Well-defined Cu(I) complexes based on [N,P]-pyrrole ligands catalyzed a highly endoselective 1,3-dipolar cycloaddition.

We herein report the synthesis and catalytic application of a new family of dinuclear Cu(I) complexes based on [N,P]-pyrrole ligands. The Cu(I) complexes (4a-d) were obtained in good yields and their catalytic properties were evaluated in the1,3-dipolar cycloaddition of azomethine ylides and electron-deficient alkenes. The air-stable complexes 4a-d exhibited high endo-diasteroselectivity to obtain substituted pyrrolidines, and the catalytic system showed excellent reactivity and wide substitution tolerance.

Synthesis of (µ2,η3-allyl-η5-oxapentadienyl)diiron pentacarbonyl complexes, an unusual reaction product from η4-(vinylketene)Fe(CO)3 complexes: structure and electron density distribution analysis.

The synthesis of a series of ferrocenylvinylketenes as stable η4-[Fe(CO)3] complexes (3a-f) was successfully accomplished through the reaction of η2-[Fe(CO)4] complexes under mild carbonylation conditions. The reactivity of 3a-f under thermal conditions afforded the unexpected formation of a novel family of (µ2,η3-allyl-η5-oxapentadienyl)diiron pentacarbonyl complexes 5a-f proposed to be formed by a sequence metathesis-haptotropic rearrangement between the starting η4-vinylketene iron(0) complex 3 and a η4-vinylcarbene iron(0) complex trapped in situ after a reversible carbonylation process favored by the thermal conditions. An electron density distribution analysis (EDD) of 5e using high-resolution X-ray diffraction data in combination with the DFT framework was performed to understand the electronic communication between the two iron fragments.

Iodine-promoted insertion of the oxygen atom from water in η4-vinylketene[Fe(CO)3] complexes.

Iodine promotes the in situ formation of iron(II) species from η4-vinylketene[Fe(CO)3] (3a-h) as a key intermediate for the synthesis of 2(5H)-furanones (4a-h) by a sequential water-insertion/carbon-oxygen coupling under mild reaction conditions. Compounds 4a-h were obtained in good to excellent yields. A possible reaction pathway was also proposed by DFT calculations. This methodology can be extended to the synthesis of (5H)-pyrrol-2-ones using anilines, with moderate yields and a few limitations.

Laplacian of the Hamiltonian Kinetic Energy Density as an Indicator of Binding and Weak Interactions.

The kinetic energy is the center of a controversy between two opposite points of view about its role in the formation of a chemical bond. One school states that a lowering of the kinetic energy associated with electron delocalization is the key stabilization mechanism of covalent bonding. In contrast, the opposite school holds that a chemical bond is formed by a decrease in the potential energy due to a concentration of electron density within the binding region. In this work, a topographic analysis of the Hamiltonian Kinetic Energy Density (KED) and its laplacian is presented to gain more insight into the role of the kinetic energy within chemical interactions. This study is focused on atoms, diatomic and organic molecules, along with their dimers. In addition, it is shown that the laplacian of the Hamiltonian KED exhibits a shell structure in atoms and that their outermost shell merge when a molecule is formed. A covalent bond is characterized by a concentration of kinetic energy, potential energy and electron densities along the internuclear axis, whereas a charge-shift bond is characterized by a fusion of external concentration shells and a depletion in the bonding region. In the case of weak intermolecular interactions, the external shell of the molecules merge into each other resulting in an intermolecular surface comparable to that obtained by the Non-covalent interaction (NCI) analysis.

Vorticity: Simplifying the analysis of the current density.

The induced current density (J(r)) provides useful information about the electronic structure of molecules under a magnetic field (B). However, the analysis of its topology is cumbersome because of its vectorial nature. We show that its tropicity (direction of rotation) and its strength can be compressed in the triple product B ⋅ ∇ × J(r) (tpJ(r)) that is a scalar field. The topology of tpJ(r) has clear similarities to the Laplacian of the electron density. Additionally, the topology of aromatic and antiaromatic compounds is notoriously different. The vorticity of J(r) is helpful to define the circulation of the current density, C, that contrary to other methods, can be easily defined for individual rings in polycyclic molecules. This allows tpJ(r) to clearly reproduce the Clar's structure of polycyclic aromatic hydrocarbons. © 2019 Wiley Periodicals, Inc.

J(Si,H) Coupling Constants of Activated Si-H Bonds.

We outline in this combined experimental and theoretical NMR study that sign and magnitude of J(Si,H) coupling constants provide reliable indicators to evaluate the extent of the oxidative addition of Si-H bonds in hydrosilane complexes. In combination with experimental electron density studies and MO analyses a simple structure-property relationship emerges: positive J(Si,H) coupling constants are observed in cases where M → L π-back-donation (M = transition metal; L = hydrosilane ligand) dominates. The corresponding complexes are located close to the terminus of the respective oxidative addition trajectory. In contrast negative J(Si,H) values signal the predominance of significant covalent Si-H interactions and the according complexes reside at an earlier stage of the oxidative addition reaction pathway. Hence, in nonclassical hydrosilane complexes such as Cp2Ti(PMe3)(HSiMe3-nCln) (with n = 1-3) the sign of J(Si,H) changes from minus to plus with increasing number of chloro substituents n and maps the rising degree of oxidative addition. Accordingly, the sign and magnitude of J(Si,H) coupling constants can be employed to identify and characterize nonclassical hydrosilane species also in solution. These NMR studies might therefore help to reveal the salient control parameters of the Si-H bond activation process in transition-metal hydrosilane complexes which represent key intermediates for numerous metal-catalyzed Si-H bond activation processes. Furthermore, experimental high-resolution and high-pressure X-ray diffraction studies were undertaken to explore the close relationship between the topology of the electron density displayed by the η2(Si-H)M units and their respective J(Si,H) couplings.

