Reductive Elimination and Oxidative Addition

Oxidative additions proceed by a great variety of mechanisms, but the fact that the electron count increases by two units in Eq. 6.1 means that a vacant 2e site is always required on the metal. We can either start with a I6e complex or a 

The Organomeialtic Chemistry of the Transition Metals, Fourth Edition, by Robert H. Crabtree Copyright 2(X)5 John Wiley Sons. Inc. 

2e site must be opened up in an 18e complex by the loss of a ligand. The change in oxidation state means that a metal complex of a given oxidation state must also have a stable oxidation state two units higher to undergo oxidative addition (and vice versa for reductive elimination). 

Equation 6.2 shows binuclear oxidative addition, in which each of two metals changes its oxidation state, electron count, and coordination number by one unit instead of two. This typically occurs in the case of a 17e complex or a binuclear 18e complex with an M—M bond where the metal has a. stable oxidation state more positive by one unit. Table 6.1 systematizes the more common types of oxidative addition reactions by d configuration and position in the periodic table. Whatever the mechanism, there is a net transfer of a pair of electnms from the metal into the t orbital of the A—B bond, and of the A—B a electrons to the 

TABLE 6.1 Conunon Types of Oxidative Addition Reaction  

Solution, (a) Ti(IV) is d° and the compound has 16 valence electrons. It cannot undergo OA because there are no oxidizable electrons on the d° metal, (b) Pt(ll) is d and the complex has 16 valence electrons and only 4 ligands. This species can undergo OA. (c) Ni(0) is so it has oxidizable electrons and plenty of space, but it is already an 18-electron compound, so it will be unable to participate in an OA reaction, (d) Th(IV) is d° and this is a 16-electron compound. It cannot undergo OA because there are no oxidizable d-electrons and the molecule is 

Example 19-2. Which of the following will be more reactive toward oxidative addition of H2 [Rh(PPh3)3CI] or [Rh(PPh3)2(CO)CI] Explain your answer. 

Solution. Both species are d LS square planar compounds with a fairly electrophilic metal in a low oxidation state. The former molecule, however, has more electron-donating groups, which will build up electron density on the Rh(l) and make it easier for the molecule to undergo oxidative addition. 

The polar S 2 OA reaction mechanism occurs with second-order reaction kinetics and large, negative entropies of activation (—40 to —50J/K mol), suggestive of an 

General types of oxidative addition reaction mechanisms. 

Moving on from discussing ligand types, we now return to reactivity questions by looking at two reactions that play a key role in most catalytic cycles as well as in many synthetic pathways. 

Oxidative additions go by a variety of mechanisms, but since the metal electron count increases by two, a vacant 2e site is always needed. We may start with a 16e complex, or a 2e site may be opened up by initial ligand loss from an 18e complex. The change in oxidation state means that to undergo Eq. 6.1, a complex must have a stable OS two units more positive (and vice versa for RE). 

First row metals typically prefer a one-unit change in oxidation state, electron count, and coordination number. Equation 6.2 shows how binuclear oxidative addition conforms to this pattern. We start with a 17e complex or an M-M bonded 18e complex that can dissociate into 17e fragments. The metal must now have a stable OS more positive by one unit for OA. Table 6.1 shows common types of OAs by d configuration and position in the periodic table. Whatever the mechanism, two electrons from M transfer into the A-B ct, while the A-B a bonding pair donate to M. This cleaves the A-B 

Note Common reductive eliminations follow the reverse paths. 

The mechanisms of oxidative addition reactions involving organic halides form the subjects of recent reviews.  

—The reaction of [Fe(CO)6] and HgCla involves two consecutive processes (1) and (2) which follow rate laws (3) and (4) respectively. A mechanism for the first 

Studies on oxidative-addition reactions of alkyl halides to square-planar iridium(i) complexes and other low-valent metal centres have shown that the reactions may either be regarded as Ss2 processes in which the metal centre acts as a nucleophile or else involve a concerted, three-centre addition. However, it has now been found that the oxidative-addition reaction of many alkyl halides to /w j-[IrCl(CO)(PMe3)2] can also proceed via a free-radical pathway. The studies show that the rates of reaction are greatly enhanced if small quantities of oxygen or a radical initiator, e.g. benzoyl peroxide, are present and that reaction rates are retarded by traces of radical scavengers, e.g. duroquinone or hydroquinone. Studies with the halides (1) show that the reaction proceeds with loss of stereochemistry at carbon. It is also found that the reaction rate 

Studies on the cobalt(i), rhodium(i), and iridium(i) complexes (2) have revealed the relative reactivity of metal complexes in this triad. The results 

