Oxidative addition of C-H bonds

Direct oxidative addition of C-H bonds to square-planar Pt(II) to form octahedral Pt(IV) alkyl hydrides (Scheme 5, a) appears to be extremely uncommon. In fact, there are no unequivocal examples of this reaction pathway. However, a couple of examples have been reported wherein such a direct mechanism may provide the best explanation for the available data. 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

Oxidative additions involving C-H bond breaking have recently been the topic of an extensive study, 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. Intramolecular oxidative additions of C-H bonds have been known for quite some time see Figure 2.15. This process is named orthometallation or cyclometallation. It occurs frequently in metal complexes, and is not restricted to "ortho" protons. It is referred to as cyclometallation and is often followed by elimination of HX, while the metal returns to its initial (lower) oxidation state. 

Comparable three-membered metallacycles were also obtained by oxidative addition of C—H bonds. Reduction of TaCls by sodium sand in PMe3 as a solvent afforded [Ta(PMe3)3(r/2-CH2PMe2)(tj2-CHPMe2)] (39 7% yield), whose structure (Figure 31) shows that C(l) is bonded... 

Zr(H)(CHiCMe,) Zr atome/iaobutane/ condense in matrix at 77 K Black solid, not fully characterized, Zr-H bond deduced from hydrolysis. Unstable on warming from matrix temp. No spectroscopic data. Oxidative addition of C-H bond to metal atom. 104... 

Scheme 2. Oxidative addition of C-H bonds to RhCI(CO)(PMe3)2 and RhCI(PMe3)3 under irradiation.

The stirring discovery, that transition metal alkyls and aryls were accessible via oxidative addition of C—H bonds to electrophilic metal centers, soon brought up the question whether dialkyl, diaryl or alkyl(aryl) transition metal... 

There are quite a number of routes available for the production of iridium(ni) alkyl compounds. In addition to the halide displacement and olefin insertion pathways noted above for iridium(l) compounds, oxidative addition of C-H bonds to iridium(l) to form iridium(in) hydrido alkyl complexes is also a possibihty. This subject will be covered in detail in Section 9 and will not be discussed here. However, there are other oxidative addition routes that lead to the formation of iridium(lll) alkyls. First, oxidative addition of O2 or HCl to some alkyl and aryl iridium(l) complexes can produce iridium(lll) alkyl or aryl compounds. In some cases, HgCl2 can add, but this appears to lead to tractable products only for the very stable pentafluorophenyl complex. Of course, oxidative addition see Oxidative Addition) of alkyl halides such as H3CI will also yield alkyl iridium(lll) compounds. Addition of Mel to Vaska s compound yields a stable iridium(III) complex, but addition of Etl does not produce a stable compound, presumably due to subsequent /J-hydride elimination see fi-Hydride Elimination). A number of mechanistic studies have been done on the oxidative addition of alkyl halides to iridium(l), especially Vaska s complex see Vaska s Complex). 

Osmium forms a wide variety of alkyl and aryl complexes including homoleptic alkyl and aryl complexes and many complexes with ancillary carbonyl (see Carbonyl Complexes of the Transition Metals), cyclopentadienyl (see Cyclopenta-dienyl), arene (see Arene Complexes), and alkene ligands (see Alkene Complexes). It forms stronger bonds to carbon and other ligands than do the lighter elements of the triad. Because of this, most reactions of alkyl and aryl osmium complexes are slower than the reactions of the corresponding ruthenium complexes. However, because osmium is more stable in higher oxidation states, the oxidative addition (see Oxidative Addition) of C-H bonds is favored for osmium complexes. The rate of oxidative addition reactions decreases in the order Os > Ru Fe. 

The oxidative addition of C H bonds of ligands is very common and this reaction forms metal alkyl or metal aryl complexes. In osmium triarylphosphine complexes, orthomet-allation gives four-membered metaUocycles. When the ortho... 

Carbon-hydrogen bonds are commonly formed by reductive elimination when an alkyl or aryl group and a hydride occupy mutually cis positions. Although intramolecular oxidative additions of C—H bonds and reactions of activated C—H bonds are well known for Ni, Pd, and Pt, additions of C—H bonds in simple alkanes and arenes are less common. 

Oxidative addition of C-H bonds leading to Tt (C)pyrrolyl(hydride) metal complexes (Eq. 6.3) has also been described [25]. 

Other studies of Pt(II) systems have directly probed the oxidative addition of C H bonds and have demonstrated the viability of this transformation. For example, the... 

A theoretical study of the reactions between methane and ML2 complexes (where M = Pd, Pt L = PH3, CO, = PH2CH2CH2PH2), led to the conclusion that for 14-electron ML2 complexes, a smaller L-M-L angle, a better electron-donating ligand, and a heavier transition metal center should be a potential model for the oxidative addition of C-H bonds [63a]. 

A similar reactivity trend has been reported for the oxidative addition of C-H bonds by binuclear iridium complexes. In fact, the Ir2 compoxmd [It2(CO)2(H) (p-l,8-(NH)2naphtha)(P Pr3)2] also undergoes the C-H bond oxidative addition of phenylacetylene to give a p-KC,q -aIkynyl-bridged complex, which, in this case. 

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