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1. C–H bond activation via concerted metalation–deprotonation at a palladium( iii ) center

   https://doi.org/10.1039/D3SC00034F  

   Bouley, Bailey S.; Tang, Fengzhi; Bae, Dae Young; Mirica, Liviu M. (April 2023, Chemical Science)

   Herein we report the direct observation of C–H bond activation at an isolated mononuclear Pd( iii ) center. The oxidation of the Pd( ii ) complex ( Me N4)Pd II (neophyl)Cl (neophyl = –CH 2 C(CH 3 ) 2 Ph; Me N4 = N , N ′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF 6 ) yields the stable Pd( iii ) complex [( Me N4)Pd III (neophyl)Cl]PF 6 . Upon the addition of an acetate source, [( Me N4)Pd III (neophyl)Cl]PF 6 undergoes Csp 2 –H bond activation to yield the cyclometalated product [( Me N4)Pd III (cycloneophyl)]PF 6 . This metalacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd( iii ) complexes. The C–H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest that either a pre-association step such as an agostic interaction may be rate limiting, or that the C–H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ 3 ligand coordination and that the flexible ligand structure is important for this transformation. Overall, this study represents the first example of discrete C–H bond activation occurring at a Pd( iii ) center through a concerted metalation–deprotonation mechanism, akin to that observed for Pd( ii ) and Pd( iv ) centers. 

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2. Linear versus branched: flow of a wormlike micellar fluid past a falling sphere

   https://doi.org/10.1039/d1sm00281c  

   Wu, Shijian; Mohammadigoushki, Hadi (April 2021, Soft Matter)

   We report experiments on flow of wormlike micellar solutions past a falling sphere. By increasing the salt-to-surfactant concentration ratio, and beyond a viscosity peak, wormlike micelles experience a transition from linear to branched microstructure. Two viscoelastic wormlike micelles with salt to surfactant concentrations on each side of the viscosity peak are considered. Our results indicate three significant differences in flows of branched and linear micelles. First, while the sphere drag correction factor rapidly decreases upon increasing Weissenberg number in linear micelles, it shows an apparent local maximum at Wi ≈ 3 in branched micelles. Second, despite its high viscoelasticity, the time-averaged flow of branched micelles around the falling sphere exhibits a fore-and-aft symmetry, while a strong negative wake is observed in linear micelles at relatively weaker flows. Third, branched micelles exhibit a stronger flow-induced birefringence than linear micelles in an otherwise identical condition. Our hypothesis is that subject to strong flows around the falling sphere, branched micelles can relax much more efficiently than linear wormlike micelles through sliding of the branched junctions. This additional stress relaxation mechanism may facilitate micellar orientation, produce a marginal sphere drag reduction and a Newtonian-like flow profile around the falling sphere. Finally, unsteady flow is observed in both linear and branched micellar solutions beyond some critical thresholds of the extensional Weissenber number. Our results corroborate a recently proposed criterion for onset of instability in flow of wormlike micelles past a falling sphere, thereby, suggesting that micellar branching does not affect the mechanism of flow instability. 

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3. Allyl‐Allyl Coupling Promoted by Catalyst Systems with two Palladium Atoms – A Plethora of Potentially Pericyclic Processes

   https://doi.org/10.1002/anie.202406095  

   De_Snoo, William; Kong, Wang‐Yeuk; Tantillo, Dean_J (June 2024, Angewandte Chemie International Edition)

   Abstract Recently, Huang and co‐workers reported a catalytic reaction that utilizes H₂as the sole reductant for a C−C coupling of allyl groups with yields up to 96 %. Here we use computational quantum chemistry to identify several key features of this reaction that provide clarity on how it proceeds. We propose the involvement of a Pd−Pd bound dimer precatalyst, demonstrate the importance of ligand π‐π interactions and counterions, and identify a new, energetically viable, mechanism involving two dimerized, outer‐sphere reductive elimination transition structures that determine both the rate and selectivity. Although we rule out the previously proposed transmetalation step on energetic grounds, we show it to have an unusual aromatic transition structure in which two Pd atoms support rearranging electrons. The prevalence of potential metal‐supported pericyclic reactions in this system suggests that one should consider such processes regularly, but the results of our calculations also indicate that one should do so with caution. 

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4. Comparing methods for detection and quantification of plasmodesmal callose in Nicotiana benthamiana leaves during defense responses

   https://doi.org/10.1094/MPMI-09-23-0152-SC  

   Sankoh, Amie Fornah; Adjei, Joseph; Roberts, Daniel; Burch-Smith, Tessa M. (February 2024, Molecular Plant-Microbe Interactions®)

   Callose, a beta-(1,3)-D-glucan polymer, is essential for regulating intercellular trafficking via plasmodesmata (PD). Pathogens manipulate PD-localized proteins to enable intercellular trafficking by removing callose at PD, or conversely by increasing callose accumulation at PD to limit intercellular trafficking during infection. Plant defense hormones like salicylic acid regulate PD-localized proteins to control PD and intercellular trafficking during immune defense responses such as systemic acquired resistance. Measuring callose deposition at PD in plants has therefore emerged as a popular parameter for assessing likely intercellular trafficking activity during plant immunity. Despite the popularity of this metric there is no standard for how these measurements should be made. In this study, we compared three commonly used methods for identifying and quantifying PD callose by aniline blue staining were evaluated to determine the most effective in the Nicotiana benthamiana leaf model. The results reveal that the most reliable method used aniline blue staining and fluorescent microscopy to measure callose deposition in fixed tissue. Manual or semi-automated workflows for image analysis were also compared and found to produce similar results although the semi-automated workflow produced a wider distribution of data points. 

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5. Suzuki–Miyaura cross-coupling of esters by selective O–C(O) cleavage mediated by air- and moisture-stable [Pd(NHC)(μ-Cl)Cl] ₂ precatalysts: catalyst evaluation and mechanism

   https://doi.org/10.1039/D1CY00312G  

   Yang, Shiyi; Zhou, Tongliang; Poater, Albert; Cavallo, Luigi; Nolan, Steven P.; Szostak, Michal (January 2021, Catalysis Science & Technology)

   null (Ed.)

   The cross-coupling of aryl esters has emerged as a powerful platform for the functionalization of otherwise inert acyl C–O bonds in chemical synthesis and catalysis. Herein, we report a combined experimental and computational study on the acyl Suzuki–Miyaura cross-coupling of aryl esters mediated by well-defined, air- and moisture-stable Pd( ii )–NHC precatalysts [Pd(NHC)(μ-Cl)Cl] 2 . We present a comprehensive evaluation of [Pd(NHC)(μ-Cl)Cl] 2 precatalysts and compare them with the present state-of-the-art [(Pd(NHC)allyl] precatalysts bearing allyl-type throw-away ligands. Most importantly, the study reveals [Pd(NHC)(μ-Cl)Cl] 2 as the most reactive precatalysts discovered to date in this reactivity manifold. The unique synthetic utility of this unconventional O–C(O) cross-coupling is highlighted in the late-stage functionalization of pharmaceuticals and sequential chemoselective cross-coupling, providing access to valuable ketone products by a catalytic mechanism involving Pd insertion into the aryl ester bond. Furthermore, we present a comprehensive study of the catalytic cycle by DFT methods. Considering the clear advantages of [Pd(NHC)(μ-Cl)Cl] 2 precatalysts on several levels, including facile one-pot synthesis, superior atom-economic profile to all other Pd( ii )–NHC catalysts, and versatile reactivity, these should be considered as the ‘first-choice’ catalysts for all routine applications in ester O–C(O) bond activation. 

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