Migratory insertion attack of nucleophiles onto coordinated ligands b h abstraction releasing substrates from metal centre. It is important to note that this does not always happen in one step. C h activation reactions of this type have been studied for sc and lu by bercaw2 and watson3 et al. The dashed lines represent bonds formed and cleaved during the. Synthesis and comparative studies of ruii complexes for. Hoxidative cleavage, and electrophilic activation scheme 1. Assuming the first agostic interaction leads to ch activation by oxidative addition ch m ch m m ch cm h.
Cartesian coordinates of all geometries and other details. There sigma bond metathesis ch activation are so many students who are in a turbulent kind of problem because they are not able to complete their term paper, thesis, and assignments by themselves. Bond metathesis reactions of water and methanol with palladium hydride and methyl. This is also the mechanism for other hydrofunctionalization. For example, oxidation addition in the example shown above would require. Douglas group members will be acknowledged, and preliminary investigations into the idea of migratory insertion or sigma bond metathesis across cyclopropane will be provided. H bond activation and can be used in the functionalization of alkanes. A catalyst lowers the activation energy of a reaction no catalyst. Organic chemistry palladiumcatalyzed carbonsulfur or carbonphosphorus bond metathesis by reversible arylation zhong lian, benjamin n. The importance of x h bond activation will encourage continued growth in. The active catalysts are lutetiummethyl or lutetiumhydride complexes.
Sigma bond metathesis for ch bonds of hydrocarbons and scr r h, alkyl, aryl bonds of permethylscandocene derivatives. Activation of ch bonds with organometallic species is of major importance in catalysis, the main advantage of such process should be mild conditions and selectivity, with oxidative addition and. Ch activation involves any of a wide range of techniques, often using catalysts in which a carbon to hydrogen covalent bond of an organic compound is cleaved to replace the hydrogen with a functional group. Increasing the acidity of the c h hydrogen, and the basicity of the ligand receiving the hydrogen, accelerates c h activation. Such complexes can not undergo an exchange of ligand by a pathway involving an oxidative addition followed by a reductive elimination. So, unlike the methane c h activation, the direct sigma bond metathesis pathway is predicted to take place predominantly. Ch activation of benzene by platinumii complexes with. Ch bond activation refers to the formation of a complex wherein the ch bond interacts directly with the metal to afford a cm intermediate in the absence of a. Selective c h activation at a molecular rhodium sigma.
Also the more common sigma bond metathetical metallation of arenes was observed eq 2. It explores the fundamentals of the field and its modern applications. Ch bond activation in organic synthesis jie jack li download. For the former, the mechanism for an experimentally known complex was elucidated and for the latter the controlling factors for a proposed catalyst were explored.
This complex was then shown to also work in acidic media, a critical aspect for product stabilization. Sigma bond metathesis reactions between substrates in coordination sphere. Chen oxidative addition typically occurs with electronrich, lowvalent. Computational study of the nicatalyzed ch oxidative. Pathways for ch activation and functionalization by group. Catalytic dehydrogenation of ethane over mononuclear cr. M interaction have not only been found in the ground state but also implicated in the transition states of many important organometallic transformations such as zieglernatta catalysis and sigma bond metathesis. Coordination of the phosphine and ch bond activation at an sp 2 carbon of the ligand with the release of methane takes place to. Sigmabond metathesis is easier to explain with a drawing than with words. The formation of 3a in the presence of hbpin was calculated to proceed via a fourcenter sigma bond metathesis. There is no more than 1 sigma bond between any two atoms.
Mechanistically, ch bond activation, metal carbene formation, migratory insertion, and reductive elimination are hypothesized as indispensable steps in the catalytic cycle scheme 4. Sigmabond metathesis reactions involving lanthanide. The past twenty years have seen many examples of ch bond activation at transitionmetal centres, often under remarkably mild conditions and with high selectivity. Assuming the first agostic interaction leads to ch activation. Electrondeficient diazo compounds are preferred for this transformation. H activation is mechanistically similar to the reaction of h. Unfortunately the reaction does not readily allow the introduction of functional groups. Sigma bond metathesis electrophilic activation metalloradical activation complexes existing in monomerdimer equilibrium can reversibly break alkane ch bonds 1,2 addition addition of an alkane to a metalnonmetal double bond labinger, j. In recent years, there has been explosive growth in this area. Development of carboncarbon sigmabond activation with. Experimental and computational evidence for a boronassisted, sigma bond metathesis pathway for alkane borylation. The study focuses on a reaction mechanism consisting of three main reaction steps. Carbonhydrogen sigmabond metathesis occurs when such a.
Similarly, cc activation often is proposed to proceed through. The relative reaction rates of sp 3 c h bond activation by felements through. Catalytic dehydrogenation of ethane over mononuclear criii surface sites on silica. Tilley department of chemistry, university of california at san diego. Sigmabond metathesis reactions involving lanthanidesilicon. Greener than conventional methods, ch activation methods have flourished during the last decade and become especially attractive to organic chemists. H bond activations mediated by rareearth metals and actinides following traditional reaction pathways, such as. Why does ch activation work well for pdii and ptii. The organometallic chemistry of the transition metals, 7th.
