Olefin insertions metal-hydride bonds

Insertions of Olefins into Metal-Hydride Bonds... 

Hydrogenation of 1,3-dienes to terminal olefins is catalyzed by HRh(PPh3)4 and [Rh(CO)2(PPh3)2]2 in the presence of excess phosphine diene insertion into a metal- hydride bond to give a-alkenyl rather than 7r-allyl intermediates was postulated for the initial step (141). Mechanistic studies of the HRh(PPh3)4 catalyst (142) and a more reactive phosphole analog (143) HRh(DBP)4 [5-phenyl-5//-dibenzophosphole (DBP), 7] for... 

Figure 20 Chain growth termination and start of a new chain by fi-H transfer to a coordinated monomer (top) and to the metal centre (bottom), followed by olefin insertion into the metal-hydride bond (RLS — rate-limiting step).

More recent studies have shown that a number of other mechanisms are operative in the hydrosilation process for different metals. Mechanistic proposals for early metals, lanthanides and actinides have been elaborated on. These involve a Chalk-Harrod like initial migratory insertion into a metal-hydride bond, followed by a a-bond metathesis step (Scheme 4). An alternative mechanism, however, was proposed for Group 4 metallocene catalysis, which involves a coordinated olefin, which undergoes a-bond metathesis with the hydrosilane. ... 

Transformation of the. r-olefin complex 7 to a o -alkyl complex 7a via insertion of the olefin into the metal-hydride bond (step 3) ... 

Insertions of olefins into metal-carbon bonds are thought to occur by cis or 1,2-syn addition, as found for analogous insertions into metal-hydride bonds. Unfortunately, few well-documented examples exist among these are reactions (a)-(j) " . 

Although 187-189 were not active catalysts for polymerization process, 187 and 189 proved to be active olefin hydrosilylation catalysts, presumably 187 first reacted with a silane to form a reactive metal hydride species. They are the first examples of d° metal complexes with non-Cp ligands in the catalytic hydrosilylation of olefins. The mechanism was believed to be consistent with that of other d° metallocene-based catalysts and included two steps 1) fast olefin insertion into the metal hydride bond and 2) a slow metathesis reaction with the silane. The catalysts exhibited a high regioselective preference for terminal addition in the case of aliphatic olefins... 

Kinetic data have been reported for cyclohexene reduction with a 1 6 Cr(acac)3- Bu3Al catalyst in heptane at 30 C, which showed a first-order dependence on catalyst and H2. Hydrogenation rates generally decrease with increasing substitution of the alkene substrate. Similar kinetic results were independently obtained for the Cr(acac)3- Bu3Al catalyst. A proposed mechanism involves alkylation of the metal-halide [equation (a)], hydride formation [equation (b)], followed by reversible insertion of the olefin substrate into the metal-hydride bond [equation (c)], and hydrogenolysis of the resulting metal-alkyl bond [equation (d)]. ... 

For electrophilic attack, Markovnikov addition is that in which the positive portion of the reagent adds to the least substituted carbon atom of the double bond undergoing reaction.) This may result from a steric preference for the least-substituted metal alkyl intermediate formed by insertion of olefin into the metal hydride bond . Vinylarenes comprise an exception, where interaction of nickel with the aromatic ring stabilizes the precursor of the branched nitrile, leading primarily to a Markovnikov addition product . 

A detailed mechanism of asymmetric hydrovinylation is discussed in order to explain the pathways of the asymmetric induction4- 51314. The 7t-allylnickel complex, as the catalyst precursor, is activated by phosphanes and ethylaluminum chloride and reacts with an olefin to give a catalytically active nickel hydride-olefin complex. The olefin then inserts into the metal hydride bond and after coordination and insertion of ethene a new alkylnickel compound is... 

This is the rate-determining step involving heterolytic splitting of the hydrogen molecule and formation of an hydridoruthenium(II) complex. The next step involves rearrangement of the hydrido-7r-olefin complex to a (T-alkyl complex via insertion of the olefin into the metal hydride bond. Finally, electrophilic attack occurs on the metal-bonded carbon... 

Because various important industrial organic processes utilize olefins, convenient methods to convert olefins into various products are vital. Transition metal catalysts with proper ligands have proved most useful in controlling the course of these reactions. Transition metal complexes catalyze skeletal isomerization, double bond isomerization, polymerization, and other processes. Insertion of a terminal olefin into a transition metal hydride bond by 1,2-inserfion or... 

Insertion reactions play an important role in the catalysis of C-C and C-H coupling [1]. Insertion of CO and olefins into metal-alkyl and metal-hydride bonds are of major importance in industrial chemistry. Insertion reactions take place according to... 

Olefin insertion is particularly facile in the case of the complexes [PtH(SnCl3)(PR3)2]. The soft ii-acceptor ligand [SnCls] stabilizes the metal-hydride bond (symbiosis of soft ligands) and hence catalyzes the insertion reaction as... 

