Transition metals oxidative-addition reactions

Han, L.-B. and Tanaka, M. Transition metal-catalyzed addition reactions of H-heteroatom and inter-heteroatom bonds to carbon-carbon unsaturated linkages via oxidative additions, Chem. Commun., 395, 1999. 

Transesterification of methyl methacrylate with the appropriate alcohol is often the preferred method of preparing higher alkyl and functional methacrylates. The reaction is driven to completion by the use of excess methyl methacrylate and by removal of the methyl methacrylate—methanol a2eotrope. A variety of catalysts have been used, including acids and bases and transition-metal compounds such as dialkjitin oxides (57), titanium(IV) alkoxides (58), and zirconium acetoacetate (59). The use of the transition-metal catalysts allows reaction under nearly neutral conditions and is therefore more tolerant of sensitive functionality in the ester alcohol moiety. In addition, transition-metal catalysts often exhibit higher selectivities than acidic catalysts, particularly with respect to by-product ether formation. 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

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

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

The abundance of accessible donor and acceptor orbitals in common transition-metal complexes facilitates low-energy bond rearrangements such as insertion ( oxidative-addition ) reactions, thus enabling the critically important catalytic potential of metals. 

The reactions of MeOH with some transition metal oxide cluster anions [M O J, where M = Mn, Fe, Co, Ni, Cu n = 1,2 x = 2—4, have been studied (254). The [M03] anions were unreactive toward MeOH, unlike [Nb03]. The addition of the hydrogen molecule to the other cluster anions was the common reaction yielding the following transformations,... 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

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. 

The metals which form this type of complex by reaction with dioxygen are mainly the later second and third row transition series metals, notably Ru°, Os°, Rh1, Ir1, Pt° and Pd°. It is not surprising to see metals noted for their ability to undergo oxidative addition reactions amongst those which form this type of mononuclear dioxygen complex as a formal two-electron reduction of dioxygen is required for complex formation. The other metals known to form mononuclear peroxo-type complexes with dioxygen are Ni°, Co1 and one example of Co . 

Oxidative cyclization is another type of oxidative addition without bond cleavage. Two molecules of ethylene undergo transition metal-catalysed addition. The intermolecular reaction is initiated by 7i-complexation of the two double bonds, followed by cyclization to form the metallacyclopentane 12. This is called oxidative cyclization. The oxidative cyclization of the a,co-diene 13 affords the metallacyclopentane 14, which undergoes further transformations. Similarly, the oxidative cyclization of the a,co-enyne 15 affords the metallacyclopentene 16. Formation of the five-membered ring 18 occurs stepwise (12, 14 and 16 likewise) and can be understood by the formation of the metallacyclopropene or metallacyclopropane 17. Then the insertion of alkyne or alkene to the three-membered ring 17 produces the metallacyclopentadiene or metallacyclopentane 18. 

The reaction of CF3I with low valent metals is remarkable for the very mild conditions that are required. Typically, trifluoromethyl iodide and the transition metal species are maintained at ambient temperatures for periods of time which range from a few minutes to a few days. When employed at all, solvents are noninteractive and, at most, only gentle heating is required. The utility of oxidative addition reactions with trifluoromethyl iodide has been demonstrated in numerous systems and representative examples are presented in Table II. 

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. 

---