Oxidative addition to metal complexes

Formation of Bonds between Hydrogen and Transition Metals 1.10.5. by Oxidative Addition to Metal Complexes... 

All the preceding reactions have involved oxidative additions to metal complexes. Similar reactions are also known for metal complexes. The Pd(0) complex, (dpm)3Pd2, whose detailed structure is unknown, also undergoes oxidative addition according to reaction 18 ... 

The high kinetic barrier associated with their oxidative addition to metal complexes and the fact that S-hydride elimination processes are often... 

Numerous examples of the application of PHIP to study catalytic processes that involve H2 activation in solution by NMR have been published and reviewed [12-14, 24-31]. Several directions in this field of research can be roughly classified as follows (i) studies of reaction products (ii) studies of reaction intermediates including those previously unknown and/or not detectable by conventional NMR and (iii) studies of pHj oxidative addition to metal complexes... 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

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). 

Perhaps the most important metal metal bonded reaction from the point of view of catalysis is addition of hydrogen. In spite of years of study, the exact mechanism of hydrogenation of even dicobalt octacarbonyl is not ftdly understood. Oxidative addition see Oxidative Addition) of hydrogen to metal-metal bonds is generally slower than oxidative addition to mononuclear complexes that have vacant sites. Two principal mechanisms have been proposed in this chemistry. The first relies on cleavage of the metal-metal bond to generate radicals (equation 75). The second mechanism involves dissociation of CO to generate a coordinatively unsaturated complex that retains the metal metal bond (equation 76). In spite of the apparent simphcity of these two possibilities, the reaction mechanism is unknown and it is likely that parallel reaction mechanisms occur. 

There are two main classes of molecules (substrates) that can perform oxidative additions to metal centers non-electrophilic and electrophilic. Oxidative addition reactions with either class of substrates are favored by metal complexes that are more electron rich. Common non-electrophilic substrates are H2, Si-H bonds, P-H bonds, S-H bonds, B-H bonds, N-H bonds, S-S bonds, C-H bonds, alkenes, and alkynes. An important criterion for these non-electrophillic substrates is that they require a sterically accessible open coordination site on the metal (16e configuration or lower) onto which they need to pre-coordinate before initiating the oxidative addition to the metal center. For these substrates, both ligand atoms typically become cisoidally coordinated to the metal center after the oxidative addition as anionic (T-donors (subsequent ligand rearrangements, of course can occur). H2 is the most important and common for catalysis and a well-studied reaction is shown in Equation (5). 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

In addition, a catalytic version of Tt-allylpalladium chemistry has been devel-oped[6,7]. Formation of the Tr-allylpalladium complexes by the oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of the complex with soft carbon nucleophiles are the basis of catalytic allylation. After the reaction, Pd(0) is reformed, and undergoes oxidative addition to the allylic compounds again, making the reaction catalytic.-In addition to the soft carbon nucleophiles, hard carbon nucleophiles of organometallic compounds of main group metals are allylated with 7r-allylpalladium complexes. The reaction proceeds via transmetallation. These catalytic reactions are treated in this chapter. 

Perfluoroalkyl or -aryl halides undergo oxidative addition with metal vapors to form nonsolvated fluonnated organometallic halides and this topic has been die subject of a review [289] Pentafluorophenyl halides react with Rieke nickel, cobalt, and iron to give bispentafluorophenylmetal compounds, which can be isolated in good yields as liquid complexes [290] Rieke nickel can also be used to promote the reaction of pentafluorophenyl halides with acid halides [297] (equation 193)... 

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