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1. Photocatalyst-Independent Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides: Stereocontrol and Mechanism

   https://doi.org/10.1039/D0SC05550F  

   Zhong, Yongliang ; Feng, Quanyou ; wang, xiaoqian ; Yang, Lei ; Korovich, Andrew G ; Madsen, Louis A ; Tong, Rong ( January 2021 , Chemical Science)

   Photoredox ring-opening polymerization of O -carboxyanhydrides allows for the synthesis of polyesters with precisely controlled molecular weights, molecular weight distributions, and tacticities. While powerful, obviating the use of precious metal-based photocatalysts would be attractive from the perspective of simplifying the protocol and enabling unexpected reactivity. Herein, we report the Co and Zn catalysts that are activated by external light to mediate efficient ring-opening polymerization of O -carboxyanhydrides, without the use of exogenous precious metal-based photocatalysts. Our methods allow for the synthesis of isotactic polyesters with high molecular weights (>200 kDa) and narrow molecular weight distributions ( M w / M n < 1.1). Mechanistic studies indicate that light activates the oxidative status of Co III intermediate that is generated from the regioselective ring-opening of the O -carboxyanhydride. We also demonstrate that the use of Zn or Hf complexes together with Co can allow for stereoselective photoredox ring-opening polymerizations of multiple racemic O -carboxyanhydrides to synthesize syndiotactic and stereoblock copolymers, which vary widely in their glass transition temperatures.
2. Olefin oligomerization by main group Ga3+ and Zn2+ single site catalysts on SiO2

   https://doi.org/10.1038/s41467-021-22512-6  

   LiBretto, Nicole J. ; Xu, Yinan ; Quigley, Aubrey ; Edwards, Ethan ; Nargund, Rhea ; Vega-Vila, Juan Carlos ; Caulkins, Richard ; Saxena, Arunima ; Gounder, Rajamani ; Greeley, Jeffrey ; et al ( April 2021 , Nature Communications)

   In heterogeneous catalysis, olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni²⁺and Cr³⁺. Here we report that silica-supported, single site catalysts containing immobilized, main group Zn²⁺and Ga³⁺ion sites catalyze ethylene and propylene oligomerization to an equilibrium distribution of linear olefins with rates similar to that of Ni²⁺. The molecular weight distribution of products formed on Zn²⁺is similar to Ni²⁺, while Ga³⁺forms higher molecular weight olefins. In situ spectroscopic and computational studies suggest that oligomerization unexpectedly occurs by the Cossee-Arlman mechanism via metal hydride and metal alkyl intermediates formed during olefin insertion and β-hydride elimination elementary steps. Initiation of the catalytic cycle is proposed to occur by heterolytic C-H dissociation of ethylene, which occurs at about 250 °C where oligomerization is catalytically relevant. This work illuminates new chemistry for main group metal catalysts with potential for development of new oligomerization processes.
3. Attempts at generating metathesis-active Fe(IV) and Co(IV) complexes via the reactions of (silox)2M(THF)2, [(silox)3M][Na(THF)2] (M = Fe, Co), and related species with propellanes and triphenylboron

   Gregory M. George, Peter T. ( January 2022 , Polyhedron)

   Parkin, Ged (Ed.)

   Metathetical syntheses of (silox)2M(THF)2 (1-M, Fe, Co), [(silox)3M]Na(THF)2 (2-M, Fe, Co), and (silox)3Fe(THF) (3) are presented, as are X-ray structural studies of 1-Co, 2-Fe, and 2-Co. Exposure of these complexes to 1.1.1- propellane (111P, C5H6) and 1,3-dehydroadamantane (AdP), which are known progenitors of ROMP-active alkylidenes with ruthenium, failed to elicit similar reactivity. A total of 28 complexes were subjected to 111P in attempts to make “Fe(IV)” alkylidenes capable of some form of olefin metathesis with no success. At best, catalytic ring-opening to 3-exo-methylene cyclobutylidene was evidenced. The addition of triphenylboron as a Lewis acid failed to aid in metal complex formation of the propellanes, but an unusual rearrangement of AdP was noted. In addition, 2-Fe catalyzed the conversion of 111P to 3-exo-methylene cyclobutylidene, which added twice to Ph3B, affording a structurally characterized product, Ph(κ-CH=CMeCH2BPh
4. Pyridinol-based CNC Pincer catalysts for carbon dioxide reduction: The big impact of one small remote group

   Papish, Elizabeth T. ; Das, Sanjit ; Boudreaux, Chance M. ; Burks, Dalton Bodine ; Rodrigues, Roberta R. ; Webster, Charles Edwin ; Delcamp, Jared H. ( April 2019 , ACS National Meeting Abstracts)

   The first examples of a CNC pincer ligands with a central pyridinol derived ring were recently reported.  The differences in catalytic reactivity between CNC ligands with a central pyridine ring vs. a pyridinol derived ring are substantial and highly active and robust catalysts have been synthesized and studied.  In these pincer ligands, the 4-substituent can be OMe, OH, or O , and these latter two options allow for altered catalyst properties as a function of proton concn.  Catalytic studies have used ruthenium(II), nickel(II), and other transition metals.  We have made metal complexes that can be protonated or deprotonated reversibly in situ to switch on or off the photocatalytic performance towards CO redn.  Furthermore, the methoxy group on the pyridine ring offers unique catalysis advantages not seen with the unsubstituted analog.  Our best catalysts offer selective CO formation, >300 turnover cycles, and a 40 h lifetime.  Highly active self-sensitized catalysts have recently been developed.  Steric and electronic ligand effects are being studied with these catalysts by exptl. and computational methods.
5. Stable abnormal N-heterocyclic carbenes and their applications

   https://doi.org/10.1039/c9cs00866G  

   Sau, Samaresh Chandra ; Hota, Pradip Kumar ; Mandal, Swadhin K. ; Soleilhavoup, Michele ; Bertrand, Guy ( February 2020 , Chemical Society Reviews)

   Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd( ii ), Cu( i ), Ni( ii ), Fe(0), Zn( ii ), Ag( i ), and Au( i / iii ) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki–Miyaura cross-coupling reaction, C–H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K( i ), Al( iii ), Zn( ii ), Sn( ii ), Ge( ii ), and Si( ii / iv ). Among them, K( i ), Al( iii ), and Zn( ii ) complexesmore »were used for the polymerization of caprolactone and rac -lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO 2 ), nitrous oxide (N 2 O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO 2 and the catalytic reduction of carbon dioxide, including atmospheric CO 2 , into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.« less