In organometallic chemistry, sigmabond metathesis is a chemical reaction wherein a metalligand sigma bond undergoes metathesis exchange of parts with. Sigmabond metathesis resulting in the elimination of cd 3oh is the initial reaction observed, with further solvation of the metal center and subsequent elimination of hydrogen occurring as additional reaction channels. The new edition of this bestselling text continues to provide chemistry students and researchers with vital information on organometallic compounds, their preparation, and use in synthesis. Sigmabond metathesis is a common reaction with early transition metals that are in their highest oxidation state. These processes are facile at room temperature and involve little or no activation energy. It can occur through oxidative addition, sigmabond metathesis, metalloradical activation, 1,2addition, or electrophilic activation.
Phosphorus, sulfur, and silicon and the related elements. Experimental and computational evidence for a boron. Palladiumiicatalyzed c h activation c c cross coupling reactions chunrui sun lee group. Catalytic sigmabond metathesis and the polymerization of 1,3. The reaction attracted much attention because hydrocarbons are normally unreactive substrates, whereas some sigma bond metatheses are facile. Kinetic studies indicate the stronger donating pme3 and poch23cet ligands increase the rate of degenerate benzene ch activation for tprulncmeph systems relative to tpruconcmeph. Organometallic c h bond activation refers to the formation of a complex wherein the c h bond interacts directly with the metal to afford a cm intermediate in the absence of a free radical or an ionic intermediate. Through these pathways, metathesis of me bonds can occur at late transition metals. Computational studies show that this alkyl c h bond activation reaction involves deprotonation of one of the c h bonds that is geminal to the agostic interaction, rather than the agostic c h bond. And finally, before we close the discussion, we will discuss two important aspects which is, one is the interaction. University of groningen lanthanide mediated activation of. Sigma bond metathesis reactions of the type of eq 3 are mechanistically. Controlling the hydrogenolysis of silicasupported tungsten.
Includes endofchapter problems and their solutions has uptodate examples of fundamental reaction steps. Hammett studies and kinetic isotope effect studies are consistent with a ch activation mechanism which proceeds by a sigma bond metathesis pathway. Ch bond activation an overview sciencedirect topics. Ch bond activation in organic synthesis crc press book. Increasing the acidity of the ch hydrogen, and the basicity of the ligand receiving the hydrogen, accelerates ch activation. We also anticipated that hydrogenolysis of the wme moieties in 1. Ch bond activation in organic synthesis jie jack li. Ch bond activation of hydrocarbons mediated by rareearth. Nov 25, 2015 catalytic sigma bond metathesis and the polymerization of 1,3dienes by rareearth metal complexes. Theoretical elucidation of the platinummediated arene ch. A mechanism involving sigma bond metathesis following coordination of benzene instead of an oxidative addition. University of groningen lanthanide mediated activation of ch. Finally, its postulated that the activation energy for oxidative addition. Bhawal, peng yu, bill morandi compounds bearing arylsulfur and arylphosphorus bonds have found numerous applications in drug development, organic materials, polymer science, and homogeneous catalysis.
Apply now sigma bond metathesis ch activation chat live 1. Sigma bond metathesis reactions involving lanthanidesilicon and lanthanidehydrogen bonds n. Amine catalyzed solvent ch bond activation as deactivation. Understanding and exploiting ch bond activation nature. First, it is often simply easier for the metal center to approach a ch sigma bond sterics. The two types of ch activation lead to activation of different ch bonds. Hammett studies and kinetic isotope effect studies are consistent with a c h activation mechanism which proceeds by a sigma bond metathesis pathway. First, unlike the majority of nicatalyzed ch functionalization reactions, which. Sigmabond metathesis reactions of scocd 1 with water.
Sigma bond metathesis is basically an exchange of a sigma bond that typically occurs with the transition metals on the first few columns the early transition metals that are in their highest oxidation state. It has been suggested that dehydrocoupling reactions proceed via sigma bond metathesis. A molecule with no rings can be represented as a tree with a number of bonds equal to the number of atoms minus one as in dihydrogen, h 2, with only one sigma bond, or ammonia, nh 3, with 3 sigma bonds. Both with sigma bond metathesis and concerted metalation deprotonation were investigated. H bond with the metal center, engaging in a 3center 2electron interaction, i. In the later chapters, nonoxidative ch activation is discussed ir complexes. This material is available free of charge via the internet at pdf. Why does ch activation work well for pdii and ptii, but not for niii. Bond metathesis reactions involving palladiumii utrecht. Chem soc rev rsc publishing royal society of chemistry. I have seen a lot of students of graduation, and postgraduation suffering miserably because of these issues. Moreover, the ch bond activation is found irreversible via deuterium experiment.
