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CH Activation (ch + activation)
Selected AbstractsDirect Synthesis of Dihydrobenzo[a]carbazoles via Palladium-Catalyzed Domino Annulation of IndolesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 2-3 2010Farnaz Jafarpour Abstract A straightforward one-step synthesis of annulated indoles via palladium-catalyzed, norbornene-mediated sequential intermolecular aryl ortho -alkylation/intramolecular indole CH activation has been devised. This method provides an efficient route to a wide variety of substituted 6,11-dihydro-5H -benzo[a]carbazoles from readily accessible 3-bromoalkylindoles and iodoarenes. [source] Intramolecular Palladium-Catalyzed Direct Arylation vs.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Heck Reactions: Synthesis of Pyrroloisoquinolines, Isoindoles Abstract The competition between CH activation and Heck reactions has been studied on 2-alkenyl-substituted o -iodobenzylpyrroles. The reaction can be switched from the alkene to the pyrrole nucleus by choosing the adequate catalytic system, regardless of the nature of the substituent on the alkene, obtaining excellent yields of pyrrolo[1,2- b]isoquinolines or pyrrolo[2,1- a]isoindoles. [source] Ruthenium-Catalyzed Oxidative Homo-Coupling of 2-ArylpyridinesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2009Xiangyu Guo Abstract A ruthenium-catalyzed oxidative homo-coupling reaction of 2-arylpyridines via CH activation was developed. The reaction could tolerate various functional groups on both the aryl and the pyridyl rings to afford a series of dimerized products with iron(III) chloride (FeCl3) as a stoichiometric oxidant. A tentative mechanism was proposed for this oxidative CH/CH homo-coupling. [source] Transition Metal-Catalysed, Direct and Site-Selective N1-, C2- or C3-Arylation of the Indole Nucleus: 20 Years of ImprovementsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 5 2009Lionel Joucla Abstract The direct and site-selective transition metal-catalysed N1-, C2- or C3-arylations of indoles have been the subject of almost continuous improvements since their discovery in early 1980s. This research area is mainly motivated by the biological relevance of this class of compounds in order to propose catalytic alternative syntheses to the well known methodologies involving the formation of the indole ring like the Fischer, Larock, Cacchi, Lautens etc. reactions. Since the late 1990s it has experienced new impulses related to the intensive development of catalytic CH activation. Today, through the intensive studies of Buchwald and Hartwig, the N1-arylation of indoles has reached sufficient maturity for both academic and industrial applications. On the other hand, the selective C2- or C3-arylation of indoles, initiated by Ohta in the middle 1980s, has become a hot research area these last years following the reports of Sames. Surprisingly, only few reports concern the use of heterogeneous catalysts; however, the application of these emerging methodologies seems to be related to the discovery of industrially attractive systems. [source] Oxidant-Free Dehydrogenation of Alcohols Heterogeneously Catalyzed by Cooperation of Silver Clusters and Acid,Base Sites on AluminaCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2009Ken-ichi Shimizu Dr. Abstract Trifunctional green catalysis: In-depth characterization shows that oxidant-free selective oxidation of alcohols by silver nanoparticles on ,-Al2O3, as a new heterogeneous catalyst, proceeds through cooperation of silver, acid, and base sites (see figure). A ,-alumina-supported silver cluster catalyst,Ag/Al2O3,has been shown to act as an efficient heterogeneous catalyst for oxidant-free alcohol dehydrogenation to carbonyl compounds at 373,K. The catalyst shows higher activity than conventional heterogeneous catalysts based on platinum group metals (PGMs) and can be recycled. A systematic study on the influence of the particle size and oxidation state of silver species, combined with characterization by Ag,K-edge XAFS (X-ray absorption fine structure) has established that silver clusters of sizes below 1,nm are responsible for the higher specific rate. The reaction mechanism has been investigated by kinetic studies (Hammett correlation, kinetic isotope effect) and by in situ FTIR (kinetic isotope effect for hydride elimination reaction