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1. On the Reactivity Enhancement of Graphene by Metallic Substrates towards Aryl Nitrene Cycloadditions

   https://doi.org/10.1002/chem.202100227  

   Yang, Xiaojian; Chen, Feiran; Kim, Min A; Liu, Haitao; Wolf, Lawrence M; Yan, Mingdi (March 2021, Chemistry – A European Journal)

   null (Ed.)

   Pristine graphene is fairly inert chemically, and as such, most application‐driven studies use graphene oxide, or reduced graphene oxide. Using substrates to modulate the reactivity of graphene represents a unique strategy in the covalent functionalization of this otherwise fairly inert material. We found that the reactivity of pristine graphene towards perfluorophenyl azide (PFPA) can be enhanced by a metal substrate on which graphene is supported. Results on the extent of functionalization, defect density, and reaction kinetics all show that graphene supported on Ni (G/Ni) has the highest reactivity toward PFPA, followed by G/Cu and then G/silicon wafer. DFT calculations suggest that the metal substrate stabilizes the physisorbed nitrene through enhanced electron transfer to the singlet nitrene from the graphene surface assisted by the electron rich metal substrate. The G/Ni substantially stabilizes the singlet nitrene relative to G/Cu and the free‐standing graphene. The product structure is also predicted to be substrate dependent. These findings open the opportunities to enhance the reactivity of pristine graphene simply through the selection of the substrate. This also represents a new and powerful approach to increasing the reactivity of singlet nitrene through direct electron communication with graphene. 

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2. Enhancing the Chemical Reactivity of Graphene through Substrate Engineering

   https://doi.org/10.1002/smll.202408116  

   Tu, Jia; Yan, Mingdi (December 2024, Small)

   Abstract Covalent functionalization of pristine graphene can modify its properties, enabling applications in optoelectronics, biomedical fields, environmental science, and energy. However, the chemical reactivity of pristine graphene is relatively low, and as such, methods have been developed to increase the reactivity of graphene. This review focuses on substrate engineering as an effective strategy to enhance the reactivity of graphene through strain and charge doping. Nanoparticles, metals with different crystal orientations, and stretchable polymers are employed to introduce strains in graphene, leading to enhanced chemical reactivity and increased degree of functionalization. Charge doping through orbital hybridization with metals and charge puddles induced by oxide substrates generally enhance the reactivity of graphene, while alkyl‐modified surfaces and 2D materials often reduce graphene reactivity via charge screening and van der Waals interactions that increase the stability of the graphene layer, respectively. This review summarizes methods for creating and characterizing strains and charge doping in graphene and discusses their effects on the chemical functionalization of graphene in various reactions. 

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3. BF3–Catalyzed Diels–Alder Reaction between Butadiene and Methyl Acrylate in Aqueous Solution—An URVA and Local Vibrational Mode Study

   https://doi.org/10.3390/catal12040415  

   Freindorf, Marek; Kraka, Elfi (April 2022, Catalysts)

