Intermolecular oxidative addition

Intermolecular oxidative additions involving C-H bond breaking is a topic which has been extensively studied recently. It is usually referred to as C-H activation the idea is that the M-H and M-hydrocarbyl bonds formed will be much more prone to functionalization than the unreactive C-H bond [42-44], Intramolecular oxidative additions of C-H bonds have been known for quite some time [45] (see Fig. 4.27). This process is termed orthometallation. It occurs frequently in metal complexes, and is not restricted to ortho protons. It has considerable importance in metal-mediated synthesis. 

Given the state of the art around 1980, Bergman s and Graham s findings were indeed a landmark since they proved for the first time that transition metal compounds were capable of intermolecular oxidative addition to single C—H... 

The gas-phase reactions of the cationic Irm complexes follow a previously unreported mechanism for their observed a-bond metathesis reactions. Previous discussions had considered a two-step mechanism involving intermolecular oxidative addition of either [Cp Ir(PMe3)(CH3)]+ or [CpIr(PMe3)(CH3)]+ to the C-H bond of an alkane or arene producing an Irv intermediate, followed by reductive elimination of methane, or a concerted a-bond metathesis reaction sim-... 

The corresponding hydrido/alkyl (and aryl) complexes v-[RuHR(L-L), ] (L-L = dppe, dppm, dmpe R = Me, Et, Ph) are readily prepared from m-[RuClR(L-L)2] and Li[AlH4]1659 whereas treatment of cis- or tvans-[RuCl2 (dmpe)2 ] with arene radical anions affords d.v-[RuH(f 1-aryl)(dmpe)2] (aryl = phenyl, 2-naphthyl, anthryl, phenanthryl).1389 In solution, these compounds are in tautomeric equilibria with significant concentrations of Ru° complexes (e.g. equation 148) although X-ray analysis for aryl = 2-naphthyl confirms the presence of the six-coordinate Ru" species (373) in the solid state.2459 Some reactions of (373) with various substrates to produce other hydrido complexes are shown in Scheme 74.44>24m Note that the compound of empirical formula [ Ru(dmpe)2 ] obtained by pyrolysis of [RuH(2-np)(dmpe)2] (reaction (iv) Scheme 74) is a binuclear Ru" hydrido complex, resulting from intermolecular oxidative addition of methyl groups to ruthenium.1390... 

Intermolecular oxidative addition of H—C usually involves activated H—C bonds. The weak acid HCN reacts with transition-metal complexes e.g., HCN and NiL lead to the hydride complexes HNi(CN)Lj (L = various phosphorus ligands). The versatile complex IrCl(CO)(PPh3)j adds HCN cleanly in CH Clj at RT to form HIr(CN)(Cl(PPhj)2. The zero-valent complexes Pt(PPhj) or Pt(PPh3)3 also add HCN to yield HPt(CN)(PPh3)j. Reactions of HMNp(dmpe)j (M = Fe, Ru, Os Np = 2-naphthyl dmpe = Me PCH CH PMej) with HCN and terminal acetylenes give HMR(dmpe)2 that contain new M—C bonds (R = — CN, — CjR ) . 

The reaction leading to the new Pt(II) complex in the case X = F, H, proceeds probably through a sequence of oxidative addition and subsequent reductive elimination. The same sequence gives C-C bond formation in intermolecular oxidative addition reactions (TOA) with Pd(II) complexes [40]. The corresponding POA leads to complete dissociation of the C-bonded ligand (see below). 

Direct formation of isolable cr-aryl complexes by intermolecular oxidative addition of an aromatic C—H bond to a transition-metal complex was first observed in an alkyl phosphine complex of Ru and has now been extended to most of the transition metals. The relevant literature is summarized in Table 1. There are reviews of C—H activation by homogeneous metal complexes and its relevance to homogeneous and heterogeneous catalysis s " . 

Other multistep reactions may be initiated by intermolecular oxidative additions. For example, Ir complexes react with cyclopentane at 80°C to give the cyclopentadienyl complex in 40% yield . 

Ru(dmpe)2 ] obtained by pyrolysis of [RuH(2-np)(dmpe)2] (reaction (iv) Scheme 74) is a binuclear Ru" hydrido complex, resulting from intermolecular oxidative addition of methyl groups to ruthenium. 

The thermodynamics of intermolecular oxidative addition of C-C bonds to an unsaturated metal center is usually considered to be less favorable than C-H activation.Take, for example, reactions (85) and (86), for which the entropy changes should be approximately identical ... 

For many years, only intramolecular C-H additions were observed because this type of reaction is favored kinetically and thermodynamically. Intermolecular additions of arenes were later observed, and arenes are more reactive than alkanes toward oxidative addition to all single-metal centers. In 1982, the isolation of an alkyl hydride complex from the oxidative addition of an alkane was first reported. - Since that time, many complexes have been reported that undergo oxidative additions of alkanes. Many of these complexes do not provide stable alkyl-hydride products, but these complexes can be induced in some cases to undergo productive transformations. The following sections describe the development of intramolecular and intermolecular oxidative addition of the C-H bonds of al%l groups, aryl groups, alkanes, and arenes. 

Bergman et al. [12] reported one of the first studies of C—H bond activation with a transition metal system capable of intermolecular oxidative addition. The reaction involved the photolysis of [Ir(ri-Cp )(PMe3)(H)J in different hydrocarbon solvents. Figure 25.5 shows that the reaction likely proceeds via loss to form a very reactive IF 16 electron metal intermediate. The C—H activation proceeds via a 3-centered transition state leads to an Ir hydrido-alkyl complex in a high yield at room temperature. The process is well described as an oxidative addition of the alkane. 

---