— The reaction of pentacarbonyliron with iodine is known to yield cis-[Fel2(CO)J. A study of the kinetics of the thermal reaction has been reported using stopped-flow techniques. The observed overall rate laws are interpreted 

Rhodium.—Sodium cyanide reacts with [RhCl(CO)a]2 in methanol to form [RhH(CN)s] . A transient intermediate, [Rh(CN)J , has now been detected in this reaction. This intermediate decays by oxidatively adding HCN to form [RhH(CN)5] . The activation parameters for this oxidative-addition reaction are A/f = 1.8 kcal mol and = —42 cal deg mol. This entropy of activation is characteristic of other oxidative-adcUtion reactions and is consistent with a considerable loss of freedom in the transition state, as would be expected for the addition of one molecule to another. For the oxidative addition of methyl iodide to [Rh(CN)4] , = 8.4 kcal mol and 

LS = -18 cal deg mol. In comparison with other oxidative-addition reactions of methyl iodide to d complexes this is a relatively small negative value for AS, and the authors claim that steric effects contribute significantly to values of AS in these systems. Thus since [Rh(CN)J will be smaller than a complex with bulky phosphines, transition states involving [Rh(CN)J will have a greater effect on the entropy of the solution. Some data have also been obtained on the reaction of methyl iodide with [Rh(CN)3(CO)] and [Rh(CN)2(CO)a] . Some acyl product [Rh(COMeXCN)s] can result from these reactions, possibly by a Ry-catalysed interconversion of a Rh species.  

Since transition metal alkyls and hydrides are quintessential organometallic species that undergo various elementary processes in catalytic reactions, information on appropriate methods for their generation is quite important. Oxidative 

Oxidative addition of organic compounds having carhon-X bond (X = heteroatom) to a low valent transition metal complex such as Pd(0) or Rh(I) with cleavage of the C-X bond often yields reactive organotransition metal complexes. Cleavage of non-polar bonds will be discussed in Chapter 2, while cleavage of polar bonds will be dealt with in Chapter 3. 

The oxidative addition of an aryl iodide to a zerovalent complex such as [Pd(PPh3)4] gives tra s-[Pd(Ar)I(PPh3)2] having a palladium-aryl and a palladium-iodide bond indeed, this is one of the oldest examples of oxidative addition of aryl iodide to Pd(0) complex (Eq. 1.1) [10]. 

Oxidahve addihon of methyl iodide to a Rh(I) complex coordinated with iodide and carbonyl ligands affords methylrhodium(III) type complex where the methyl and the iodide hgands are situated in mutually trans positions (Eq. 1.2). 

The former type of oxidahve addition can be coupled with other subsequent processes such as transmetallation to give diorganotransihon metal complexes. Since reduchve eliminahon often follows the transmetallation to liberate a product where the two organic moieties are coupled, a quite useful process catalyzed by 

Metal-Carbon Bond Formation and Cleavage, Including Oxidative Addition and Reductive Elimination 

Metal-to-carbon r-bonds may be formed and cleaved by various reactions, of which oxidative addition and reductive elimination are only one aspect. Oxidative addition involves various mechanisms and extends beyond organometallic chemistry so that reactions not involving M—C bonds will also be included here. 

The reductive elimination of Ha has been studied mechanistically in a few cases. The protonation of [CoHfPfORjgjg] gives [CoH2 P(OR)s 4]+, which exchanges Hg with Da without formation of HD, so that an oxidative addition/ reductive elimination pathway is proposed. The reaction of the dihydride cation with phosphite is also believed to occur via a preliminary reductive elimination 

H—H bond is being formed. This avoids the need to give [ZrH4(C5H6)a] by oxidative addition. The related hydrogenolysis of Zr—Me of [ZrHMe(C6H5)a] has been treated similarly. 

Reductive elimination of H2 as shown below occurs by two parallel pathways, one first-order and the other second-order in complex concentration. The first- 

Oxidative addition is a reaction where the metal undergoes formal oxidation and atoms, groups of atoms, or molecules are added to the metal center. Reductive elimination is the exact opposite of oxidative addition—the metal ion is formally reduced with elimination of ligands. A few examples are shown in Fig. 2.6. In all these examples the forward reactions are oxidative addition 

All the forward reactions are important steps in commercial homogeneous catalytic processes. Reaction 2.2 is a step in methanol carbonylation (see Chapter 4), while reaction 2.3 is a step in the hydrogenation of an alkene with an acetamido functional group. This reaction, as we will see in Chapter 9, is 

Formation of metal alkyl by addition of RX Kinetic instability of metal alkyl, /1-hydride elimination 

As already mentioned, the reverse reactions of Fig. 2.6 are reductive elimination reactions. By the principle of microscopic reversibility, the existence of an oxidative addition reaction means that reductive elimination, if it were to take place, would follow the reverse pathway. The reductive elimination of an alkane from a metal-bonded alkyl and hydride ligand in most cases poses a mechanistic problem. This is because clean oxidative addition of an alkane onto a metal center to give a hydrido metal alkyl, such as a reaction like Reaction 2.5, is rare. 

