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1. Modular Approach to Bio-Based Poly(enol ether)s with Tunable Thermal Properties and Degradability

   https://doi.org/10.1021/acsapm.3c03196  

   Ibrahim, Tarek; Sun, Hao (February 2024, ACS Applied Polymer Materials)

   Biomass-derived polymer materials are emerging as sustainable and low-carbon footprint alternatives to the current petroleum-based commodity plastics. In the past decade, the ring-opening metathesis polymerization (ROMP) technique has been widely used for the polymerization of cyclic olefin monomers derived from biorenewable resources, giving rise to a diverse set of biobased polymer materials. However, most synthetic biobased polymers made by ROMP are nondegradable because of their all-carbon backbones. Herein, we present a modular synthetic strategy to acid-degradable poly(enol ether)s via ring-opening metathesis copolymerization of biorenewable oxanorbornenes and 3,4-dihydropyran (DHP). 1H NMR analysis reveals that the percentage of DHP units in the resulting copolymers gradually increases as the feed ratio of DHP to oxanorbornene increases. The composition of the copolymers plays a pivotal role in governing their thermal properties. Thermogravimetric analysis shows that an increasing percentage of DHP results in a decrease in the decomposition temperatures, suggesting that the incorporation of enol ether groups in the polymer backbone reduces the thermal stability of the copolymers. Moreover, a wide range of glass transition temperatures (16–165 °C) can be achieved by tuning the copolymer composition and the oxanorbornene structure. Critically, all of the poly(enol ether)s developed in this study are degradable under mildly acidic conditions. A higher incorporation of DHP in the copolymer leads to enhanced degradability, as evidenced by smaller final degradation products. Altogether, this study provides a facile approach for synthesizing biorenewable and degradable polymer materials with highly tunable thermal properties desired for their potential industrial applications. 

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2. Biomass-Derived Degradable Polymers via Alternating Ring-Opening Metathesis Polymerization of Exo-Oxanorbornenes and Cyclic Enol Ethers

   https://doi.org/10.1021/acsmacrolett.3c00608  

   Sun, Hao; Ibrahim, Tarek; Ritacco, Angelo; Durkee, Katie (November 2023, ACS Macro Letters)

   Degradable polymers made via ring-opening metathesis polymerization (ROMP) hold tremendous promise as eco-friendly materials. However, most of the ROMP monomers are derived from petroleum resources, which are typically considered less sustainable compared to biomass. Herein, we present a synthetic strategy to degradable polymers by harnessing alternating ROMP of biomass-based cyclic olefin monomers including exo-oxanorbornenes and cyclic enol ethers. A library of well-defined poly(enol ether)s with modular structures, tunable glass transition temperatures, and controlled molecular weights was achieved, demonstrating the versatility of this approach. Most importantly, the resulting copolymers exhibit high degrees of alternation, rendering their backbones fully degradable under acidic conditions. 

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3. Oxidative Nitrogen Insertion into Silyl Enol Ether C═C Bonds

   https://doi.org/10.1021/jacs.4c07111  

   Lin, Alex; Ghosh, Arghya; Yellen, Simon; Ball, Zachary T; Kürti, László (July 2024, Journal of the American Chemical Society)

   Here, we demonstrate a fundamentally new reactivity of the silyl enol ether functionality utilizing an in situ generated iodonitrene-like species. The present transformation inserts a nitrogen atom between the silyl enol ether olefinic carbons with the concomitant cleavage of the CC bond. Overall, this facile transformation converts a C-nucleophilic silyl enol ether to the corresponding C-electrophilic N-acyl-N,O-acetal. This unprecedented access to α-amido alkylating agents enables modular derivatization with carbon and heteroatom nucleophiles and the unique late-stage editing of carbon frameworks. The reaction efficiency of this transformation is well correlated with enol ether nucleophilicity as described by the Mayr N scale. Applications presented herein include late-stage nitrogen insertion into carbon skeletons of natural products with previously unattainable regioselectivity as well as modified conditions for 15N labeling of amides and lactams. 

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4. Exploring the Formation of Copper–Ruthenium Bimetallic Complexes in Olefin Metathesis

   https://doi.org/10.1021/acs.organomet.3c00338  

   Almuzaini, Hanan N; Slebodnick, Carla; Schulz, Michael D (December 2023, Organometallics)

   Copper(I) halides are often added to olefin metathesis reactions to inhibit catalyst degradation, control product isomerization, enhance catalyst activation, or facilitate catalyst dimerization. In each of these examples, the copper salt is presumed to operate as an independent species, separate from the ruthenium center. We have discovered, however, that certain copper salts can form complexes with the ruthenium catalyst itself, forming hetero-bimetallic copper-ruthenium olefin metathesis catalysts. We confirmed the formation of two complexes through single-crystal X-ray crystallography and NMR spectroscopy. The crystal structure revealed the presence of a four-member ring containing ruthenium, carbon, copper, and chlorine or bromine. The hetero-bimetallic catalyst displayed higher latency and lower activity in the ring-opening metathesis polymerization (ROMP) of norbornene compared to analogous monometallic catalysts. For example, norbornene polymerization catalyzed by the monometallic complex reached 80 % conversion after 4 h, but only 12% conversion when catalyzed by the hetero-bimetallic copper-ruthenium complex under the same conditions. Conversion increased to 63 % when the temperature increased to 50 °C for 1 h, indicating that the bimetallic complex retains activity but requires a higher temperature to activate. The formation of these copper-ruthenium bimetallic complexes suggests the possibility of multi-metallic olefin metathesis catalysts, potentially with different activity and properties than their traditional monometallic counterparts. 

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5. A Rigidified Macrocyclic Grubbs Complex: A Rare Example of In - and Out -Isomers that Show a Dramatic Difference in Catalytic Reactivity

   https://doi.org/10.1021/acs.organomet.4c00331  

   Davalos-Morinigo, Anibal R; Vemulapalli, Srini; Dudding, Travis; Diver, Steven T (September 2024, Organometallics)

   Chirik, Paul (Ed.)

   The design of a rigidified macrocyclic N-heterocyclic carbene (NHC) ligand led to the formation and structural characterization of in- and out-Ru carbene complexes. In this study, introduction of a conformational lock was used to rigidify heteroaryl-aryl bonds and thereby enforce a more perpendicular dihedral angle. A forcing metalation step was needed to form the isomeric Ru carbene complexes (Grubbs complexes). The major isomer had the Ru carbene fragment located outside the macrocyclic ring whereas the minor isomer had the Ru carbene inside the macrocyclic ring. The two new Ru carbene complexes are the first examples of in- and out-isomers of a Grubbs-type complex. The solid state structures of each isomeric ruthenium carbene complex was determined by x-ray diffraction studies. The two Ru complexes showed significantly different catalytic reactivity in the ring-closing metathesis (RCM) of the benchmark substrate, diethyl diallylmalonate. We performed computational studies to determine rotational barriers; scalable energetic barriers were found in the unmetallated NHC ligand, favoring the in-isomer by 2.4 kcal/mol. These calculations, coupled with attempted interconversion of isomers, support a mechanism featuring rotational isomerization of the NHC nucleophile in a preequilibrium step before metalation. 

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