Although polyethylene (PE) and polypropylene (PP) are by far the world’s largest volume plastics, only a tiny fraction of these energy-rich polyolefins are currently recycled. Depolymerization of PE ...to its constituent monomer, ethylene, is highly endothermic and conventionally accessible only through unselective, high-temperature pyrolysis. Here, we provide experimental demonstrations of our recently proposed tandem catalysis strategy, which uses ethylene to convert PE to propylene, the commodity monomer used to make PP. The approach combines rapid olefin metathesis with rate-limiting isomerization. Monounsaturated PE is progressively disassembled at modest temperatures via many consecutive ethenolysis events, resulting selectively in propylene. Fully saturated PE can be converted to unsaturated PE starting with a single transfer dehydrogenation to ethylene, which produces a small amount of ethane (1 equiv per dehydrogenation event). These principles are demonstrated using both homogeneous and heterogeneous catalysts. While selectivity under batch conditions is limited at high conversion by the formation of an equilibrium mixture of olefins, high selectivity to propylene (≥94%) is achieved in a semicontinuous process due to the continuous removal of propylene from the reaction mixture.
An inhibitory role of 1,5-cyclooctadiene (COD) in nickel-catalyzed C–H functionalization processes was identified and studied. The bound COD participates in C–H activation by capturing the hydride, ...leading to a stable off-cycle π-allyl complex that greatly diminished overall catalytic efficiency. Computational studies elucidated the origin of the effect and enabled identification of a 1,5-hexadiene-derived pre-catalyst that avoids the off-cycle intermediate and provides catalytic efficiencies that are superior to those of catalysts derived from Ni(COD)2.
A detailed mechanistic investigation of N-heterocyclic carbene-nickel-catalyzed hydroarylation via C–H functionalization is described. These catalysts are complicated, in part, by undesired ...reactivity stemming from common olefinic ligands such as cyclooctadiene (COD) that stabilize the precatalyst. This reaction adds diversity to the overall reactive landscape by permitting multiple types of ligand-to-ligand hydrogen transfer (LLHT) steps to activate the substrate arene C–H bonds. In one case, stable π-allyl complexes can be formed via LLHT to the olefin, hindering catalysis, and in the other, LLHT to the alkyne substrate leads to productive catalysis. Here, a useful map is built from extensive computational and experimental studies to guide subsequent investigations on the productive use of Ni catalysis. In addition to showing the details of catalyst deactivation, activation, and operating regimes, this article suggests the following: 1. Reductive elimination is rate-limiting and assisted by an additional alkyne ligand; 2. The resting state for catalysis is an alkyne-ligated Ni center; and 3. The reaction rate is under thermodynamic control, showing a good correlation with thermodynamics of C–H addition to the metal center (R 2 = 0.95).
The efficient copolymerization of acrylates with ethylene using Ni catalysts remains a challenge. Herein, we report two neutral Ni(II) catalysts (POP-Ni-py (1) and PONap-Ni-py (2)) that exhibit high ...thermal stability and significantly higher incorporation of polar monomer (for 1) or improved resistance to tert-butylacrylate (tBA)-induced chain transfer (for 2), in comparison to previously reported catalysts. Nickel alkyl complexes generated after tBA insertion, POP-Ni-CCO(py) (3) and PONap-Ni-CCO(py) (4), were isolated and, for the first time, characterized by crystallography. Weakened lutidine vs pyridine coordination in 2-lut facilitated the isolation of a N-donor-free adduct after acrylate insertion PONap-Ni-CCO (5) which represents a novel example of a four-membered chelate relevant to acrylate polymerization catalysis. Experimental kinetic studies of six cases of monomer insertion with aforementioned nickel complexes indicate that pyridine dissociation and monomer coordination are fast relative to monomer migratory insertion and that monomer enchainment after tBA insertion is the rate limiting step of copolymerization. Further evaluation of monomer insertion using density functional theory studies identified a cis–trans isomerization via Berry-pseudorotation involving one of the pendant ether groups as the rate-limiting step for propagation, in the absence of a polar group at the chain end. The energy profiles for ethylene and tBA enchainments are in qualitative agreement with experimental measurements.
The nickel-catalyzed coupling of enones or enals with alkynes in the presence of silane and titanium alkoxide reductants provides direct access to skipped diene products. The process involves a net ...four-electron reductive coupling and proceeds with deoxygenation of the starting enone or enal. A new class of well-defined nickel(0) precatalysts bearing an unhindered N-heterocyclic carbene ligand, which was developed in optimization of the process, is essential for the efficiency of the transformation. The strategy allows the high reactivity of α,β-unsaturated carbonyl substrates to be utilized in couplings with simultaneous extrusion of the oxygen atom, thus enabling a traceless strategy for alkene installation.