from surface alkoxide species), and the following mechanism is proposed: 1),reaction between the alcohol and a basic OH group on the alumina to yield alkoxide on alumina and an adsorbed water molecule, 2),CH activation of the alkoxide species by the silver cluster to form a silver hydride species and a carbonyl compound, and 3),H2 desorption promoted by an acid site in the alumina. The proposed mechanism provides fundamental reasons for the higher activities of silver clusters on acid,base bifunctional support (Al2O3) than on basic (MgO and CeO2) and acidic to neutral (SiO2) ones. This example demonstrates that catalysts analogous to those based on of platinum group metals can be designed with use of a less expensive d10 element,silver,through optimization of metal particle size and the acid,base natures of inorganic supports. [source] Synthesis and Reactivity of Rare Earth Metal Alkyl Complexes Stabilized by Anilido Phosphinimine and Amino Phosphine LigandsCHEMISTRY - A EUROPEAN JOURNAL, Issue 3 2007Bo Liu Abstract Anilido phosphinimino ancillary ligand H2L1 reacted with one equivalent of rare earth metal trialkyl [Ln{CH2Si(CH3)3}3(thf)2] (Ln=Y, Lu) to afford rare earth metal monoalkyl complexes [L1LnCH2Si(CH3)3(THF)] (1,a: Ln=Y; 1,b: Ln=Lu). In this process, deprotonation of H2L1 by one metal alkyl species was followed by intramolecular CH activation of the phenyl group of the phosphine moiety to generate dianionic species L1 with release of two equivalnts of tetramethylsilane. Ligand L1 coordinates to Ln3+ ions in a rare C,N,N tridentate mode. Complex l,a reacted readily with two equivalents of 2,6-diisopropylaniline to give the corresponding bis-amido complex [(HL1)LnY(NHC6H3iPr2 -2,6)2] (2) selectively, that is, the CH activation of the phenyl group is reversible. When 1,a was exposed to moisture, the hydrolyzed dimeric complex [{(HL1)Y(OH)}2](OH)2 (3) was isolated. Treatment of [Ln{CH2Si(CH3)3}3(thf)2] with amino phosphine ligands HL2-R gave stable rare earth metal bis-alkyl complexes [(L2-R)Ln{CH2Si(CH3)3}2(thf)] (4,a: Ln=Y, R=Me; 4,b: Ln=Lu, R=Me; 4,c: Ln=Y, R=iPr; 4,d: Ln=Y, R=iPr) in high yields. No proton abstraction from the ligand was observed. Amination of 4,a and 4,c with 2,6-diisopropylaniline afforded the bis-amido counterparts [(L2-R)Y(NHC6H3iPr2 -2,6)2(thf)] (5,a: R=Me; 5,b: R=iPr). Complexes 1,a,b and 4,a,d initiated the ring-opening polymerization of d,l -lactide with high activity to give atactic polylactides. [source] Transition-State Energy and Position along the Reaction Coordinate in an Extended Activation Strain Model,CHEMPHYSCHEM, Issue 8 2007G. Theodoor de Jong Dr. Abstract We investigate palladium-induced activation of the CH, CC, CF, and CCl bonds in methane, ethane, cyclopropane, fluoromethane, and chloromethane, using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Our purpose is to arrive at a qualitative understanding, based on accurate calculations, of the trends in activation barriers and transition state (TS) geometries (e.g. early or late along the reaction coordinate) in terms of the reactants' properties. To this end, we extend the activation strain model (in which the activation energy ,E, is decomposed into the activation strain ,E,strain of the reactants and the stabilizing TS interaction ,E,int between the reactants) from a single-point analysis of the TS to an analysis along the reaction coordinate ,, that is, ,E(,)=,Estrain(,)+,Eint(,). This extension enables us to understand qualitatively, trends in the position of the TS along , and, therefore, the values of the activation strain ,E,strain=,Estrain(,TS) and TS interaction ,E,int=,Eint(,TS) and trends therein. An interesting insight that emerges is that the much higher barrier of metal-mediated CC versus CH activation originates from steric shielding of the CC bond in ethane by CH bonds. Thus, before a favorable stabilizing interaction with the CC bond can occur, the CH bonds must be bent away, which causes the metal,substrate interaction ,Eint(,) in CC activation to lag behind. Such steric shielding is not present in the metal-mediated activation of the CH bond, which is always accessible from the hydrogen side. Other phenomena that are addressed are anion assistance, competition between direct oxidative insertion (OxIn) versus the alternative SN2 pathway, and the effect of ring strain. [source] | ||||||||||