The mechanism of reductive elimination of a hydrido alkyl complex is therefore often approached in an indirect manner. The hydrido-alkyl complex is made not by oxidative addition of the alkane but by some other route. The decomposition of the hydrido-alkyl complex to give alkane is then studied for mechanistic information. Reductive eliminations of an aldehyde from an acyl-hydrido complex, Reaction 2.7, and acetyl iodide from an iodo-acyl complex, 

Note that in (2.3.1.2) the organic molecule acts as a bidentate chelating ligand. The double bond of the alkene and the oxygen lone pair of the acetamido group are used for this purpose. Reaction 2.3.1.3 is OA of HCN to NiLj. It is the first step in the Ni-catalyzed hydrocyanation of butadiene. 

A few points about OA and RE are to be noted. First, in the products of OA reactions, the atoms or groups of atoms that are added to the metal center must be cis to each other. It is for this reason the two hydrides in reaction 2.3.1.2, are cis to each other. Similarly, the hydride and the cyano group in reaction 2.3.1.3 are cis to each other. A corollary of this is that the atoms or groups of atoms that take part in RE must also be cis to each other. 

Second, there are other molecules, such as (X =halogens), O, RCOX, and ArX, that can also add oxidatively. Some of these reactions are of direct relevance to homogeneous catalysis and will be discussed later. Third, to be able to undergo OA, the metal complex must be electronically unsaturated, i.e., its electron count should be less than 18. All the complexes that undergo OAs in reactions 2.3.1.1- 2.3.1.3 have electron counts of 16. 

The reviewer sympathizes with Crabtree and Alatky/ who have highlighted problems in distinguishing between types of additions, e.g., whether they are oxidative, ligand, or even reductive. They suggest that the addition of a ligand to a metal should be described numerically, e.g., 3,2, in terms of the number of atoms involved and the number of electrons donated by the ligand to the metal, viz. 3 and 2, respectively, here. Change in oxidation state of the metal, if any, is not included. 

we will look at reactions in which the oxidation state of the metal in the overall process rises by only 1. Three closely related systems are noteworthy, two involving cobalt and one chromium. 

This gives an overall rate equation, - d[Co( )]ldt = 2 i[Co(II)] [RX]. Bi2r reacts more slowly than some but not all of its models (see Table 8.6). The reactions of iodides in aqueous solution are second order in B r and two possible mechanisms are proposed. 

Goh and Goh have shown that the rate of reaction of [Co(CN)5] with various bridgehead iodides is first order in each of these two reactants in the presence of acrylonitrile (as is the same reaction with simple organic ions in the 

Moving to processes in which the oxidation state changes by 2, one can find examples of all four mechanisms which have been cited for oxidative addition and reductive elimination, namely, 5n2, radical, radical chain, and concerted. 

Molybdenum.— The reaction of Gel4 with [Mo(CO)4(o-phen)] follows the rate law -d[Mo(CO)4(o-phen)]/dr = A 2[Mo(CO)4( phen)][GeI4] 

Adduct formation is considered to be the rate-determining step, the overall mechanism being  

The interest in individual steps, particularly in the formation of the radical anion type of species which were featured in Volume I, e.g., Ni(0) + 

RX - [Ni(I), RX ], is superseded by more examples of the four types of processes concerted, radical, radical-chain, and two-step ionic. Under the mantle of reductive elimination, perhaps not quite correctly, /3 elimination of hydrogen is included since this process seems able to take place under very similar conditions to those under which a concerted reaction occurs. It is interesting to see the use of Hoffmann-type calculations in rationalizing elimination reaction pathways. Mention should be made also of the mechanism of processes involving two metal centers. 

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Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

Let us consider the general trends of the reactivity of C-C, C-S, and C-Q (Q = Cl, Br, I) bonds towards oxidative addition and reductive elimination (Scheme 7-25). In many cases, either C-C bond-forming reductive elimination from a metal center (a) or the oxidative addition of a C-Q bond to a low-valent metal center is a thermodynamically favorable process (c). On the other hand, the thermodynamics of the C-S bond oxidative addition and reductive elimination (b) lies in between these two cases. In other words, one could more easily control the reaction course by the modulation of metal, ligand, and reactant Further progress for better understanding of S-X bond activation will be achieved by thorough stoichiometric investigations and computational studies. 

