Selective Cross-Metathesis Versus Ring-Closing Metathesis of Terpenes, Taking the Path Less Travelled.

A diiodo ruthenium olefin metathesis pre-catalyst was employed to achieve remarkably selective cross-metathesis reactions of prenylated 1,6-dienes, effectively overcoming the entropically favored intramolecular ring-closing metathesis. This reaction was investigated using Density Functional Theory (DFT) computations and fine-tuned through the application of a Design of Experiments (DoE) approach. The potential of this innovative process was demonstrated through the unprecedented functionalization of various terpene natural products via cross-metathesis, resulting in the synthesis of new derivatives in a single step.

Hydrophenoxylation of alkynes by gold catalysts: a mini review.

CONTEXT: The field of chemistry has significantly evolved, with catalysis playing a crucial role in transforming chemical processes. From Valerius' use of sulfuric acid in the sixteenth century to modern advancements, catalysis has driven innovations across various industries. The introduction of gold as a catalyst marked a pivotal shift, expanding its applications beyond ornamentation to homogeneous catalysis. Gold's unique properties, such as its electrophilic nature and flexibility, have enabled its use in synthesizing complex molecules, including those in nanomedicine and sustainable chemical processes. The development of gold-based complexes, particularly in hydroalkoxylation and hydroamination reactions, showcases their efficiency in forming carbon-oxygen bonds under mild conditions. Recent studies on dual gold catalysis and heterobimetallic complexes further highlight gold's versatility in achieving high turnover rates and selectivity. This evolution underscores the potential of gold catalysis in advancing environmentally sustainable methodologies and enhancing the scope of modern synthetic chemistry. The debate about the nature of monogold and dual-gold catalysis is open. METHODS: DFT calculations have played a key role in promoting the activation of alkynes, in particular the hydrophenoxylation of alkynes by metal-based catalysts. They not only help identify the most efficient and selective catalysts but also aid in screening for those capable of performing a dual metal catalytic mechanism. The most commonly used functionals are BP86 and B3LYP, with the SVP and 6-31G(d) basis sets employed for geometry optimizations, and M06 with TZVP or 6-311G(d,p) basis sets used for single-point energy calculations in a solvent. Grimme dispersion correction has been explicitly added either in the solvent single point energy calculations or in the gas phase geometry optimizations or in both. To point out that M06 implicitly includes part of this dispersion scheme.

From (Sub)Porphyrins to (Sub)Phthalocyanines: Aromaticity Signatures in the UV-Vis Absorption Spectra.

The development of novel synthetic methods has greatly expanded the toolbox available to chemists for engineering porphyrin and phthalocyanine derivatives with precise electronic and optical properties. In this study, we focus on the UV-vis absorption characteristics of substituted phthalocyanines and their contracted analogs, subphthalocyanines, which feature nonplanar, bowl-shaped geometries. These macrocycles, which are central to numerous applications in materials science and catalysis, possess extensive π-conjugated systems that drive their unique electronic properties. We explore how the change from a metalloid (B) to a metal (Zn) and the resulting coordination environments influence the aromaticity and, consequently, the spectroscopic features of these systems. A combined computational and experimental approach reveals a direct correlation between the aromaticity of the external conjugated pathways and the Q bands in the UV-vis spectra. Our findings highlight key structural modifications that can be leveraged to fine-tune the optical properties of porphyrinoid systems, offering new pathways for the design of advanced materials and catalysts with tailored functionalities.

Tuning the steric hindrance of alkylamines: a predictive model of steric editing of planar amines.

