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1. Reaction Mechanism Underlying Pd(II)-Catalyzed Oxidative Coupling of Ethylene and Benzene to Form Styrene: Identification of a Cyclic Mono-Pd II Bis-Cu II Complex as the Active Catalyst

   https://doi.org/10.1021/acs.organomet.2c00183  

   Musgrave, Charles B. ; Bennett, Marc T. ; Ellena, Jeffrey F. ; Dickie, Diane A. ; Gunnoe, T. Brent ; Goddard, William A. ( July 2022 , Organometallics)

   A recent advance in the synthesis of alkenylated arenes was the demonstration that the Pd(OAc)2 catalyst precursor gives >95% selectivity toward styrene from ethylene and benzene under optimized conditions using excess Cu(II) carboxylate as the in situ oxidant [ Organometallics 2019, 38(19), 3532−3541]. To understand the mechanism underlying this catalysis, we applied density functional theory (DFT) calculations in combination with experimental studies. From DFT calculations, we determined the lowest-energy multimetallic Pd and Pd–Cu mixed metal species as possible catalyst precursors. From the various structures, we determined the cyclic heterotrinuclear complex PdCu2(μ-OAc)6 to be the global minimum in Gibbs free energy under conditions of excess Cu(II). For cyclic PdCu2(μ-OAc)6 and the parent [Pd(μ-OAc)2]3, we evaluated the barriers for benzene C–H activation through concerted metalation deprotonation (CMD). The PdCu2(μ-OAc)6 cyclic trimer leads to a CMD barrier of 33.5 kcal/mol, while the [Pd(μ-OAc)2]3 species leads to a larger CMD barrier at >35 kcal/mol. This decrease in the CMD barrier arises from the insertion of Cu(II) into the trimetallic species. Because cyclic PdCu2(μ-OAc)6 is likely the predominant species under experimental conditions (the Cu to Pd ratio is 480:1 at the start of catalysis) with a predicted CMD barrier within the range of the experimentally determined activation barrier, we propose that cyclic PdCu2(μ-OAc)6 is the Pd species responsible for catalysis and report a full reaction mechanism based on DFT calculations. For catalytic conversion of benzene and ethylene to styrene at 120 °C using Pd(OAc)2 as the catalyst precursor and Cu(OPiv)2 (OPiv = pivalate) as the oxidant, an induction period of ∼1 h was observed, followed by catalysis with a turnover frequency of ∼2.3 × 10–3 s–1. In situ1H NMR spectroscopy experiments indicate that during the induction period, Pd(OAc)2 is likely converted to cyclic PdCu2(η2-C2H4)3(μ-OPiv)6, which is consistent with the calculations and consistent with the proposal that the active catalyst is the ethylene-coordinated heterotrinuclear complex cyclic PdCu2(η2-C2H4)3(μ-OPiv)6. 

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2. Iron-catalyzed C(sp 2 )–C(sp 3 ) cross-coupling at low catalyst loading

   https://doi.org/10.1039/C8CY02374C  

   Bisz, Elwira ; Kardela, Marlena ; Piontek, Aleksandra ; Szostak, Michal ( March 2019 , Catalysis Science & Technology)

   The iron-catalyzed C(sp 2 )–C(sp 3 ) cross-coupling provides a highly economical route to exceedingly valuable alkylated arenes that are widespread in medicinal chemistry and materials science. Herein, we report an operationally-simple protocol for the selective C(sp 2 )–C(sp 3 ) iron-catalyzed cross-coupling of aryl chlorides with Grignard reagents at low catalyst loading. A broad range of electronically-varied aryl and heteroaryl chlorides underwent the cross-coupling using challenging alkyl organometallics possessing β-hydrogens with high efficiency up to 2000 TON. A notable feature of the protocol is the use of environmentally-friendly cyclic urea ligands. A series of guidelines to predict cross-coupling reactivity of aryl electrophiles is provided. 

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3. Dynamic π-Bonding of Imidazolyl Substituent in a Formally 16-Electron Cp*Ru(κ 2 - P , N ) + Catalyst Allows Dramatic Rate Increases in ( E )-Selective Monoisomerization of Alkenes

   https://doi.org/10.1021/acscatal.8b04345  

   Paulson, Erik R. ; Moore, Curtis E. ; Rheingold, Arnold L. ; Pullman, David P. ; Sindewald, Ryan W. ; Cooksy, Andrew L. ; Grotjahn, Douglas B. ( June 2019 , ACS Catalysis)
4. Iron‐Catalyzed C( sp 2 )−C( sp 3 ) Cross‐Coupling of Chlorobenzamides with Alkyl Grignard Reagents: Development of Catalyst System, Synthetic Scope, and Application

   https://doi.org/10.1002/adsc.201800849  

   Bisz, Elwira ; Szostak, Michal ( November 2018 , Advanced Synthesis & Catalysis)

   Direct preparation of alkylated amide‐derivatives by cross‐coupling chemistry using sustainable protocols is challenging due to sensitivity of the amide functional group to reaction conditions. Herein, we report the synthesis of alkyl‐substituted amides by iron‐catalyzed C(sp²)−C(sp³) cross‐coupling of Grignard reagents with aryl chlorides. The products of these reactions are broadly used in the synthesis of pharmaceuticals, agrochemicals and other biologically‐active molecules. Furthermore, amides are used as versatile intermediates that can participate in the synthesis of valuable ketones and amines, providing access to motifs of broad synthetic interest. The reaction is characterized by its good substrate scope, tolerating a range of amide substitution, including sterically‐bulky, sensitive and readily modifiable amides. The reaction is compatible with challenging organometallics possessing β‐hydrogens, and proceeds under very mild, operationally‐simple conditions. Optimization of the catalyst system demonstrated the beneficial effect of O‐coordinating ligands on the cross‐coupling. The reaction was found to be fully chemoselective for the mono‐substitution at the less sterically‐hindered position. Mechanistic studies establish the order of reactivity and provide insight into the role of amide to control mono‐selectivity of the alkylation. The protocol provides the possibility for convenient access to alkyl‐amide structural building blocks using sustainable cross‐coupling conditions with high efficiency.

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5. Catalyst Controlled Site- and Stereoselective Rhodium(II) Carbene C(sp 3 )–H Functionalization of Allyl Boronates

   https://doi.org/10.1021/acs.orglett.2c03335  

   Boni, Yannick T. ; Vaitla, Janakiram ; Davies, Huw M. ( January 2023 , Organic Letters)