Insertion and -elimination

By insertion, a ix-bound 2e ligand, A=B, inserts into an M-X bond to give M-(AB)-X, where AB has formed a new bond with both M and X. There are two main types of insertion, either 1,1 (Eq. 71) or 1,2 (Eq. 7.2). In 1,1 insertion, M and X end up bound to the same atom of AB, but in the 1,2 type, M and X end up on adjacent atoms of AB. The type of insertion in any given case depends on the nature of A=B. For example, CO gives only 1,1 insertion where both M and X end up bound to CO carbon. On the other hand, ethylene gives only 1,2 insertion, where the product, MCH2CH2X, has M and X on adjacent atoms of the ligand. 

The Organometallic Chemistry of the Transition Metals, Sixth Edition. Robert H. Crabtree. 

In principle, insertion is reversible, and reversibility is indeed seen experimentally, but just as we saw for OA and RE in Chapter 6, in many cases, only the thermodynamically favored direction is ever observed. For example, SO2 commonly inserts into M-R bonds to give alkyl sulfinate complexes, but these rarely eliminate SO2. Conversely, N2 readily eliminates from diazoarene complexes, but the reverse is not seen. 

In one useful picture of insertion, the X ligand migrates with its M-X bonding electrons (e.g., as H or Me ) to attack the tt orbital of the A=B ligand. In this intramolecular nucleophilic attack on A=B, the 

Oxidative addition and substitution allow u to assemble le and 2e ligands on the metal. With insertion, and its reverse reaction, elimination, we can now combine and transform these ligands within the coordination. sphere, and ultimately expel these transformed ligands to form free organic compounds. In insertion, a coordinated 2e ligand, A=B, can insert itself into an M—X bond to give M—(AB)—X, where ABX is a new le ligand in which a bond has been fmmed between AB and both M and X. 

The OrgammeudUc Chendsay of the Transiiim Mends, Fourth Edition, by Roben H. Crabtree Copyright 2005 John Wiley Sons, Inc. 

1- InseTtion (Eq. 7.1) occurs with -bound ligands such as CO. 

In principle, insertion reactions are reversible, but just as we saw for oxidative addition and reductive elimination in Chapter 6, for many ligands only one of the two possible directions is observed in practice, probably because this direction is strongly favored thermodynamically. For example, SO2 commonly inserts into M—R bonds to give alkyl sulhnate complexes, but these rarely eliminate S02- Conversely, diazoarene complexes readily eliminate N2, but N2 has not yet been observed to insert into a metal-aryl bond. 

Departamento de Qumica Inorgdnica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain 

The cis-1,2-addition of M-X bonds to unsaturated A=B bonds and its reverse, the -elimination of X from M-B-A-X, are fundamental elementary steps of catalytic reactions such as hydrogenation, hydroformylation, oligomerization, polymerization, hydrosilation, hydrocyanation, or alkene isomerization processes, as well as the Heck reaction. Most of the reactions described in the literature involve M-H or M-C bonds, and alkenes or alkynes. Besides them there are processes where the unsaturated substrate is different from alkene or alkyne This includes CO2, CS2, aldehydes and ketones, imine, or nitrile. Also, there are processes involving M-Si, M-Sn, M-B, M-N, M-P, or M-M bonds. The insertion of alkenes into M-carbene bonds is not essentially different in their intimate mechanism, but it is not discussed in this chapter. 

The heart of the cw-1,2-addition, at a first level of approximation, is common to most of the processes to be considered. It consists of an orbital interaction in a four-center transition state (TS) from which the bond rearrangement occurs (Eq. 6.1). The result of the c/s-1,2-addition can also be looked at as arising from insertion of the unsaturated group A=B into the M-X bond, and for this reason the reaction is also named insertion. Another way to describe the process is as an endo attack of X to the coordinated A=B reagent (often an alkene). 

Depending on all the actors playing a role in Eq. 6.1, each particular case can show important variations in the details of the insertion. Thus, the kind of transition metal, its oxidation state, the nature of X and of A=B will produce dramatic changes in the kinetics and thermodynamics of this step. Some cases have been studied in detail and examples are discussed later. 

Current Methods in Inorganic Chemistry, Volume 3 Editors H. Kurosawa and A. Yamamoto 2003 Elsevier Science B.V. All rights reserved 

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As already pointed out, the presence of a macrocyclic ligand, which occupies the four equatorial coordination sites of an octahedral complex, will tend to limit insertion and elimination reactions of the Co—C bond to those which require only that single coordination site. The presence of unidentate ligands in the pentacyanides, on the other hand, will offer greater opportunities for reactions which require a second, adjacent site. 

The following types of insertion and elimination reaction have been... 

Insertion and -elimination. A catalytic cycle that involves only one type of elementary reaction must be a very facile process. Isomerisation is such a process since only migratory insertion and its counterpart P-elimination are required. Hence the metal complex can be optimised to do exactly this reaction as fast as possible. The actual situation is slightly more complex due to the necessity of vacant sites, which have to be created for alkene complexation and for P-elimination. 

The general catalytic cycle thus typically consists of the four steps addition, association, insertion, and elimination. 