Amines are one of the most prevalent functional groups in chemistry. Perhaps even more importantly, amines represent one of the most ubiquitous moieties within the realm of bioactive natural products and life-saving pharmaceuticals. The archetypal geometrical property of amines is their sp3 hybridization with the lone pair of nitrogen occupying the apex of the pyramid. Herein, we present a blueprint for quantifying the properties of extremely sterically hindered alkylamines. These amines reach planarity around the nitrogen atom due to the excessive steric hindrance, which results in a conformational re-modeling of the amine moiety. Crucially, the steric properties of amines are characterized by the %V Bur index, which we show is a general predictive parameter for evaluating the properties of sterically hindered amines. Computational studies on the acidic nature and the reactivity of organometallic Au and Pd complexes are outlined. Density functional theory calculations permit for predictive catalysis, ordering the mapping of extremely hindered tertiary amines by employing artificial intelligence via machine learning. Overall, the study outlines the correlation between the unusual geometry and the key thermodynamic and kinetic properties of extremely hindered alkylamines. The steric hindrance, as quantified by %V Bur, is the crucial factor influencing the observed trends and the space required to accommodate sterically hindered tertiary amines.

Iron-catalyzed synthesis of substituted 3-arylquinolin-2(1H)-ones via an intramolecular dehydrogenative coupling of amido-alcohols.

Here we report an iron-complex-catalyzed synthesis of various mono- and di-substituted quinolin-2(1H)-ones achieved via the intramolecular acceptorless dehydrogenative cyclization of amido-alcohols. This approach for the synthesis of N-heterocycles has provided access to underdescribed disubstituted quinolinones and represents an alternative to the well-known palladium-catalyzed coupling reactions.

Repurposing a supramolecular iridium catalyst via secondary Zn⋯O[double bond, length as m-dash]C weak interactions between the ligand and substrate leads to ortho-selective C(sp2)-H borylation of benzamides with unusual kinetics.

The iridium-catalyzed C-H borylation of benzamides typically leads to meta and para selectivities using state-of-the-art iridium-based N,N-chelating bipyridine ligands. However, reaching ortho selectivity patterns requires extensive trial-and-error screening via molecular design at the ligand first coordination sphere. Herein, we demonstrate that triazolylpyridines are excellent ligands for the selective iridium-catalyzed ortho C-H borylation of tertiary benzamides and, importantly, we demonstrate the almost negligible effect of the first coordination sphere in the selectivity, which is so far unprecedented in iridium C-H bond borylations. Remarkably, the activity is dramatically enhanced by exploiting a remote Zn⋯O[double bond, length as m-dash]C weak interaction between the substrate and a rationally designed molecular-recognition site in the catalyst. Kinetic studies and DFT calculations indicate that the iridium-catalyzed C-H activation step is not rate-determining, this being unique for remotely controlled C-H functionalizations. Consequently, a previously established supramolecular iridium catalyst designed for meta-borylation of pyridines is now compatible with the ortho-borylation of benzamides, a regioselectivity switch that is counter-intuitive regarding precedents in the literature. In addition, we highlight the role of the cyclohexene additive in avoiding the formation of undesired side-products as well as accelerating the HBpin release event that precedes the catalyst regeneration step, which is highly relevant for the design of powerful and selective iridium borylating catalysts.

Improving Environmental Stress Cracking Resistance of High-Density Polyethylene Grades by Comonomer Addition and Nanocomposite Approach.

The aim of this paper is to determine the effect of polymer density, correlated to the comonomer content, and nanosilica addition on the mechanical and Environmental Stress Cracking Resistance (ESCR) characteristics of high-density polyethylene (HDPE). In this regard, five HDPE samples with similar Melt Flow Index (MFI) and molar mass but various densities were acquired from a petrochemical plant. Two polymerization reactors work in series and differ only in the amount of 1-buene comonomer fed to the second reactor. To ascertain the microstructure of the studied samples, GPC and SSA (successive self-nucleation and annealing) analyses were accomplished. All samples resulted having similar characteristics but slightly various SCB/1000 C=7.26-9.74 (SCB=Short Chain Branching). Consequently, meanwhile studied HDPEs reveal similar notched impact and stress at yield values, the tensile modulus, stress-at-break, and elongation-at-break tend to demonstrate different results with the SCB content. More significantly, ESCR characteristic varied considerably with SCB/1000 C extent, so that higher amount of SCB acknowledged advanced ESCR. Notably, blending HDPE sample containing higher amount of SCB/1000 C, with 3 wt.% of chemically modified nanosilica enhanced ESCR characteristic by 40 %. DFT (Density Functional Theory) calculations unveiled the role of the comonomer, quantitatively by binding energies and qualitatively by Non Covalent Interaction (NCI) plots.

