Intermetallic Cooperation in Olefin Polymerization Catalyzed by a Binuclear Samarocene Hydride: A Theoretical Study

The cooperative effect in bi- and multinuclear metal complexes is of great interest in catalysis since such a cooperative effect often gives the complexes unique catalytic performance unavailable in mononuclear analogues. However, the related mechanism of bi- and multinuclear cooperative catalysis remained almost unexplored. Herein, the detailed mechanism of ethylene polymerization by a binuclear samarocene hydride complex has been computationally modeled. The results have not only revealed new aspects of the mechanism of olefin insertion reactions but also provided theoretical evidence for electronic communication between the metal centers during the polymerization, where the bridging hydride ligand plays an important role in such an intermetallic cooperation

Enantioselective Reduction of Prochiral Ketones with NaBH₄/Me₂SO₄/(<i>S</i>)-Me-CBS

<div><p></p><p>The enantioselective reduction of prochiral ketones with NaBH₄/Me₂SO₄/(<i>S</i>)-Me-CBS is described. Borane is generated in situ via the reaction of NaBH₄ with Me₂SO₄ in tetrahydrofuran, which is as efficient as the commercial one. Such in situ–generated borane reagent was applied to reduce prochiral ketones in the presence of chiral oxazaborolidine catalyst directly. The corresponding chiral secondary alcohols were obtained with excellent enantiomeric excesses (93–99% <i>ee</i>) and good to excellent yield (80–99%).</p> </div

Synthesis of <i>gem</i>-Difluoroallylboronates via FeCl₂‑Catalyzed Boration/β-Fluorine Elimination of Trifluoromethyl Alkenes

The first ferrous chloride catalyzed boration/β-fluorine elimination of trifluoromethyl alkenes is described. Thus, a full range of <i>gem</i>-difluoroallylboronates were obtained in high yield under mild conditions. As an important fluorinated building block, <i>gem</i>-difluoroallylboronate can be readily converted into diverse difluoro-substituted species

Mechanistic Insights into the Copper-Cocatalyzed Sonogashira Cross-Coupling Reaction: Key Role of an Anion

The Sonogashira cross-coupling reaction is one of the most important and widely used sp²–sp carbon–carbon bond formation reactions in organic synthesis. Up to now, the exact mechanism of the palladium/copper-catalyzed Sonogashira reaction is far from being fully understood, mainly due to the difficulties in clarifying the combination behavior of the two metal catalysts. In this study, DFT calculations have been performed to elucidate the mechanism of the copper-cocatalyzed Sonogashira cross-coupling reaction, where bis­(triphenylphosphino)palladium was used as a catalyst and Cs₂CO₃ was applied as a base. In an agreement between theory and experiment, the Cu cycle could favorably generate an I^–-coordinated copper acetylide as the catalytically active species rather than the generally considered neutral copper acetylide. In addition, the transmetalation is calculated to be the rate-determining step. The results reported herein are expected to have broad mechanistic implications for other bimetal-catalyzed reactions employing metal salts as additives

Mechanistic Investigation on Scandium-Catalyzed C–H Addition of Pyridines to Olefins

This paper reports computational studies on the ortho alkylation of pyridines via C–H addition to olefins catalyzed by cationic half-sandwich rare-earth alkyl species. A detailed mechanism concerning the generation of catalytically active species and C–H addition has been computationally investigated at the molecular and electronic levels. The results support the mechanism based on experiments, which involves the initial generation of a metal pyridyl active species, followed by the coordination and insertion of an olefin and the subsequent pyridine C–H activation by a metal–carbon bond. The <i>o-</i>methyl sp³ C–H activation product of α<i>-</i>picoline has been also calculated, and the results suggest that the sp³ C–H activation product mainly results from the conversion of the sp² C–H activation product of α-picoline rather than from the direct reaction of the cationic species (η⁵-C₅Me₅)­Sc­(CH₂C₆H₄NMe₂<i>-o</i>)⁺ with α-picoline, and such a conversion is reversible. The reaction rate of the whole process is controlled by the generation of active species and an insertion step. The formation of the branched product is both kinetically and energetically favorable over that of the linear product, which is in agreement with the experimental observation. Both steric and electronic factors account for the regioselectivity. An analysis of energy decomposition provides new insights into the stability of the 1-hexene insertion transition states involved in such processes. A comparison between the successive olefin insertion and the C–H activation of pyridine has also been computationally carried out. In addition, it is predicted that the cationic scandium pyridyl species (η⁵-C₅Me₅)­Sc­(MeC₅H₃N)⁺ has a shorter induction period than the initial aminobenzyl analogue (precursor) (η⁵-C₅Me₅)­Sc­(CH₂C₆H₄NMe₂<i>-o</i>)⁺ for the initiation step of ethylene polymerization

