Hydrogenation Reactions Catalyzed by Transition Metal Complexes

Hydrogenation Reactions Catalyzed by Transition Metal Complexes... 

Hydrogenation Reactions Catalyzed by Transition Metal Complexes, 17, 319 Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199 Infrared and Raman Studies of ir-Complexes, 1, 239 Insertion Reactions of Compounds of Metals and Metalloids, 5, 225 Insertion Reactions of Transition Metal-Carbon Bonded Compounds 1. Carbon Monoxide Insertion, 11, 87... 

Hydrogenation Reactions Catalyzed by Transition Metal Complexes, 17, 319 Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199... 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

Prior chapters have covered the use of transition metals in asymmetric hydrogenations ( 6.2 and 7.1), hydroborations ( 7.3), hydrosilylations and hydro-cyanations ( 6.3, 6.4, 7.4 and 7.5), cyclopropanations ( 7.19), aldol reactions ( 6.11), allylations of carbanions ( 5.3.2), and some sigmatropic rearrangements ( 10.3). This chapter covers other reactions catalyzed by transition metal complexes including coupling of organometallic reagents with vinyl, aryl or allyl derivatives, Heck reactions allylamine isomerizations, some allylation reactions, car-bene insertions into C-H bonds and Pauson-Khand reactions. 

Low-density polyethylene (LDPE) is industrially synthesized by a free-radical process using peroxide initiators at high temperatures and pressures. LDPE has a highly branched structure due to hydrogen atom shifts that convert a primary alkyl radical to a secondary alkyl radical. Plants that can accomodate this high-pressure process are expensive to build and operate. Therefore, LDPE formed by radical reactions is slowly being replaced by LLDPE prepared by reactions catalyzed by transition metal complexes. 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

Homogeneous Hydrogenation. The mild reaction conditions used in the homogeneous hydrogenation of C02 catalyzed by transition metal complexes allows the partial hydrogenation of C02 to yield formic acid and derivatives ... 

In recent years there has been much interest in homogeneous hydrogenations catalyzed by transition metal complexes (7). One facet of research in this area is the search for chiral catalysts (catalysts that are dissymmetric, i.e., optically active) that can be used to produce chiral compounds via asymmetric reactions. In this review, we survey asymmetric homogeneous hydrogenation reactions, that is reactions that create asymmetric carbon atoms by the addition of hydrogen across multiple bonds under the influence of soluble chiral catalysts. 

Over the last 50 years numerous reactions of organic compounds catalyzed by transition metal complexes have been developed (e. g., olefin oxidation -Wacker Process, hydroformylation, carbonylation, hydrogenation, metathesis, Ziegler-Natta polymerization and oligomerization of olefins) in which the reactivity of metal-carbon bonds in the active intermediate (organometallics) is crucial. 

In catalytic conversions of (organo)silicon compounds only the hydrosilylation is a well-known process of industrial importance [1-5]. However, since 1985 other reactions of silicon compounds catalyzed by transition metal complexes have been revealed and spectacularly developed. Most of them occur via a mechanism involving metal-silicon and metal-hydrogen bonds, only occasionally accompanied by (or sided) metal-carbon bonds. 

The ability to interconvert one functional group into another is of fundamental importance in organic synthesis. Often, these interconversions involve reduction or oxidation of a functional group, and such transformations also may either create or destroy a stereogenic center. The first part of Section 12-1 will explore transition metal-catalyzed hydrogenations of C=C and C=0 bonds, which can exhibit a high degree of stereoselectivity. The second part will consider oxidation reactions that are also catalyzed by transition metal complexes, which can lead to enantioenriched products. 

In view of these observations it is not surprising that there occur a wide variety of catalytic reactions, particularly involving hydrocarbons. For example, the isomerization of olefins, homogeneous hydrogenations, group substitution in the vicinity of double bonds, and polymerization reactions are all catalyzed by transition metal complexes. 

The 1,2-insertion of alkenes into transition metal-carbon o-bond leads to C-C bond formation under mild conditions, as described in Chapter 6. This reaction is considered to be a crucial step in the coordination polymerization and carbometalation of alkenes catalyzed by transition metal complexes. A common and important carbometalation is the Heck-type arylation or vinylation of alkene catalyzed by Pd complexes [118], The arylation of alkene, most typically, involves the formation of arylpalladium species and insertion of alkene into the Pd-aryl bond as shown in Scheme 5.20. The arylpalladium species is formed by the oxidative addition of aryl halides to Pd(0) complexes or the transmetalation of aryl compounds of main group metals with Pd(II) complexes. Insertion of alkene into the Pd-aryl bond produces 2-arylalkylpalladium species whose y6-hydrogen elimination leads to the arylalkene. Oxidative chlorination of the 2-arylalkylpalladium intermediate forms chloroalkanes as the product. 

Hydrogenation of olefins catalyzed by transition metal complexes dissolved in ionic liquid solvents have been reported using rhodium-, and ruthenium- and cobalt-containing catalysts. In these studies it was shown that hydrogenation rates where up to five times higher than the comparable reactions when carried out in propanone. The solubilities of the alkene reagents, TOFs, and product distributions where all strongly influenced by the nature of the anion in the ionic liquid solvent... 

Hydrocarbyl ligands, such as those shown in Figure 3.1, lie at the heart of transition metal organometallic chemistry. Virtually all reactions catalyzed by transition metals, such as hydrogenation (Chapter 15), cross-coupling (Chapter 19), hydroformylation (Chapter 16), and olefin polymerization (Chapter 22), involve the formation of such complexes. Stable hydrocarbyl complexes are known for all transition metals, and the early work in organometallic chemistry focused on their synthesis. 

Isomerization of olefins which takes place as a result of the transfer of hydrogen atoms with concomitant migration of the double bond is catalyzed by transition metal complexes which may react with olefins to form organometallic compounds. Often, it is necessary that cocatalysts be present as sources of hydrogen. Most commonly utilized cocatalysts are acids, water, alcohol, hydrogen, silanes, etc. The formation of hydrido complexes during isomerization reactions is crucial. The following mechanisms of olefin isomerization reactions are known ... 

The hydrosilylation of alkenes and alkynes catalyzed by transition metal complexes is often accompanied by side reactions such as isomerization, polymerization, and hydrogenation of unsaturated compounds and/or redistribution and dehydrogenation of silicon hydrides as well as reactions in which both substrates take part, such as dehydrogenative silylation (4,13). The latter reaction, which in certain conditions permits direct production of unsaturated silyl compound, has been a subject of intense study over the last two decades. 

Intermolecular oxidative addition of sp CH bonds are rare [7] and, until recently, were limited to organic molecules with CH bonds activated by electronic effects (reactions 3—5) [12, 13, 14], A particular example is the hydrogen transfer reaction (reaction 4) [13] catalyzed by transition metal complexes. 

Hydrogenation of CO2 to formic acid (H2 -I- CO2 -< HC(O)OH) catalyzed by transition metal complexes is one of interesting and attractive reactions for COj fixation. Experimentally, various metal complexes, especially those of Ru and Rh, have been found to be able to efficiently catalyze this reaction [10]. Studies show that metal hydride complexes display excellent catalytic activity. This is understandable as reactions of metals with Hj easily generate metal hydride species. Computationally, hydrogenation of COj to formic acid is also among one of the most studied reactions involving COj [11,12]. 

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