Polymicrobial biofilms are a hallmark of chronic wound infection. The forces governing assembly and maturation of these microbial ecosystems are largely unexplored but the consequences on host ...response and clinical outcome can be significant. In the context of wound healing, formation of a biofilm and a stable microbial community structure is associated with impaired tissue repair resulting in a non-healing chronic wound. These types of wounds can persist for years simmering below the threshold of classically defined clinical infection (which includes heat, pain, redness, and swelling) and cycling through phases of recurrent infection. In the most severe outcome, amputation of lower extremities may occur if spreading infection ensues. Here we take an ecological perspective to study priority effects and competitive exclusion on overall biofilm community structure in a three-membered community comprised of strains of Staphylococcus aureus, Citrobacter freundii, and Candida albicans derived from a chronic wound. We show that both priority effects and inter-bacterial competition for binding to C. albicans biofilms significantly shape community structure on both abiotic and biotic substrates, such as ex vivo human skin wounds. We further show attachment of C. freundii to C. albicans is mediated by mannose-binding lectins. Co-cultures of C. freundii and C. albicans trigger the yeast-to-hyphae transition, resulting in a significant increase in neutrophil death and inflammation compared to either species alone. Collectively, the results presented here facilitate our understanding of fungal-bacterial interactions and their effects on host-microbe interactions, pathogenesis, and ultimately, wound healing.
Emerging pathogen
causes nosocomial outbreaks of life-threatening invasive candidiasis. It is unclear how this species colonizes skin and spreads in health care facilities. Here, we analyzed
growth ...in synthetic sweat medium designed to mimic axillary skin conditions. We show that
demonstrates a high capacity for biofilm formation in this milieu, well beyond that observed for the most commonly isolated
sp.,
The
biofilms persist in environmental conditions expected in the hospital setting. To model
skin colonization, we designed an
porcine skin model. We show that
proliferates on porcine skin in multilayer biofilms. This capacity to thrive in skin niche conditions helps explain the propensity of
to colonize skin, persist on medical devices, and rapidly spread in hospitals. These studies provide clinically relevant tools to further characterize this important growth modality.
The emerging fungal pathogen
causes invasive infections and is spreading in hospitals worldwide. Why this species exhibits the capacity to transfer efficiently among patients is unknown. Our findings reveal that
forms high-burden biofilms in conditions mimicking sweat on the skin surface. These adherent biofilm communities persist in environmental conditions expected in the hospital setting. Using a pig skin model, we show that
also forms high-burden biofilm structures on the skin surface. Identification of this mode of growth sheds light on how this recently described pathogen persists in hospital settings and spreads among patients.
The living, chain-growth polymerization of (hetero)aryl monomers by carbene-ligated palladium precatalysts gives conjugated polymers with narrow dispersities as well as specified molecular weights ...and sequences. Despite this success, the mechanism for Pd precatalyst initiation and subsequent polymer growth is unknown. A quantum chemical study is presented herein to provide insight into thiophene homopolymerization initiated by (IPr)PdCl2(3-chloropyridine) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) using novel reaction discovery tools for generating and evaluating mechanistic hypotheses. This study reveals the role for the “throw-away” 3-chloropyridine ligand during initiation via a stepwise transmetalation pathway that involves pyridine dissociation, followed by pyridine reassociation to liberate the Grignard byproduct (MgCl2) and stabilize the resulting Pd intermediate. In contrast, 3-chloropyridine association to Pd during propagation hinders catalyst turnover by producing a low-energy, off-cycle intermediate. Throughout these studies, explicit solvent molecules (THF) coordinated to the Grignard reagent are shown to be needed for accurate modeling. Finally, the energetic span model (ESM) reveals that the turnover-limiting step for initiation is transmetalation with pyridine dissociation/reassociation, while the turnover-limiting step for propagation is also transmetalation that is inhibited by pyridine coordination.
Copper, an earth-abundant metal, has reemerged as a viable alternative to the versatile Pd-catalyzed C–N coupling. Coupling sterically hindered reaction partners, however, remains challenging. ...Herein, we disclose the discovery and development of a pyrrole-ol ligand to facilitate the coupling of ortho-substituted aryl iodides with sterically hindered amines. The ligand was discovered through a library screening approach and highlights the value of mining heteroatom-rich pharmaceutical libraries for useful ligand motifs. Further evaluation revealed that this ligand is uniquely effective in these challenging transformations. The reaction enables the coupling of sterically hindered primary and secondary amines, anilines, and amides with broad functional group tolerance.
Nickel(0) catalysts of N-heterocyclic carbenes (NHCs) that are stabilized by electron-deficient alkenes possess desirable properties of air tolerance and ease of handling while also retaining high ...catalytic activities. Since catalyst stability often comes at the expense of catalytic activity, we have undertaken a detailed study of the activation mechanism of an IMes-nickel(0) catalyst stabilized by di(o-tolyl) fumarate that converts the stable precatalyst form into a catalytically active species. Computational evaluation provided evidence against a simple ligand exchange as the activation mechanism for this catalyst, and a stoichiometric activation process that covalently modifies the stabilizing ligand was identified. A detailed computational picture for the activation process was developed, with predictive insights that elucidate an unexpected catalyst activation pathway that operates when ligand exchange is thermodynamically unfavorable.
PDF
