Exploring the Nonenzymatic Origin of Duclauxin-like Natural Products.

Chemical-biological efforts to increase the diversity of duclauxin ()-like molecules for medicinal chemistry purposes unveiled the reactivity of duclauxin () toward amines and alcohols. To expand the compound class, a semisynthetic strategy conjugating amines to duclauxin () was employed. Insights gained from this approach led to the hypothesis that certain duclauxin-like "natural products" such as talaromycesone B (), bacillisporin G (), xenoclauxin (), bacillisporins F (/), bacillisporins J (/), bacillisporins I (/), and verruculosin A () may be isolation artifacts rather than enzymatic products.

Stereoelectronic interactions are too weak to explain the molecular conformation in solid state of -2--butyl-5-(-butylsulfonyl)-1,3-dioxane.

-2--Butyl-5-(-butylsulfonyl)-1,3-dioxane (-1) exhibits a high degree of eclipsing in the H-C5-S-C segment in the solid state, the origin of which remains unexplained. The eclipsed conformation that corresponds to an energetic minimum in the solid state practically corresponds to a rotational transition state in solution, which allows an approach to understand transitions states. The difference in the enthalpy of sublimation Δ between -1 and the more stable -1 is 8.

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 (μ,η-allyl-η-oxapentadienyl)diiron pentacarbonyl complexes, an unusual reaction product from η-(vinylketene)Fe(CO) complexes: structure and electron density distribution analysis.

The synthesis of a series of ferrocenylvinylketenes as stable η-[Fe(CO)] complexes (3a-f) was successfully accomplished through the reaction of η-[Fe(CO)] complexes under mild carbonylation conditions. The reactivity of 3a-f under thermal conditions afforded the unexpected formation of a novel family of (μ,η-allyl-η-oxapentadienyl)diiron pentacarbonyl complexes 5a-f proposed to be formed by a sequence metathesis-haptotropic rearrangement between the starting η-vinylketene iron(0) complex 3 and a η-vinylcarbene iron(0) complex trapped 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.

Formation of the quasi-planar B boron cluster: topological path from B and disk aromaticity.

Formation and stability of the B boron cluster were investigated using a topological approach and the disk aromaticity model. An extensive global energy minimum search for the B system which was carried out by means of the Mexican Enhanced Genetic Algorithm (MEGA) in conjunction with density functional theory computations, confirms a quasi-planar structure as its energetically most stable isomer. Such a structural motif is derived by applying a topological leapfrog operation to a B form.

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

Iodine promotes the formation of iron(II) species from η-vinylketene[Fe(CO)] (3a-h) as a key intermediate for the synthesis of 2(5)-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.

Triangulenes and theirs ions: reaching the limits of Clar's rule.

The triangulenes and their closed-shell ions are a family of polycyclic aromatic hydrocarbons (PAH) that have possible applications in fields as different as spintronics or catalysis. However, the electron delocalization in such systems is not well understood because there are several differences to classical PAHs. We found that the triangulene cations are always more delocalized than the radicals or the anions, independently of the π e count.

How to Bend a Cumulene.

Allenes (carbodicarbenes) and [3]cumulenes are linear carbon chains that can be bent when the terminal group has a strong carbene nature. This bending can be quite pronounced in allenes but not in [3]cumulenes. In this study, how N-heterocyclic or cyclic (alkyl)(amino) carbene (NHC and CAAC, respectively) terminal groups can modify the linear structure of [n]cumulenes has been analyzed.

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.

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.

Origin of the Photoinduced Geometrical Change of Copper(I) Complexes from the Quantum Chemical Topology View.

Copper(I) complexes (CICs) are of great interest due to their applications as redox mediators and molecular switches. CICs present drastic geometrical change in their excited states, which interferes with their luminescence properties. The photophysical process has been extensively studied by several time-resolved methods to gain an understanding of the dynamics and mechanism of the torsion, which has been explained in terms of a Jahn-Teller effect.

Are boat transition states likely to occur in Cope rearrangements? A DFT study of the biogenesis of germacranes.

It has been proposed that elemanes are biogenetically formed from germacranes by Cope sigmatropic rearrangements. Normally, this reaction proceeds through a transition state with a chair conformation. However, the transformation of schkuhriolide (germacrane) into elemanschkuhriolide (elemane) may occur through a boat transition state due to the final configuration of the elemanschkuhriolide, but this transition state is questionable due to its high energy.

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.

Delocalized and localized donating-accepting Mn-C interactions in half-sandwich cyclopentadienyl and pentadienyl complexes.

The widely used cyclopentadienyl (Cp) ligand and the pentadienyl (Pdl) ligand have very similar molecular orbitals (MOs) but they have significantly different chemical behavior. The extensively mixing of the MOs in the Pdl ligand has prevented a quantitative explanation of the observed differences between these two ligands. In a series of synthesized close/open half sandwich manganese phosphine carbonyls, the quantum theory of atoms in molecules and the interacting quantum atom model allow us to quantify the accepting-donating capacities of both ligands and to break down the M-dienyl bond into individual atomic interactions.

Torquoselectivity in Cyclobutene Ring Openings and the Interatomic Interactions That Control Them.

Torquoselectivity has explained diasteromeric preferences of a number electrocyclic ring openings. The quantum theory of atoms in molecules (QTAIM), the electron localizability indicator (ELI-D), and the interacting quantum atoms (IQA) energy partition method are used to evaluate qualitatively and quantitatively the atomic interactions behind the torquoselectivity of a series of 3-substituted cyclobutenes. ELI-D topology and IQA energies show that the interaction between the distal terminus carbon atom of cyclobutene (C4) with the substituent at C3 (R5) in the transition state governs torquoselectivities.

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.

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.

A unifying bonding concept for metal hydrosilane complexes.

Experimental and theoretical charge density studies and molecular orbital analyses suggest that the complexes [Cp2Ti(PMe3)SiH2Ph2] (1) and [Cp2Ti(PMe3)SiHCl3] (2) display virtually the same electronic structures. No evidence for a significant interligand hypervalent interaction could be identified for 2. A bonding concept for transition-metal hydrosilane complexes aims to identify the true key parameters for a selective activation of the individual M-Si and Si-H bonds.

On the chemical shifts of agostic protons.

Agostic hydrogen atoms in planar d(8) transition metal complexes display a remarkable wide range of chemical shifts from +5 to -10 ppm in the proton NMR spectra. It is therefore surprising that a simple recipe can be elaborated to predict the influence of the local electronic structure of the metal atom on the shielding of the coordinating protons: In cases where the agostic hydrogen atom is pointing to a local Lewis acidic center at the metal the (1)H NMR signal is shifted upfield relative to the scenario where the proton is opposing a local charge concentration at the metal. To trace the physical origin of this empirical relationship, a systematic study has been performed to understand how the (i) topology of the electron density and (ii) orientation of the magnetic field vector, B0, control the paratropic or diatropic characteristics of the induced current density at the metal atom and thus the shielding or deshielding of the agostic protons.