Unveiling structural and energetic characterization of the emissive RNA alphabet anchored in the methylthieno[3,4-d]pyrimidine heterocycle core.

This study presents a comprehensive theoretical exploration of the fluorescent non-natural emissive nucleobases- mthA, mthG, mthC, and mthU derived from the methylthieno[3,4-d]pyrimidine heterocycle. Our calculations, aligning with experimental findings, reveal that these non-natural bases exert minimal influence on the geometry of classical Watson-Crick base pairs within an RNA duplex, maintaining H-bonding akin to natural bases. In terms of energy, the impact of the modified bases, but for mthG, is also found to be little significant. We delved into an in-depth analysis of the photophysical properties of these non-natural bases. This investigation unveiled a correlation between their absorption/emission peaks and the substantial impact of the modification on the energy levels of the highest unoccupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbital (LUMO). Notably, this alteration in energy levels resulted in a significant reduction of the HOMO-LUMO gap, from approximately 5.4-5.5 eV in the natural bases, to roughly 3.9-4.7 eV in the modified bases. This shift led to a consequential change in absorption and emission spectra towards longer wavelengths, elucidating their bathochromic shift.

Rhenium Alkyne Catalysis: Sterics Control the Reactivity.

Metathesis reactions, including alkane, alkene, and alkyne metatheses, have their origins in the fundamental understanding of chemical reactions and the development of specialized catalysts. These reactions stand as transformative pillars in organic chemistry, providing efficient rearrangement of carbon-carbon bonds and enabling synthetic access to diverse and complex compounds. Their impact spans industries such as petrochemicals, pharmaceuticals, and materials science. In this work, we present a detailed mechanistic study of the Re(V) catalyzed alkyne metathesis through density functional theory calculations. Our findings are in agreement with the experimental evidence from Jia and co-workers and unveil critical factors governing catalyst performance. Our work not only enhances our understanding of alkyne metathesis but also contributes to the broader landscape of catalytic processes, facilitating the design of more efficient and selective transformations in organic synthesis.

From Stilbenes to carbo-Stilbenes: an Encouraging Prospect.

Beyond previously described carbo-naphthalene and carbo-biphenyl, a novel type of bis-carbo-benzenic molecules is envisaged from the stilbene parent. The synthesis, structure, spectroscopic and electrochemical properties of two such carbo-stilbenes are described at complementary experimental and computational DFT levels. In the selected targets, the bare skeletal carbo-mer of carbo-stilbene is decorated by 8 or 10 phenyl groups, 0 or 2 tert-butyl groups, and 2 n-octyl chains, the later substituents being introduced to compensate anticipated solubility issues. As in the parent stilbene series, isomers of the phenylated carbo-stilbenes are characterized. The cis- and trans-isomers are, however, formed in almost equal amounts and could not be separated by either chromatography or crystallization. Nevertheless, due to a slow interconversion at the NMR time scale (up to 55 °C) the 1H NMR signals of both isomers of the two carbo-stilbenes could be tentatively assigned. The calculated structure of the cis-isomer exhibits a helical shape, consistent with the observed magnetic shielding of phenyl p-CH nuclei residing inside the shielding cone of the facing C18 ring. The presence of the two isomers in solution also gives rise to quite broad UV-vis absorption spectra with main bands at ca 460, 560 and 710 nm, and a significant bathochromic shift for the decaphenylated carbo-stilbene vs the di-tert-butyl-octaphenylated counterpart. Square wave voltammograms do not show any resolution of the two isomers, giving a reversible reduction wave at -0.65 or -0.58 V/SCE, and an irreversible oxidation peak at 1.11 V/SCE, those values being classical for most carbo-benzene derivatives. Calculated NICS values (NICS(1)=-12.5±0.2 ppm) also indicate that the aromatic nature of the C18 rings is not markedly affected by the dialkynylbutatriene (DAB) connector between them.

Ring Opening Polymerization of Six- and Eight-Membered Racemic Cyclic Esters for Biodegradable Materials.

