppe > dppp > dppe > dppm, than in CD(2)Cl(2). All five of the phosphine systems described are less active than those based on bcope [where bcope is (C(8)H(14))PCH(2)-CH(2)P(C(8)H(14))] and (t)bucope [where (t)bucope is (C(8)H(14))PC(6)H(4)CH(2)P((t)Bu)(2)]. cis, cis-1,2,3,4-Tetraphenyl-buta-1,3-diene is detected as a minor reaction product with Pd(LL')(PhCH-CHPh-CPh[double bond, length as m-dash]CHPh)(+) () also being shown to play a role in the alkyne dimerisation step.     

The magnetic concentration gradient force—Is it real?

There are suggestions in the electrochemical literature that a body force F _∇c acts when an electrolyte with a non-uniform concentration c of paramagnetic ions is subject to a uniform magnetic field. We demonstrate, experimentally and theoretically, that no such magnetic body force exists, to first order. A second-order correction associated with the demagnetizing field does lead to a very small concentration-dependent body force, which is not expected to produce any observable effect in electrochemistry. Contribution to special issue on Magnetic field effects in Electrochemistry.

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Diastereoselective ring-closing metathesis: synthesis of P-stereogenic phosphinates from prochiral phosphinic acid derivatives

[reaction: see text] The preparation of phosphorus-containing trienes featuring two diastereotopic vinyl moieties followed by a diastereoselective ring-closing metathesis is described. This methodology allowed for the synthesis of novel highly functionalized P-stereogenic heterocycles featuring both an exo- and an endocyclic double bond. An investigation into the factors influencing the diastereochemical outcome of the ring-closing metathesis is also presented, revealing that the geometry of the double bonds conjugated to phosphorus is important and that 1,3-stereoinduction is superior to 1,4-stereoinduction for these reactions.     

Lengthscale-dependent,elastically anisotropic,physically-based hcp crystal plasticity: Application to cold-dwell fatigue in Ti alloys

A crystal plasticity model for hcp materials is presented which is based on dislocation glide and pinning. Slip is assumed to occur on basal and prismatic systems, and dislocation pinning through the generation of geometrically necessary dislocations (GNDs). Elastic anisotropy and, through the coupling of GNDs with slip rate, physically-based lengthscale effects are included.     

Collective Effects in Ionic Liquid [emim][Tf2N] and Ionic Paramagnetic Nitrate Solutions without Long-Range Structuring

Mesoscopic shear elasticity has been revealed in ordinary liquids both experimentally by reinforcing the liquid/surface interfacial energy and theoretically by nonextensive models. The elastic effects are here examined in the frame of small molecules with strong electrostatic interactions such as room temperature ionic liquids [emim][Tf2N] and nitrate solutions exhibiting paramagnetic properties. We first show that these charged fluids also exhibit a nonzero low-frequency shear elasticity at the submillimeter scale, highlighting their resistance to shear stress. A neutron scattering study completes the dynamic mechanical analysis of the paramagnetic nitrate solution, evidencing that the magnetic properties do not induce the formation of a structure in the solution. We conclude that the elastic correlations contained in liquids usually considered as viscous away from any phase transition contribute in an effective way to collective effects under external stress whether mechanical or magnetic fields.     

Full-trajectory diagnosis of laser-driven radiative blast waves in search of thermal plasma instabilities

Experimental investigations into the dynamics of cylindrical, laser-driven, high-Mach-number shocks are used to study the thermal cooling instability predicted to occur in astrophysical radiative blast waves. A streaked Schlieren technique measures the full blast-wave trajectory on a single-shot basis, which is key for observing shock velocity oscillations. Electron density profiles and deceleration parameters associated with radiative blast waves were recorded, enabling the calculation of important blast-wave parameters including the fraction of radiated energy, epsilon, as a function of time for comparison with radiation-hydrodynamics simulations.