Migratory insertions are one step of many different types of catalytic processes, several of which are conducted on large industrial scales and are presented in later chapters of this text. For example, the mechanism of carbonylation processes, such as hydroformylation, includes ttie insertion of CO into a metal-carbon bond. Likewise, catalytic hydrogenation occurs by insertion of an olefin into a metal-hydride bond, and olefin polymerizations and couplings of olefins with haloarenes occur by insertions of olefins into metal-carbon bonds. The reverse of these reactions, p-hydride, p-alkyl, and p-aryl eliminations, are principal pathways for the decomposition of metal-alkyl complexes. 

The insertion of alkynes into metal-hydride bonds occurs during a number of catalytic processes, including alkyne hydrogenation, hydrosilylation, silylformylation, hydroesterification and dimerization. This insertion chemistry is more complex mechanistically than the insertions of olefins into metal hydrides. In some cases, ds addition products have... 

The simplest pathway for the insertion of alk)mes into metal-alkyl bonds involves a migratory insertion through a four-centered transition state akin to that for the insertion of olefins into metal hydrides. Selected examples of this insertion process are shown in Equations 9.48-9.50. The formation of ds products from insertion into iridium and rhodium hydrides is shoivn in Equations 9.48-9.49. In these cases, the open coordination site for binding of an aUcyne prior to the actual insertion step is thought to be generated by indenyl ring slip. A case in which the isomerization of the initially formed cis olelfin to the trans isomer was observed directly is shown in Equation 9.50. - ... 

The chemistry of the cobalt complexes in Figure 9.3 illustrates the different rates for these two types of insertions. The hydride complex undergoes rapid intramolecular insertion of the olefin into the hydride, as revealed by isotopic labeling, but the methyl complex does not undergo the analogous insertion of olefin into the metal-methyl bond. The chemistry of the rhodium complexes in Equations 9.57 and 9.58 provides a more quantitative comparison of the relative rates for insertion of olefins into metal-hydride and... 

The insertions of alk5mes into metal-carbon o-bonds are less common than either the insertions of olefins into metal-carbon bonds or the insertions of alkynes into metal-hydride bonds. Nevertheless, several examples of this reaction have been studied, and many examples are part of catalytic processes. Most of the insertions of alkynes into metal-carbon bonds occur by concerted migratory insertion pathivays and provide products from cis addition of the metal and hydrocarbyl group across the carbon-carbon multiple bond, as predicted on theoretical groimds by Thom and Hoffmann. In some cases, the products from trans addition are observed, but these kinetic products are thought to result from isomerization of the vinyl group in reaction intermediates formed by cis addition. 

A few studies of isolated metal-silyl complexes and ttie computational study of rhodium-sUyl complexes illustrate the insertion of olefins into metal-silicon bonds. Wrighton studied the photochemical reaction of iron-silyl complexes witti ettiylene (Scheme 9.13). Photolysis of Cp FefCOl fSiMej) in the presence of ettiylene forms Cp Fe(CO)(CjHJ(SiMej). This complex appears to insert ethylene, but ttie 16-electron insertion product is unstable and forms the corresponding vinylsilane and iron hydride complexes as products. Photolysis of Cp Fe(CO)j(SiMe3) in the presence of ethylene and CO forms ttie p-silylaDcyl complex containing two CO ligands. 

The mechanism of hydrosilylation involves a sequence of elementary reactions described in the earlier chapters of the book. The most commonly cited mechanism for hydrosilylation was first described by Chalk and Harrod and involves oxidative addition of the silane, insertion of an olefin into the metal-hydride bond, and reductive elimination to form the silicon-carbon bond in the organosilane product. More recently, a related but distinct mechanism involving insertion of the olefin into the silyl group has been recognized, and this mechanism is often called the modified Chalk-Harrod mechanism. Before these steps are described, some of the mechanistic issues regarding the specific systems of Speier s catalyst and Karstedt s catalyst are described briefly. 

A quantum dynamical modehng of olefin insertion into a metal-hydride bond and its reverse reaction is the topic of Chapter 1 by Klatt and Koppel. They employ the wave packet methodology, which is based on the time-dependent Schrodinger equation and allows the description of coherence and tunneling effects as well as... 

Coordination of the olefin at the metal hydride center and subsequent insertion of the carbon-carbon double bond of the coordinated olefin into the metal-hydride bond can be related to the initial step of a classical polymerization. The metal-carbon bond formed in this way inserts a second monomer molecule previously coordinated into the same metal center (the propagation step). The dimer is formed by a p-hydride subtraction, a common cleavage reaction of transition metal-carbon bonds. The p-hydrogen of this alkyl group attached to the... 

Both CO and olefin insertion can proceed into a metal-alkyl bond, and olefins also easily insert into metal-hydride bonds, but CO insertion into a metal-hydride bond is thermodynamically unfavorable and thus very rarely encountered. 

Olefins can insert into metal-alkyl bonds as well as into metal-hydride bonds provided that a vacant coordination site is available on the metal center. This reaction can thus be repeated even when the alkyl ligand lengthens, which makes the proposed mechanism a Ziegler-Natta olefin polymerization. This type of polymerization is detailed in Chap. 15.1 ... 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

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