The low percentage of recyclability of the polymeric materials obtained by olefin transition metal (TM) polymerization catalysis has increased the interest in their substitution with more eco-friendly materials with reliable physical and mechanical properties. Among the variety of known biodegradable polymers, linear aliphatic polyesters produced by ring-opening polymerization (ROP) of cyclic esters occupy a prominent position. The polymer properties are highly dependent on the macromolecule microstructure, and the control of stereoselectivity is necessary for providing materials with precise and finely tuned properties. In this review, we aim to outline the main synthetic routes, the physical properties and also the applications of three commercially available biodegradable materials: Polylactic acid (PLA), Poly(Lactic-co-Glycolic Acid) (PLGA), and Poly(3-hydroxybutyrate) (P3HB), all of three easily accessible via ROP. In this framework, understanding the origin of enantioselectivity and the factors that determine it is then crucial for the development of materials with suitable thermal and mechanical properties.

Unveiling the Reaction Mechanisms of the Synthesis and the Excited State Intramolecular Proton Transfer of 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine.

Given its wide variety of applications in the pharmaceutical industry, the synthesis of imidazo[1,2-a]pyridines has been extensively studied since the beginning of the last century. Here, we disclose the mechanism for the synthesis of imidazo[1,2-a]pyridines by means of the Ortoleva-King reaction. We also reveal the reaction pathway leading to the formation of a iodinated byproduct, demonstrating the challenge of preventing the formation of such a byproduct because of the low energy barrier to access it. Moreover, quantum chemistry tools were employed to investigate the mechanism of intramolecular proton transfer in the excited state, and connections with aromaticity were explored.

In-situ photo-crosslinkable elastomer based on polyalphaolefin/halloysite nanohybrid.

In this research, new injectable and in situ photocurable elastomeric nanohybrids have been fabricated from polyalphaolefin (PAO) resins and halloysite nanofiller. In this regard, the co-oligomerization of long α-olefin monomers (C6, C8 and C10) with alkenol counterparts was carried out via a simple cationic route to provide OH-functionalized PAOs. The newly formed PAO type copolymer resins as well as halloysite nanoclay were then equipped with photocurable CC bonds containing an acrylate moiety. After the characterization of the final chemical substances and also of the intermediate structures, experimentally and computationally by means of Density Functional Theory (DFT) calculations, the neat treated PAO and PAO/halloysite nanohybrids were subjected to a curing process by visible light irradiation (λ âˆ¼ 475 nm, blue light). The crosslinking efficiency of the neat resins and the formed nanohybrid was evaluated using shrinkage strain-time curves and equilibrium swelling method. The suggested nanohybrid is not only biocompatible (96 % in the MTT assay), and hydrophilic (with a water contact angle of 61°), but also exhibits an easy, fast and robust curing process with great potential for coating and sealing technologies for medical devices.

Blue-Light Induced Iron-Catalyzed Synthesis of γ,δ-Unsaturated Ketones.

A visible-light-induced iron-catalyzed α-alkylation of ketones with allylic and propargylic alcohols as pro-electrophiles is reported. The diaminocyclopentadienone iron tricarbonyl complex plays a dual role by harvesting light and facilitating dehydrogenation and reduction steps without the help of any exogenous photosensitizer. γ,δ-Unsaturated ketones can now be accessed through this borrowing hydrogen methodology at room temperature. Mechanistic investigations revealed that the steric hindrance on the δ-position of either the dienone or ene-ynone intermediate is the key feature to prevent or decrease the competitive 1,6-reduction (and consequently the formation of the saturated ketone) and to favor the synthesis of a set of non-conjugated enones and ynones.

%VBur index and steric maps: from predictive catalysis to machine learning.

