Oxidative addition defined

The Pd(0)-catalyzed addition of trimethylsilyl iodide to an alkyne, followed by capture with alkynylstannane, affords the stereo-defined enyne 186. The reaction is explained by the oxidative addition of iodosilane, the insertion of an alkyne to generate the vinylpalladium 185, and the capture of 185 with the alkynylstannane 184[102]. 

The use of well-defined complexes has been widespread in this reaction, despite intriguing studies by Beller and others that have shown that in situ catalytic systems often give better yields in comparison to isolated carbene-Pd(O) complexes [147-149]. Since the mechanism consists of an oxidative addition on a Pd(0)-monocarbene species, efforts in catalyst synthesis have been directed towards Pd(ll)-monocarbene complexes with other labile groups that can be easily released leading to the formation of Pd(0). This is the case for dimers of the type [Pd( j,-C1)C1(NHC)]2, a family of pre-catalysts effective under aerobic conditions [150], the [Pd(acac)Cl(NHC)] complexes [151] and related palladacycles [152-154],... 

Oxidation is defined as the reduction of electron state by addition of oxygen or removal of electrons. Thermodynamic balance requires balance. For every oxidation, there must be a corresponding reduction. Thus, for oxidation to occur, there must be a compound capable of receiving the transferred electrons. Electron acceptor compounds can include oxygen, sulfate, Fe3+, phosphate, nitrate, C02, and certain organics. 

These findings have stimulated enormously the search for intermolecular activation of C-H bonds, in particular those of unsubstituted arenes and alkanes. In 1982 Bergman [2] and Graham [3] reported on the reaction of well-defined complexes with alkanes and arenes in a controlled manner. It was realised that the oxidative addition of alkanes to electron-rich metal complexes could be thermodynamically forbidden as the loss of a ligand and rupture of the C-H bond might be as much as 480 kl.mol, and the gain in M-H and M-C... 

During the chemisorptions of Ru3(CO)i2 or Os3(CO)i2 on silica, the first step with the surface silanols was to produce a covalent bonding with the silica surface by oxidative addition of the silanol group to the metal-metal bond of the clusters. The nature of surface molecular species [=Si-0)(M3( x-H)(CO)io)j covalently linked to the silica surface (M = Ru, Os) was clearly defined and structurally characterized by a series of physical and chemical techniques, including mass balance taking into account the evolution of two molecules of CO and one molecule of hydrogen [27, 33, 35]. 

Transition metal-catalyzed allylic alkylation is generally considered to involve mechanistically four fundamental steps as shown in Scheme 1 coordination, oxidative addition, ligand exchange, and reductive elimination. A key step of the catalytic cycle is an initial formation of a (7r-allyl)metal complex and its reactivity. The soft carbon-centered nucleophiles, defined as those derived from conjugate acids whose pAj, < 25, usually attack the allyl ligand from the opposite side... 

The short-lived [MH2(Cp)2] and [TaH4(dmpe)2] have been obtained from the Mv hydrides using photogenerated r-butoxy radicals, and were characterized by low temperature ESR.575 On the other hand, thermally stable, well-defined dinuclear or mononuclear MIV hydrides have been prepared by oxidative addition of H2 to dinuclear Mm or mononuclear Mn halide phosphine adducts, respectively. They constitute attractive entries to lower oxidation state compounds, and will be reviewed in Sections 34.4.3.l.i and 34.6.1.2.i. 

Extensive studies by Amatore, Jutand, and co-workers have shed light on the structure and oxidative addition chemistry of a number of synthetically important palladium complexes [42], In particular, these workers have shown that the major species in a solution of Pd(dba)2 and BINAP is Pd(dba)BINAP and that oxidative addition of Phi to this complex generates (Bl-NAP)Pd(Ph)I [42d,43], In addition, it has been demonstrated that palladium halide complexes such as (PhjP jaryljPdCl do not dissociate the halide ligand in DMF solution [44], whereas the corresponding triflate complex is completely dissociated [44,45], As noted earlier, the nature of the oxidative addition intermediates defines two mechanistic pathways for the Heck reaction the neutral pathway for unsaturated halide substrates and the cationic pathway for unsaturated triflate substrates [2c-g,3,7-9]. Further, it is possible for halide substrates to be diverted to the cationic pathway by addition of Ag(I) orTh(I) salts [3], and it is possible to divert some triflate substrates to the neutral pathway by addition of halide additives [38]. Individual steps of these two pathways have recently received some scrutiny. 

Those interactions mentioned above lead to jr-complexation and oxidative addition, representing two of the several most widely employed routes to organopalladium derivatives along with transmetallation with Pd, hy-dropalladation, and heteropalladation defined as addition of Pd - X bonds to 7r-bonds, where X is any element other than C or H [2],... 

Note that while oxidative addition reactions (Section 21-2) could be defined as insertions of metals into R—X bonds, the reactions under discussion here only deal with insertions into M—X bonds, without changes in the formal metal oxidation state. 

These reactions are commonly interpreted to be composed of three main steps, namely a) oxidative addition of an aryl-X species to palladium(0) with formation of an arylpalladiumffi) bond b) insertion of a terminal olefin and c) reductive elimination regenerating palladium(0). To achieve a catalytic cycle, the rates of these steps have to match each other. The basic process was discovered by Heck in 1968. The mechanism has not yet been well defined and several variants have been proposed. A widely accepted scheme is reported in Figure 6. 

Catalytic reactions can be analyzed into a cyclic series of stoichiometric steps, for each of which there are many well-understood model systems. The most frequently encountered steps are ligand substitution oxidative addition ligand migration (or migratory insertion) nucleophilic attack reductive elimination and p-and a-elimination. Catalytic cycles are defined by a sequence of several such reactions at the metal centre the organometallic steps are often preceeded or followed by purely organic reactions. 

The Monsanto carbonylation of methanol to acetic acid catalyzed by Rh/H is a well-understood example of an organometallic catalytic cycle and can act as a good model with well defined steps (shown schematically in Chapter 4, Section 4.2.4). The starting material is the square planar Rh(I) complex, [Rh(CO)2l2] which is easily accessible by reaction of rhodium trichloride in solution with CO in the presence of iodide. This undergoes oxidative addition with Mel very readily to give the methyl-Rh(III) complex [Rh(Me)(CO)2l3] as an unstable... 

While good procedures are available for the initial oxidative addition to give allyl-PdX complexes, and the nucleophile addition has been folly developed, this process has not been amenable to catalysis. The Pd is reduced to Pd° during the basic pathway, and no compatible reoxidation system has been defined. 

When molecular chlorine is introduced into an alcoholic solution of [Tc(diars)2Cl2]C104 at room temperature, the solution turns rapidly deep red and a dark brown solid is subsequently deposited. This reaction is defined as an oxidative addition process in which the six-coordinate, d, Tc(III) is converted into eight-coordinate, d, Tc(V) (equation 84). The product yields, by metathesis with NaPFg, the dark brown [Tc(diars)2ClJPF . 

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