Finally the ability of metals to undergo oxidative addition and reductive elimination during heterocyclic synthesis is highlighted. This process is of... 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

It is anticipated that many of the catalytic Cp2Zr(II) reactions that might have been considered to proceed via oxidative addition and reductive elimination, such as hydrosila-tion [224] and hydrogenation [225], may actually proceed via a couple of o-bond metatheses, i. e. transmetallation and p-H abstraction, as exemplified by the two contrasting mechanisms for the hydrosilation of alkenes (Scheme 1.70). 

Fig. 23. Volume profile for the combined oxidative addition and reductive elimination reaction [PdMe2(bpy)] + Mel — [Pd(I)Me3(bpy)] — [Pd(I)Me(bpy)J + C2H6.

The reductive elimination/oxidative addition is of practical importance in catalytic cycles, for example the rhodium/methyl iodide catalysed carbonylation of methanol. In organic synthesis the palladium or nickel catalysed cross-coupling presents a very common example involving oxidative addition and reductive elimination. A simplified scheme is shown in Figure 2.19 [17],... 

These authors propose as the mechanism for this reaction a reversible oxidative addition of the aryl-phosphido fragments to a low valent rhodium species. A facile aryl exchange has been described for complexes Pd(PPh3)2(C6H4CH3)I. The authors [35] suggest a pathway involving oxidative additions and reductive eliminations. The mechanism outlined below, however, can also explain the results of these two studies. 

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

While the reductive elimination is a major pathway for the deactivation of catalytically active NHC complexes [127, 128], it can also be utilized for selective transformations. Cavell et al. [135] described an interesting combination of oxidative addition and reductive elimination for the preparation of C2-alkylated imida-zohum salts. The in situ generated nickel catalyst [Ni(PPh3)2] oxidatively added the C2-H bond of an imidazolium salt to form a Ni hydrido complex. This complex reacts under alkene insertion into the Ni-H bond followed by reductive elimination of the 2-alkylimidazolium salt 39 (Fig. 14). Treatment of N-alkenyl functionalized azolium salts with [NiL2] (L = carbene or phosphine) resulted in the formation of five- and six-membered ring-fused azolium (type 40) and thiazolium salts [136, 137]. 

There are several pathways by which one ligand may replace another in a square planar complex, including nucleophilic substitution, electrophilic substitution, and oxidative addition followed by reductive elimination. The first two of these are probably familiar from courses in organic chemistry. Oxidative addition and reductive elimination reactions will be covered in detail in Chapter 15. All three of these classes have been effectively illustrated by Cross for reactions of PtMeCItPMe-Ph),.-... 

Mercury(II) halides electrophilically attack Pt(CF3C CCF3)(PMePh2)2 at the alkynic carbon (equation 278).8SS Site migrations between PtCl(G=CR)(CO)L and Hg(C=CR )2 involve oxidative addition and reductive elimination sequences.856... 

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

Vaska s complex trans-IrCl(CO)(PPh ) has served as an important model for mechanistic investigation of catalytically relevant reactions such as the oxidative addition and reductive elimination of small molecules(15). The latter processes have also been the subject of some photochemical investigation. For example, the reductive elimination of H2 depicted in Equation 5, which is a relatively slow thermally activated process (k = 3.8 x 10- s l in 25° benzene solution (15)), has been shown to occur readily when the dihydride complex was subjected to continuous photolysis with 366 nm light(16). However, Vaska s compound itself was reported to be... 

Presumably, the formation of (CF3)2Pd(P(OMe)3)2 from Pd(P(OMe)3)4 (52) and the synthesis of (CF3)2Pt(COD) from (CH3)2Pt(COD) (54) are the results of a series of successive oxidative addition and reductive elimination steps. In these instances the triphenylphosphite and the cyclooctadiene ligands appear to be insufficiently basic to allow the postulated tetravalent metal species to exist as much more than reactive intermediates. 

Table VI lists various ways in which the elimination of small molecules has been used to produce silicon-transition-metal bonds most can be pictured as proceeding via consecutive processes of oxidative addition and reductive elimination. Dihydrogen may result from reaction between compounds with M-H and Si-H bonds (entries 1-10).

Organometallic compounds are used widely as homogeneous catalysts in the chemical industry. For example, if the alkene insertion reaction continues with further alkene inserting into the M C bond, it can form the basis for catalytic alkene polymerisation. Other catalytic cycles may include oxidative addition and reductive elimination steps. Figure above shows the steps involved in the Monsanto acetic acid process, which performs the conversion... 

Figure 2.6 Representative examples of oxidative addition and reductive elimination reactions.

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