Steric indices are parameters used in chemistry to describe the spatial arrangement of atoms or groups of atoms in molecules. They are important in determining the reactivity, stability, and physical properties of chemical compounds. One commonly used steric index is the steric hindrance, which refers to the obstruction or hindrance of movement in a molecule caused by bulky substituents or functional groups. Steric hindrance can affect the reactivity of a molecule by altering the accessibility of its reactive sites and influencing the geometry of its transition states. Notably, the Tolman cone angle and %VBur are prominent among these indices. Actually, steric effects can also be described using the concept of steric bulk, which refers to the space occupied by a molecule or functional group. Steric bulk can affect the solubility, melting point, boiling point, and viscosity of a substance. Even though electronic indices are more widely used, they have certain drawbacks that might shift preferences towards others. They present a higher computational cost, and often, the weight of electronics in correlation with chemical properties, e.g. binding energies, falls short in comparison to %VBur. However, it is worth noting that this may be because the steric index inherently captures part of the electronic content. Overall, steric indices play an important role in understanding the behaviour of chemical compounds and can be used to predict their reactivity, stability, and physical properties. Predictive chemistry is an approach to chemical research that uses computational methods to anticipate the properties and behaviour of these compounds and reactions, facilitating the design of new compounds and reactivities. Within this domain, predictive catalysis specifically targets the prediction of the performance and behaviour of catalysts. Ultimately, the goal is to identify new catalysts with optimal properties, leading to chemical processes that are both more efficient and sustainable. In this framework, %VBur can be a key metric for deepening our understanding of catalysis, emphasizing predictive catalysis and sustainability. Those latter concepts are needed to direct our efforts toward identifying the optimal catalyst for any reaction, minimizing waste, and reducing experimental efforts while maximizing the efficacy of the computational methods.

Chiral NHC Ligands for Enantioselective Gold(I) Catalysis Under Aerobic Conditions: the Importance of Conformational Flexibility and Steric Hindrance of NHC Ligand on Reactivity.

Gold(I) catalysis has been recognized as a valuable tool for the unique transformation of multiple carbon-carbon bonds. Enantioselective π-catalysis based on gold(I) complexes is, however, still underdeveloped due to lack of privileged ligands. Herein, we present an accessible method to a new family of stable yet catalytically active chiral NHC-Au(I)-Cl complexes. The key to preserving a simultaneous fine balance between reactivity and stability in this newly developed family appears to be sterically hindered, but conformationally flexible NHC ligands. These could be easily accessed on a multigram scale by merging sterically hindered anilines with commercially available amino alcohols and amines via a four-steps synthetic sequence without the need for chromatographic purification. Further investigations of the catalytic activity of NHC-Au-Cl complexes identified the OH functionality incorporated into the NHC core as crucial for the level of enantioselectivity as well as the TsO- anion responsible for the activation of NHC-Au(I)-Cl. Finally, NMR studies and X-ray investigations revealed for the first time that the widely accepted ion metathesis (NHC-Au-Cl to NHC-Au-OSO2 R) responsible for the activation of NHC-Au-Cl complexes does not take place (or it is very slow) in commonly used MeNO2 in contrast to DCM.

Self-organization of metallo-supramolecular polymer networks by free formation of pyridine-phenanthroline heteroleptic complexes.

Nature employs spontaneous self-organization of supramolecular bonds to create complex matter capable of adaptation and self-healing. Accordingly, the self-sorting of unlike ligands towards a cooperative heteroleptic complex or narcistic homoleptic association in a mixed ligand system is frequently employed to form interchangeable stimuli-responsive complex geometries with a wide range of applications. This notion is however just rarely employed in the organization of polymer networks. In this paper, we report the free-formation of heteroleptic complexes between tetra-am poly(ethylene glycol) (tetraPEG) precursors functionalized either with pyridine (tetraPy) or phenanthroline (tetraEPhen). Among a wide range of studied metal ions, tetraPy could form a network only in combination with Pd2+, presumably with a square-planar geometry, highlighting the importance of complex strength and stability in forming gels with monodentate ligands. Also, mixed networks with tetraEPhen form only in combination with Pd2+ and Fe2+, with strengths surpassing those of individual components and stabilities incomparable to those of parent networks, indicative of heteroleptic complexation. Extensive rheological, UV-vis, and DFT simulation studies revealed the coexistence of different coordination geometries, with an octahedral arrangement prevailing in the presence of Fe2+ and a square-planar geometry in the presence of Pd2+. Therefore, this study offers new opportunities for the development of stimuli-responsive topology-switching polymer networks.

