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1. Catalysis with Palladium Complexes Photoexcited by Visible Light

   https://doi.org/10.1002/anie.201813523  

   Chuentragool, Padon; Kurandina, Daria; Gevorgyan, Vladimir (May 2019, Angewandte Chemie International Edition)

   Abstract Palladium catalysis induced by visible light is an emerging field of catalysis. In contrast to classical reactions catalyzed by Pd complexes in the ground state, which mostly proceed through two‐electron redox processes, the mechanisms of these new methods based on photoexcited Pd complexes usually operate through transfer of a single electron. Such processes lead to putative hybrid Pd/radical species, which exhibit both radical and classical Pd‐type reactivity. This Minireview highlights the recent progress in this rapidly growing area. 

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2. The mechanism of directed Ni( ii )-catalyzed C–H iodination with molecular iodine

   https://doi.org/10.1039/C7SC04604A  

   Haines, Brandon E.; Yu, Jin-Quan; Musaev, Djamaladdin G. (January 2018, Chemical Science)

   The density functional theory method is used to elucidate the elementary steps of Ni( ii )-catalyzed C(sp 2 )–H iodination with I 2 and substrates bearing N , N ′-bidentate directing centers, amide-oxazoline (AO) and 8-aminoquinoline (AQ). The relative stability of the lowest energy high- and low-spin electronic states of the catalyst and intermediates is found to be an important factor for all of the steps in the reaction. As a result, two-state reactivity for these systems is reported, where the reaction is initiated on the triplet surface and generates a high energy singlet nickelacycle. It is shown that the addition of Na 2 CO 3 base to the reaction mixture facilitates C–H activation. The presence of I 2 in the reaction provides the much needed driving force for the C–H activation and nickelacycle formation and ultimately reacts to form a new C–I bond through either a redox neutral electrophilic cleavage (EC) pathway or a one-electron reductive cleavage (REC) pathway. The previously proposed Ni( ii )/Ni( iv ) and homolytic cleavage pathways are found to be higher in energy. The nature of the substrate is found to have a large impact on the relative stability of the lowest electronic states and on the stability of the nickelacycle resulting from C–H activation. 

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3. An unusual stereoretentive 1,3-quaternary carbon shift resulting in an enantioselective Rh ^II -catalyzed formal [4+1]-cycloaddition between diazo compounds and vinyl ketenes

   https://doi.org/10.1039/c8sc00020d  

   Rodriguez, Kevin X.; Pilato, Tara C.; Ashfeld, Brandon L. (January 2018, Chemical Science)

   Enantioselective quaternary carbon construction in the assembly of cyclopentenones employing a Rh II -catalyzed, formal [4+1]-cycloaddition is described. A Rh 2 ( S -TCPTTL) 4 -catalyzed cyclopropanation of a vinyl ketene with a disubstituted diazo compound initiates a stereoretentive, accelerated ring expansion to provide the cycloadduct in good to excellent yields and enantioselectivity. 

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4. Selenium-Catalyzed Regioselective Intermolecular Diamination of Dienes and Dienolate Derivatives

   https://doi.org/10.1021/acs.orglett.4c03431  

   Dohoda, Alexander F; Zottarelli, Victoria L; Hajikedir, Dureti; Michael, Forrest E (September 2024, Organic Letters)

   We report a selenium-catalyzed diamination of dienes using sulfamates as a convenient nitrogen source. This reaction proceeds regioselectively for 1,2-addition at the less substituted alkene, without the need for a tethered diamine. We also report the first diamination of dienyl phosphates and tosylates, affording synthetically useful α,β-diaminoketone derivatives with high syn diastereoselectivity. Density functional theory calculations support a mechanism proceeding via [4+2] cycloaddition of the diene with a selenium bis(imide), followed by ring-opening aminolysis and [2,3]-sigmatropic rearrangement. 

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5. Tyrosine, cysteine, and proton coupled electron transfer in a ribonucleotide reductase-inspired beta hairpin maquette

   https://doi.org/10.1039/C9CC04067F  

   McCaslin, Tyler G.; Pagba, Cynthia V.; Hwang, Hyea; Gumbart, James C.; Chi, San-Hui; Perry, Joseph W.; Barry, Bridgette A. (August 2019, Chemical Communications)

   Tyrosine residues act as intermediates in proton coupled electron transfer reactions (PCET) in proteins. For example, in ribonucleotide reductase (RNR), a tyrosyl radical oxidizes an active site cysteine via a 35 Å pathway that contains multiple aromatic groups. When singlet tyrosine is oxidized, the radical becomes a strong acid, and proton transfer reactions, which are coupled with the redox reaction, may be used to control reaction rate. Here, we characterize a tyrosine-containing beta hairpin, Peptide O, which has a cross-strand, noncovalent interaction between its single tyrosine, Y5, and a cysteine (C14). Circular dichroism provides evidence for a thermostable beta-turn. EPR spectroscopy shows that Peptide O forms a neutral tyrosyl radical after UV photolysis at 160 K. Molecular dynamics simulations support a phenolic/SH interaction in the tyrosine singlet and radical states. Differential pulse voltammetry exhibits pH dependence consistent with the formation of a neutral tyrosyl radical and a p K a change in two other residues. A redox-coupled decrease in cysteine p K a from 9 (singlet) to 6.9 (radical) is assigned. At pD 11, picosecond transient absorption spectroscopy after UV photolysis monitors tyrosyl radical recombination via electron transfer (ET). The ET rate in Peptide O is indistinguishable from the ET rates observed in peptides containing a histidine and a cyclohexylalanine (Cha) at position 14. However, at pD 9, the tyrosyl radical decays via PCET, and the decay rate is slowed, when compared to the histidine 14 variant. Notably, the decay rate is accelerated, when compared to the Cha 14 variant. We conclude that redox coupling between tyrosine and cysteine can act as a PCET control mechanism in proteins. 

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