J(Si,H) Coupling Constants in Nonclassical Transition-Metal Silane Complexes.

We will outline that the sign and magnitude of J(Si,H) coupling constants provide a highly sensitive tool to measure the extent of Si-H bond activation in nonclassical silane complexes. Up to now, this structure-property relationship was obscured by erroneous J(Si,H) sign determinations in the literature. These new findings also help to identify the salient control parameters of the Si-H bond activation process in nonclassical silane complexes.

The role of induced current density in Steroelectronic effects: Perlin effect.

The normal and reverse Perlin effect is usually explained by the redistribution of electron density produced by hyperconjugative mechanisms, which increases the electron population within axial or equatorial proton in normal or reverse effect, respectively. Here an alternative explanation for the Perlin effect is presented on the basis of the topology of the induced current density, which directly determines the nuclear magnetic shielding. Current densities around the C-H bond critical point and intra-atomic and interatomic contributions to the magnetic shielding explain the observed Perlin effect. The balance between intra-atomic and interatomic contributions determines the difference in the total atomic shielding. Normal Perlin effect is dominated by intra-atomic part, whereas reverse effect is dominated by interatomic contribution.

Anagostic interactions under pressure: attractive or repulsive?

Square-planar d(8)-ML4 complexes might display subtle but noticeable local Lewis acidic sites in axial direction in the valence shell of the metal atom. These sites of local charge depletion provide the electronic prerequisites to establish weakly attractive 3c-2e M⋅⋅⋅H-C agostic interactions, in contrast to earlier assumptions. Furthermore, we show that the use of the sign of the (1)H NMR shifts as major criterion to classify M⋅⋅⋅H-C interactions as attractive (agostic) or repulsive (anagostic) can be dubious. We therefore suggest a new characterization method to probe the response of these M⋅⋅⋅H-C interactions under pressure by combined high pressure IR and diffraction studies.

Role of carbocation's flexibility in sesquiterpene biosynthesis: computational study of the formation mechanism of terrecyclene.

Two mechanisms have been proposed in the literature to explain the formation of the skeleton of terrecyclic acid from farnesyl diphosphate. Both mechanisms satisfy the experimental data obtained using isotopic labeling, but computational results at the mPW1B95/6-31+G(d,p) level of theory allow the differentiation between them. While one of the mechanisms is basically a carbocation cascade, the other one requires several steps that imply high energetic demands. Specifically, there is a [1,3] hydride shift that requires approximately 100 kcal/mol making this mechanisms unlikely. The other mechanism is more plausible, and it suggests the participation of two secondary carbocation as intermediates, but these were not observed as minimums on the potential energy surface analyzed; they only appear as a point near the transition state in the intrinsic reaction coordinate. Both mechanisms proposed a [1,3] hydride shift, but in the less likely mechanism, the rigidity of the intermediate that undergoes the hydride shift greatly increases the energy of the corresponding transition state.

Conformational properties of the germacradienolide 6-epidesacetyllaurenobiolide by theory and NMR analyses.

Knowing the conformational properties of 1(10),4-cyclodecadiene gamma-lactones is important because of the biogenetic and evolutionary implications on the different groups of sesquiterpene lactones. Despite their importance, there are no physicochemical data on the conformational dynamic and the potential energy surface associated with the conformational changes of the cyclodecadiene ring. Fischer's biogenetic theory on the origin of ambrosanolides and helenanolides has support in the results presented since the conformers that yield two groups of sesquiterpene lactones coexist in solution as demonstrated by dynamic NMR experiments. These results are important on the basis of Fischer's proposal that states that the biosynthesis of each group of pseudoguaianolides requires a specific enzyme to select the right conformer to start the electrophilic cyclization. The germacra-1(10),4-dien-12,8alpha-olides can exist as a mixture of four different conformations, [(15)D(5),(1)D(14)], [(15)D(5),(1)D(14)], [(15)D(5),(1)D(14)], and [(15)D(5),(1)D(14)], and it is also proposed that the configuration of trans-annular cyclization depends on the conformation of the precursor. The results of the study presented herein show that 6-epi-desacetyllaurenobiolide exists in solution at room temperature as a mixture of two stable conformers, [(15)D(5),(1)D(14)] and [(15)D(5),(1)D(14)], which are more stable due to the diminishing of the so-called allylic strain. Analysis of the potential energy surface associated with the conformational interchange showed two other conformers that are intermediaries in the equilibria between [(15)D(5),(1)D(14)]/[(15)D(5),(1)D(14)] and [(15)D(5),(1)D(14)]/[(15)D(5),(1)D(14)]. This indicates the presence of six different conformers participating in the global process instead of the four that have been proposed. The experimental values of DeltaH(++), DeltaG(++), DeltaH(conf), and DeltaG(conf) of the conformational exchange and those calculated at the mPW1B95/6-31+G(d,p) level of theory are very similar, indicating that such level of theory is adequate for the description of this conformational equilibrium.