   In this study we investigate the Diels–Alder reaction between methyl acrylate and butadiene, which is catalyzed by BF3 Lewis acid in explicit water solution, using URVA and Local Mode Analysis as major tools complemented with NBO, electron density and ring puckering analyses. We considered four different starting orientations of methyl acrylate and butadiene, which led to 16 DA reactions in total. In order to isolate the catalytic effects of the BF3 catalyst and those of the water environment and exploring how these effects are synchronized, we systematically compared the non-catalyzed reaction in gas phase and aqueous solution with the catalyzed reaction in gas phase and aqueous solution. Gas phase studies were performed at the B3LYP/6-311+G(2d,p) level of theory and studies in aqueous solution were performed utilizing a QM/MM approach at the B3LYP/6-311+G(2d,p)/AMBER level of theory. The URVA results revealed reaction path curvature profiles with an overall similar pattern for all 16 reactions showing the same sequence of CC single bond formation for all of them. In contrast to the parent DA reaction with symmetric substrates causing a synchronous bond formation process, here, first the new CC single bond on the CH2 side of methyl acrylate is formed followed by the CC bond at the ester side. As for the parent DA reaction, both bond formation events occur after the TS, i.e., they do not contribute to the energy barrier. What determines the barrier is the preparation process for CC bond formation, including the approach diene and dienophile, CC bond length changes and, in particular, rehybridization of the carbon atoms involved in the formation of the cyclohexene ring. This process is modified by both the BF3 catalyst and the water environment, where both work in a hand-in-hand fashion leading to the lowest energy barrier of 9.06 kcal/mol found for the catalyzed reaction R1 in aqueous solution compared to the highest energy barrier of 20.68 kcal/mol found for the non-catalyzed reaction R1 in the gas phase. The major effect of the BF3 catalyst is the increased mutual polarization and the increased charge transfer between methyl acrylate and butadiene, facilitating the approach of diene and dienophile and the pyramidalization of the CC atoms involved in the ring formation, which leads to a lowering of the activation energy. The catalytic effect of water solution is threefold. The polar environment leads also to increased polarization and charge transfer between the reacting species, similar as in the case of the BF3 catalyst, although to a smaller extend. More important is the formation of hydrogen bonds with the reaction complex, which are stronger for the TS than for the reactant, thus stabilizing the TS which leads to a further reduction of the activation energy. As shown by the ring puckering analysis, the third effect of water is space confinement of the reacting partners, conserving the boat form of the six-member ring from the entrance to the exit reaction channel. In summary, URVA combined with LMA has led to a clearer picture on how both BF3 catalyst and aqueous environment in a synchronized effort lower the reaction barrier. These new insights will serve to further fine-tune the DA reaction of methyl acrylate and butadiene and DA reactions in general. 

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4. Synthesis and Thermo-Selective Recycling of Diels–Alder Cyclopentadiene Thermoplastics

   https://doi.org/10.1021/jacs.4c05952  

   Tran, Thi M; Read_de_Alaniz, Javier (July 2024, Journal of the American Chemical Society)

   Catalyst-free and reversible step-growth Diels–Alder (DA) polymerization has found a wide range of applications in polymer synthesis and is a promising method to fabricate recyclable thermoplastics. The effectiveness of polymerization and de-polymerization relies on the chemical building blocks, often utilizing furan as the diene and maleimide as the dienophile. Compared to the traditional diene–dienophile or two-component approaches that requires perfect stoichiometry, cyclopentadi-ene (Cp) can serve a dual role via self-dimerization. This internally balanced platform offers a route to access high-molecular-weight polymers and a dynamic handle for polymer recycling, which remains unexplored. Herein, through the reactivity in-vestigation of different telechelic Cp derivatives, the uncontrolled cross-linking of Cp was addressed, revealing the first suc-cessful DA homopolymerization. To demonstrate the generality of our methodology, we synthesized and characterized six Cp homopolymers with backbones derived from common thermoplastics, such as polydimethylsiloxane, hydrogenated poly-butadiene, and ethylene phthalate. Among these materials, the hydrogenated polybutadiene-Cp analog can be thermally de-polymerized (Mn = 68 to 23 kDa) and re-polymerized to the parent polymer (Mn = 68 kDa) under solvent- and catalyst-free conditions. This process was repeated over three cycles without intermediate purification, confirming the efficient thermo-selective recyclability. The varied degradable properties of other four Cp-incorporated thermoplastics were also examined. Overall, this work provides a general methodology to access a new class of reversible homopolymers, potentially expanding the designs and construction of sustainable thermoplastics. 

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5. Stereochemical effects on the mechanochemical scission of furan–maleimide Diels–Alder adducts

   https://doi.org/10.1039/C9CC06361G  

   Wang, Zi; Craig, Stephen L. (October 2019, Chemical Communications)

   Clarifying the correlation between the chemical structure of mechanophores and their mechanical reactivity informs the design of mechanochemical systems. One specific correlation that has received much recent attention is that between stereoisomerism and mechanical reactivity. Here, we report previously unobserved differences in the mechanical reactivity of furan–maleimide Diels–Alder (DA) stereoisomers. We evaluated the internal competition between the mechanically triggered retro-DA reaction and the mechanochemical ring opening of gem -dichlorocyclopropane mechanophores in the pulsed sonication of polymer solutions. The relative extent of the two sonomechanochemical reactions in the same polymer shows that the endo DA isomer exhibits greater mechanical lability than its exo isomer. This result contrasts with recent measurements of the relative rates of scission in a similar system and points to potential enhanced sensitivity obtained through the use of internal competition as opposed to absolute rates in assessing mechanical reactivity in sonication studies. 

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