Ene-type products are obtained by Co- and Fe-catalysed reaction of dienynes. Cocatalysed cyclization of substrate 111 proceeds smoothly with respect to the diene, acetylene and allylic ether moiety to afford 114. In this cyclization, the 7i-allyl complex 112 is formed by insertion of the diene to Co—H, followed by domino insertions of the triple and double bonds to give 113. The final step is the elimination of the /J-alkoxidc group from 113 to form 114 [47], The six-membered ene-type products 117 and 118 are obtained from the reaction of 115 catalysed by an Fe bipyridyl complex. The reaction seems to involve oxidative cyclization to form 116. Subsequent -elimination and reductive elimination provide 117 and 118. As another possibility, insertion of the diene to Fe—H gives a 7i-allyl complex. Then double bond insertion and -elimination should give 117 and 118 [48],... 

Oxidative cross-coupling with alkenes is possible with Pd(OAc)2 [109], The reaction proceeds by the palladation of benzene to form phenylpalladium acetate (164), followed by alkene insertion and elimination of /1-hydrogen. Heteroaromatics such as furan and thiophene react more easily than benzene [109]. Stilbene (177) is formed by the reaction of benzene and styrene. The complex skeleton of paraberquamide 179 was obtained in 80% yield by the Pd(II)-promoted coupling of the indole ring with the double bond in 178, followed by reduction of the intermediate with NaBELt [110]. 

Transmetallation of silyl enol ethers of ketones and aldehydes with Pd(II) generates Pd(II) enolates, which are usefull intermediates. Pd(II) enolates undergo alkene insertion and -elimination. The silyl enol ether of 5-hexen-2-one (241) was converted to the Pd enolate 242 by transmetallation with Pd(OAc)2, and 3-methyl-2-cyclopentenone (243) was obtained by intramolecular insertion of the double bond and -elimination [148], Formally this reaction can be regarded as carbopalladation of alkene with carbanion. Preparation of the stemodin intermediate 246 by the reaction of the silyl enol ether 245, obtained from 244, is one of the many applications [149]. Transmetallation and alkene insertion of the silyl enol ether 249, obtained from cyclopentadiene monoxide (247) via 248, afforded 250, which was converted to the prostaglandin intermediate 251 by further alkene insertion. In this case syn elimination from 250 is not possible [150]. However, there is a report that the reaction proceeds by oxypalladation of alkene, rather than transmetallation of silyl enol ether with Pd(OAc)2 [151]. 

The reaction is catalysed by many transition-metal complexes, and a mechanism for the hydrosilylation of an alkene under transition-metal catalysis is depicted in Figure Si5.7. Initial coordination of the alkene to the metal is followed by cis addition of the silicon-hydrogen bond. A hydride migratory insertion and elimination of the product silane complete the cycle. 

P-Hydrogen elimination from amido complexes is a process that people assumed was rapid, but that had not been observed directly with monomeric amido complexes until recently. Fryzuk and Piers have studied the related insertion of imines into a dimeric, bridging hydride of Rh1 [69]. Their results showed that imine insertion was reversible when the imine was isoquinoline, suggesting that insertion and elimination processes are nearly thermoneutral. 

Among the many reactions of organometallic compounds, ones involving insertion and elimination of ligands are important in applications to synthesis and catalysis. An example of a carbonyl insertion is ... 

Many oxidative addition complexes are stable because four and higher coordination inhibits decomposition via de-insertion and elimination. For benzylhalides that are pendant to a polymer main chain, compounds that exhibit T 3-bonding to the benzyl group are possible. The small molecule analog is the T 3-benzylpalladium chloride dimer. This complex is neither electronically nor coordinately saturated and is therefore not inert to ligand addition. 

This is the second chapter of a two-part review concerned with insertion reactions of transition metal-carbon a-bonded compounds. The first chapter, which appeared in Volume 11 of this series (137), provided a broad introduction to the subject of insertion reactions in general and a detailed treatment of the carbon monoxide insertion and decarbonylation. Presented herein are the insertion and elimination reactions of sulfur dioxide and of a few other unsaturated molecules. The reactions of sulfur dioxide are accorded a complete literature coverage, whereas those of the other inserting species are treated selectively. Metal-carbon a-bonded compounds of the main group elements are discussed only in the context of comparisons with their transition metal analogs. 

The glyoxylative-decarbonylative coupling rationalizes as follows (Scheme 27). After the oxidative addition of indole-3-glyoxylyl chloride 38, adduct 39 undergoes a migratory de-insertion and elimination of carbon monoxide furnishing the acyl-Pd... 

Flash vacuum pyrolysis of o-alkoxybenzylidene chlorides gives benzofu-rans 230, which is consistent with intermediate carbene formation followed by intramolecular a-C—H insertion and elimination of hydrogen chloride (83TL609). 

Multiple insertion reactions [reaction (f), 11.5.1] give first an allophanate. Then, by a further insertion, and elimination of the original M—O bond, a simple or mixed isocyanurate is formed h... 

Carbenoid species such as CO and RNC also undergo insertions and eliminations (e.g., M-X + CO M-C(O)-X). These are 1,1-insertions, as opposed to the more common 1,2-insertions. Again, there is no change in total electron count or oxidation state of the metal, except transiently as CO coordinates to the metal before insertion. The insertion of CO into a M C bond is a key step in many important reactions. Again, the insertion is reversible. 

Equation 8.14 illustrates how the equilibrium between insertion and elimination can be shifted in favor of the metal alkyl by adding an electron-rich trialkyl phosphine ligand to a Ru(II) complex.18 The alkyl complex is positively charged, but electron-rich phosphines stabilize it. This means that loss of one of the phosphines to create an open coordination site necessary for (3-elimination is less likely than in a neutral complex. 

The stereochemistry associated with the Heck reaction is consistent with what we now expect with a process involving 1,2-insertion and -elimination—syn addition to the olefin and syn elimination to form a new alkene. When monosubstituted... 

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