Oxidative-addition reactions of transition metal complexes

Oxidative-addition reactions of transition metal complexes. J. Halpern, Acc. Chem. Res., 1970, 3, 386-392 (66). 

Oxidative-addition reactions of transition metal complexes with the C-H bonds of saturated or unsaturated organic groups, whereby organometal... 

A characteristic and advantageous feature of the oxidative addition reactions of transition metal complexes compared with those of nontransition metal compounds is their reversibility. Grignard reactions cannot be made reversible, but the reverse reaction of oxidative addition is possible with transition metals in many cases. This reverse reaction is called reductive elimination. The ability of the transition metals to donate as well as to accept elec-... 

Halpern, J. "Oxidative-Addition Reactions of Transition Metal Complexes." Acc. Chem. Res., 3,386 (1968). 

This chapter deals with references concerning oxidative addition reactions of transition-metal complexes, the closely related reaction of oxidative elimination, and the converse reaction, which is reductive elimination. Oxidative addition is simply what the name implies, e.g. 

A suitable example is the oxidative-addition reaction of transition metals and their complexes... 

Table 2-2 Oxidative addition reactions on transition metal complexes classification of the adding compounds...

The possibility of the bonding of the hydrogen molecule to transition metals was considered many years ago by E. Singleton in connection with the nature of RuH4(PPh3)2, which readily lost H2 in solution. The compound FeH4(PR3)2 acted similarly and showed an unusual ir band at ca. 2400 cm-1. In addition the oxidative-addition reactions of many d8 complexes, reactions such as... 

An example of H2 oxidative addition to early transition metal complexes is the reaction with WX2(PMe3)4 to give W(H)2X2(PMe3)4. The product is stable to reductive elimination if X = Cl but H2 addition is reversible if X = I.29... 

A second important selectivity issue arises when there are several different types of C—H bond in the molecule, typically, primary, secondary and tertiary C—H bonds. Since tertiary radicals and caibonium ions are more stable than their secondary or primary analogs, many functionalization processes have an intrinsic selectivity pattern tertiary > secondary > primary. Steric effects favor attack at primary positions, which is seen for very bulky reagents or in reactions in which the C—bond to be broken is brought side-on to the functionalizing group, and therefore makes the transition state very sensitive to steric effects. The best example is oxidative addition to a transition metal complex. 

Oxidative-addition and reductive elimination reactions of transition metal complexes are crucial to many homogeneously catalyzed reactions and are important for bond formation. Reactant molecules such as H2 and Oj undergo oxidative addition to many transition metal centers. Oxidative addition of carbon-hydrogen bonds has been an active area of research. Reductive elimination is the process whereby products are eliminated from transition metal centers. 

During the past decade, considerable progress has been made in the area of transition metal-catalyzed cleavage and functionalization of the inert C-Cl bond in nonactivated chloroaromatic compounds. This new and important field of chemistry is reviewed in the present chapter, which describes both mechanistic and synthetic aspects of C-Cl activation. Oxidative addition reactions of chloroarenes to complexes of catalytic metals are discussed, along with their applications in a wide variety of reductive dechlorination, nucleophilic displacement, olefin arylation, coupling, and carbonylation reactions. 

Additional examples of the reactions of transition metal complexes with PhICl2 include the oxidation of heterobimetaUic Pt(II)-Au(I) complexes to Pt(III)-Au(II) complexes [75] and the chlorinations or oxidations of palladium [76-78], cobalt [79], vanadium [80], iridium [81] and gold [82] complexes. 

The stoichiometric reactions of transition-metal complexes Chapter 3 - Redox reactions, oxidative addition and a-bond metathesis.81... 

Although in this chapter we shall be restricting coverage to reactions of transition-metal complexes, the phenomenon of oxidative addition is not confined to this type of compound. Such reactions are also well established for non-transition metals—a recently reported example concerns the oxidative addition of methyl bromide to indium(i) bromide to give InBr2Me— and for non-metals, as in the reaction of phosphorus trichloride with chlorine, to cite a very familiar example. Likewise, reductive eliminations are known and studied outside the area of transition-metal complexes. One example has been mentioned in Chapter 1 of Part II of this volume, namely the elimination of alkyl halides from the thallium(iii) compounds TlRXa. ... 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

There are few reports of oxidative addition to zerovalent transition metals under mild conditions three reports involving group 10 elements have appeared. Fischer and Burger reported the preparation of aTT -allylpalladium complex by the reaction of palladium sponge with allyl bromide(63). The Grignard-type addition of allyl halides to aldehydes has been carried out by reacting allylic halides with cobalt or nickel metal prepared by reduction of cobalt or nickel halides with manganese/iron alloy-thiourea(64). 

The oxidative addition at a transition metal is utilized in the synthesis of these complexes. The opposite reaction, i.e. reductive elimination, is a general route to the cleavage of the metal-metal bond, especially in complexes also containing a hydrogen-transition metal bond. 

The major route to -cyclopropenylium complexes L M(C3R3) (metallatetrahedranes) is by oxidative addition reactions of cyclopropenylium salts to transition metal complexes of groups 5 (V), 6 (Mo, W), 8 (Fe, Ru), 9 (Co, Rh, Ir) and 10 (Ni, Pd, Pt). The addition is frequently accompanied by loss of one or more carbonyl, olefin or halogen auxiliary ligand. Concurrent formation of oxocyclobutenyl complexes by carbonyl insertion into the cyclopropenyl ring is often observed in reactions with group 9 cobalt triad and early transition metal complexes. 

As indicated in Table II, the complexes examined thus far have all contained coordinatively unsaturated dH or d10 metal ions, as has most commonly been the case in oxidative addition reactions of alkyl halides with transition metal substrates. Almost all of the products of these reactions are immediately recognizable as having arisen from an oxidative addition reaction, but in some instances the species isolated, e.g., CF3Au(P3) (59), (CF3)2Pt(COD) (54), and CF3Pt(PEt3)2I (55), were found to be in the same oxidation state as the reagents that had been originally employed. 

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