Mechanistic Insights into the Methylenation of Ketone by a Trinuclear Rare-Earth-Metal Methylidene Complex

Trinuclear rare-earth-metal methylidene (CH₂^2–) complexes are an emerging class of compounds that serve as methylidene transfer agents for methylenation of carbonyl compounds. Herein, the reaction of a trinuclear scandium methylidene complex with acetophenone was used as a model reaction of the multimetallic-cooperating methylidene transfer case, and its detailed mechanism has been investigated by the DFT approach. The analyses of Wiberg bond index, electron occupation, the frontier molecular orbital, and natural charge provide us a clear and comprehensive understanding of the CH₂^2–/O^2– group interchange process assisted by cooperating multimetal sites. The mechanism presented here is markedly different from conventional Wittig and transition-metal-mediated Wittig-type reactions. In addition, the behavior of μ₃-CH₂ in a multinuclear complex system is also demonstrated. This study not only enriches the chemistry of metal Wittig-type reactions but also sheds light on the intermetallic cooperation for methylidene transfer

Synthesis of Trifluoromethyl Ketones via Tandem Claisen Condensation and Retro-Claisen C–C Bond-Cleavage Reaction

A highly efficient, operationally simple approach to trifluoromethyl ketones has been developed that builds on the use of a tandem process involving Claisen condensation and retro-Claisen C–C bond cleavage reaction. Enolizable alkyl phenyl ketones were found to react readily with ethyl trifuoroacetate under the promotion of NaH to afford trifluoroacetic ester/ketone exchange products, trifluoromethyl ketones, which were quite different from the general Claisen condensation products, β-diketones. This procedure uses readily available starting materials and can be extended to the preparation of perfluoroalkyl ketones in excellent yield

Reactivity and Kinetics of Vinyl Sulfone-Functionalized Self-Assembled Monolayers for Bioactive Ligand Immobilization

A new vinyl sulfone (VS) disulfide, 1,2-bis­(11-(vinyl sulfonyl)­undecyl)­disulfane, was synthesized to enable the preparation of VS-presenting self-assembled monolayers (VS SAMs) on Au substrates. The VS SAMs were used as a model system to assess the reaction kinetics of bioactive ligands, i.e., glutathione (GSH), <i>N</i>-(5-amino-1-carboxypentyl)­iminodiacetic acid (ab-NTA), and mannose, toward the VS groups on the SAM surface. The VS SAMs and the ligand immobilization were characterized by X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and protein-binding experiments using a quartz crystal microbalance (QCM). Kinetic studies showed that the surface VS groups undergo pseudo-first-order reactions with various ligands, with the observed rate constant being 0.057 min^–1 for GSH at pH 7.5, 0.011 min^–1 for ab-NTA at pH 8.5, and 0.009 min^–1 for mannose at pH 10.5. This work advanced our understanding of the reactivity of VS-bearing functional surfaces and further demonstrated the versatile potential of VS chemistry to prepare ligand-immobilized bioactive surfaces

DFT Studies on the Silver-Catalyzed Carboxylation of Terminal Alkynes with CO₂: An Insight into the Catalytically Active Species

DFT calculations on the Ag-catalyzed carboxylation of phenyl acetylene with CO₂ indicate that the true catalytically active species is a CsCO₃^–-coordinated Ag complex rather than neutral PhCCAg conventionally considered for such a process. The energy barrier for the insertion of CO₂ into the C–Ag bond of PhCCAg (28.8 kcal/mol) is higher than that of PhCCAgI^– and PhCCAg­CsCO₃^– anions (19.0 and 23.6 kcal/mol, respectively). Such an anion as a key intermediate is the predominant feature of the carboxylation process. The electronic effect plays a crucial role in stabilizing such transition states. In addition, the presence of an organic ligand slightly hampers generation of the active species and, therefore, reduced the yield of the final carboxylation product, which was observed experimentally

Friedel–Crafts-Type Allylation of Nitrogen-Containing Aromatic Compounds with Allylic Alcohols Catalyzed by a [Mo₃S₄Pd(η³-allyl)] Cluster

With the direct use of allylic alcohols as allylating agents, the Friedel–Crafts-type allylic alkylation of nitrogen-containing aromatic compounds catalyzed by a [Mo₃S₄Pd­(η³-allyl)] cluster is achieved. With a 3 mol % catalyst loading in acetonitrile at reflux or 60 °C, a variety of <i>N</i>,<i>N</i>-dialkylanilines and indoles reacted smoothly with allylic alcohols to afford the Friedel–Crafts-type allylation products in good to excellent yields with high levels of regioselectivity