Fundamentals and recent developments giovanni poli giovanni. Bond metathesis for carbonhydrogen bonds of hydrocarbons and scr r h, alkyl, aryl bonds of permethylscandocene derivatives. B carboxylateassisted metalation deprotonation at palladium. May 30, 2002 the past twenty years have seen many examples of ch bond activation at transitionmetal centres, often under remarkably mild conditions and with high selectivity. Decoordination substitution reductive elimination sigma bond metathesis reverse of introduction.
Ch bond activation of hydrocarbons mediated by rareearth metals and. Organic chemistry palladiumcatalyzed carbonsulfur or. Another interesting study from the same work is the regioselectivity of toluene activation. Alkane activation 336 cc bond cleavage in 342 dehydrogenation, homogeneous catalysis of 339 metathesis, homogeneous catalysis of 340 alkene coupling 82 hydroboration catalysis 246 hydroformylation, homogeneous. Edited by a practioner in this rapidly developing field, ch bond activation in organic synthesis provides an overview of this exciting playgroun. As shown in the drawing, a sigma bonded ligand is replaced through reaction with the sigma bond of an incoming ligand. However, it is difficult to achieve selectivity when multiple inequivalent ch bonds are present in the target molecule. Oxidative addition vs sigma bond metathesis b ond metathesis. Sigmabond metathesis electrophilic activation metalloradical activation complexes existing in monomerdimer equilibrium can reversibly break alkane ch bonds 1,2 addition addition of an alkane to a metalnonmetal double bond labinger, j. Selective c h activation at a molecular rhodium sigmaalkane. Metalfree catalytic ch bond activation and borylation of. Notice that this fourcenter mechanism does not involve a change in oxidation state. C metalfree c h activation of heteroarenes using flp catalysts. Functionalization of unactivated carbonhydrogen ch single bonds is an efficient strategy for rapid generation of complex molecules from simpler ones.
The first presents identifying features and reactivity characteristics of this pivotal mechanistic step for high oxidation state metals, which are given with some historical context, though not in. It has been suggested that dehydrocoupling reactions proceed via sigmabond metathesis. Indeed, using dft b3pw91 calculations for the study of the ch 4 activation by the cp 2 lnme complex where cp is modeled by cl and ln is a lanthanide. Second, a statistical abundance of ch bonds will always be prevalent with hydrocarbons. Proposed fourcentered transition states in c h activation of rh by cp,lnh, complexes ln sc, y, lu. Pathways for ch activation and functionalization by group 9. Consequences of metaloxide interconversion for ch bond activation during. The ch sigma bond activation of methane and the nh sigma bond activation of ammonia by me3sio2tinsime3 1 were theoretically investigated with dft, mp2 to mp4sdq, and ccsdt methods.
Nov 25, 2009 amine catalyzed solvent c h bond activation as deactivation route for cationic decamethylzirconocene olefin polymerization catalysts. We attribute this new mode of c h activation to a weak endon interaction between the linear nitrile group and the metal centre. H bond activation examples are discussed in detail in order to understand the. Ch activation for functionalization is the state of the art in organic chemistry. Insertion of a c h bond into the ligand of a transition metal tm complex. Includes endofchapter problems and their solutions has uptodate examples of fundamental reaction steps and emphasis on key topics. C h r u cl cl cy 3 p c h n n o o mo r r n r r r schrock range of biphen binaphtholates. Sigma bond metathesis lanthanides pretzel festival. The activation under mild conditions of alkane ch bonds had itself long been considered a challenging goal. Evidence for the noninvolvement of the pi system in electrophilic activation of aromatic and vinylic ch bonds. We show here by a combination of experimental and theoretical studies that the unoccupied p orbital of dioxaboryl ligands are intimately involved in the ch bond activation step and that this hydrogen transfer to boron occurs by a boronassisted, metalmediated sigma bond metathesis. Ch bond activation of unsaturated hydrocarbons by a niobium methyl cyclopropyl precursor. Ch activation of benzene by platinum ii complexes with cyclometalated phosphine ligands.
Activation of remote metach bonds assisted by an endon. Sigma bond metathesis reactions involving lanthanidesilicon and lanthanidehydrogen bonds. A activation of c h bonds in borylation transformations using ir catalysts. In the later chapters, nonoxidative c h activation is discussed ir complexes. The barrier associated with direct sigma bond metathesis is 14. Palladiumiicatalyzed ch activation cc cross coupling. Clues that it is possible to interconvert sigma ligands without a change in oxidation state derive from c h activation reactions effecting isotope exchange and from dynamic rearrangements of sigma complexes see frontispiece. Here we report a class of easily removable nitrilecontaining templates that direct the activation of distal meta c h bonds more than ten bonds away of a tethered arene. Catalytic dehydrogenation of ethane over mononuclear criii. In early experimental studies of transitionmetal activation of methane two alternative mechanisms were invoked. In 1983, watson reported one of the first lanthanocene catalyzed ch bond activation reactions.
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