Single─Not Double─3D-Aromaticity in an Oxidized Closo Icosahedral Dodecaiodo-Dodecaborate Cluster.

3D-aromatic molecules with (distorted) tetrahedral, octahedral, or spherical structures are much less common than typical 2D-aromatic species or even 2D-aromatic-in-3D systems. Closo boranes, [BnHn]2- (5 ≤ n ≤ 14) and carboranes are examples of compounds that are singly 3D-aromatic, and we now explore if there are species that are doubly 3D-aromatic. The most widely known example of a species with double 2D-aromaticity is the hexaiodobenzene dication, [C6I6]2+. This species shows π-aromaticity in the benzene ring and σ-aromaticity in the outer ring formed by the iodine substituents. Inspired by the hexaiodobenzene dication example, in this work, we explore the potential for double 3D-aromaticity in [B12I12]0/2+. Our results based on magnetic and electronic descriptors of aromaticity together with 11B{1H} NMR experimental spectra of boron-iodinated o-carboranes suggest that these two oxidized forms of a closo icosahedral dodecaiodo-dodecaborate cluster, [B12I12] and [B12I12]2+, behave as doubly 3D-aromatic compounds. However, an evaluation of the energetic contribution of the potential double 3D-aromaticity through homodesmotic reactions shows that delocalization in the I12 shell, in contrast to the 10σ-electron I62+ ring in the hexaiodobenzene dication, does not contribute to any stabilization of the system. Therefore, the [B12I12]0/2+ species cannot be considered as doubly 3D-aromatic.

Site selective gold(i)-catalysed benzylic C-H amination via an intermolecular hydride transfer to triazolinediones.

Triazolinediones are known as highly reactive dienophiles that can also act as electrophilic amination reagents towards enolisable C-H bonds (ionic pathway) or weak C-H bonds (free radical pathway). Here, we report that this C-H amination reactivity can be significantly extended and enhanced via gold(i)-catalysis. Under mild conditions, several alkyl-substituted aryls successfully undergo benzylic C-H aminations at room temperature. The remarkable site selectivity that is observed points towards strong electronic activation and deactivation effects, that go beyond a simple weakening of the C-H bond. The observed catalytic C-H aminations do not follow the expected trends for a free radical-type C-H amination and show complementarity to existing methods. Density functional theory (DFT) calculations and distinct experimental trends provide a clear mechanistic rationale for observed selectivity patterns, postulating a novel pathway for triazolinedione-induced aminations via a carbon-to-nitrogen hydride transfer.

Suzuki-Miyaura Cross-Coupling of Amides by N-C Cleavage Mediated by Air-Stable, Well-Defined [Pd(NHC)(sulfide)Cl2] Catalysts: Reaction Development, Scope, and Mechanism.

The Suzuki-Miyaura cross-coupling of amides by selective N-C acyl bond cleavage represents a powerful tool for constructing biaryl ketones from historically inert amide bonds. These amide bond activation reactions hinge upon efficient oxidative addition of the N-C acyl bond to Pd(0). However, in contrast to the well-researched activation of aryl halides by C(sp2)-X oxidative addition, very few studies on the mechanism of C(acyl)-N bond oxidative addition and catalyst effect have been reported. Herein, we report a study on [Pd(NHC)(sulfide)Cl2] catalysts in amide N-C bond activation. These readily prepared, well-defined, air- and moisture-stable Pd(II)-NHC catalysts feature SMe2 (DMS = dimethylsulfide) or S(CH2CH2)2 (THT = tetrahydrothiophene) as ancillary ligands. The reaction development, kinetic studies, and reaction scope are presented. Extensive DFT studies were conducted to gain insight into the mechanism of C(acyl)-N bond oxidative addition and catalyst activation. We expect that [Pd(NHC)(sulfide)Cl2] precatalysts featuring sulfides as well-defined, readily accessible ancillary ligands will find application in C(acyl)-X bond activation in organic synthesis and catalysis.