Hydrocarbyl complexes oxidative addition

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. 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

Double oxidative additions occur to generate hydrocarbyl-bridged complexes ... 

As stated earlier, (11.3.1), the multiple insertion of carbon monoxide into the same metal-hydrocarbyl bond is a rather elusive reaction. On the other hand, multiple insertion of isocyanide has been reported for nickel(II). For example, when the nickelfO) derivative Ni(t-BuNC)4 was treated with Mel in hexane at RT, consecutive insertion of three RNC groups was observed to give the product of reaction (e), as a consequence of a primary oxidative addition of the alkyl iodide to the nickel(O) complex. It is interesting that one of the two terminal fragments of the five-membered metallacycle is reminiscent of an arrangement of the first insertion product. 

The conversion of the Ir(III) cyclohexyl hydride complex to an Ir/cyclohexane system involves a change in the formal oxidation state of Ir from + 3 to +1 (i.e., a formal two-electron reduction). As a result, this elementary reaction step is generally called a reductive coupling (Chart 11.4). From a metal hydrocarbyl hydride complex (i.e., M(R)(H)), the overall process of C H bond formation and dissociation of free hydrocarbon (or related functionalized molecule) is called reductive elimination (Chart 11.4). The reverse process, metal coordination of a C—H bond and insertion into the C—H bond, is called oxidative addition. Note Oxidative addition and reductive elimination reactions are not limited to reactions involving C and H.)... 

N-heterocyclic carbenes (NHC) are considered extremely effective hgands for homogeneous catalysis (Figure 6.2). These specific carbenes often lead to high efficiencies in metal-catalyzed reactions compared to traditional phosphines [49, 50). NHC complexes are usually considered to be very stable, due to their electronic properties and the unusually high bond dissociation energies (BDE) associated with NHCs [51]. Previous work has shown that Ni-hydrocarbyl complexes of NHCs readily decompose by reductive elimination to yield the 2-substituted imidazohum salts [52, 53). Later studies have shown that the reverse reaction, oxidative addition of imidazohum salts to zerovalent Group 10 metals, is feasible [53]. 

One important property of CF3-transition metal complexes became apparent almost immediately when all of the low-valent, late transition metal trifluoromethylated compounds then known were found to be significantly more thermally and oxidatively stable than the analogous methylated species. Tetracarbonyl(trifluoromethyl)cobalt(I), for example, was isolated by distillation at 91°C, whereas the hydrocarbyl Co(CO)4(CH3) decomposes at subambient temperatures (72). Additionally, while the reverse of the decarbonylation reaction, CO insertion, is commonly observed in methylated transition metal species, these reactions are essentially unknown for trifluoromethyl metal complexes (13). Prior to 1980, evidence for CO insertion into an M—CF3 bond had been reported in only one case. That reaction employed the photolysis of Mn(C05)(CF3) in an argon matrix at 17 K, and the identity of the product was not determined (14). The clear implication of the above results is that MCF3 metal—carbon bonds are significantly less reactive and thus presumably stronger than MCH3 metal—carbon bonds. 

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