Oxidative addition of alkane C-H bonds

The oxidative addition of alkane C-H bonds to Pt(II) has also been observed in these TpRa -based platinum systems. As shown in Scheme 19, methide abstraction from the anionic Pt(II) complex (K2-TpMe2)PtMe2 by the Lewis acid B(C6F5)3 resulted in C-H oxidative addition of the hydrocarbon solvent (88). When this was done in pentane solution, the pentyl(hydrido)platinum(IV) complex E (R = pentyl) was observed as a... 

Much kinetic data has been obtained that support alkane complexes as intermediates in the oxidative addition of alkane C-H bonds. Flash photolysis experiments have even provided rates of reactions for alkane complexes (Equation 2.26). In addition, studies have been conducted to identify alkane complexes generated with metal centers that do not cleave the alkane C-H bond (Equation 2.27). Most studies have focused on studying alkane complexes of the [M(CO)j] (M = Cr or W) fragment. Photoacoustic calorimetry showed that the strength of the binding of heptane to [Cr(CO)5) is 9.6 kcal/moP and to [Mo(CO)j] is 11 kcal/mol, with errors that are roughly 20-25% of the magnitude of the... 

The first direct observation of oxidative addition of the C-H bond of a saturated hydrocarbon to a transition metal center was reported in 1982 by Janowicz and Berman and by Hoyano and Graham (Equation 6.27). Jones reported the oxidative addition of alkane C-H bonds to the analogous rhodium complexes at nearly the same time (Equation 6.28). These reports have provided the foimdation for himdreds of subsequent reports of C-H bond cleavage by late transition metal complexes and studies of the mechanism by which a metal can cleave an alkane C-H bond under mild conditions. In fact, cleavage of the strong C-H bond in methane - (104 kcal/mol) occurs even at 12 K in a CH matrix. ... 

These important considerations have not been overlooked by organometallic chemists, since the selective activation of hydrocarbons (7-11) and fluorocarbons (12,13) by transition metals is currently a topic of intense research activity. Intermolecular oxidative addition of alkane C-H bonds has been achieved... 

Reactivity from the naphthyl hydride ruthenium complex is believed to occur via initial reductive elimination of naphthalene followed by oxidative addition of a C-H bond. These activation processes (requiring reductive elimination) occur at temperatures of 150°C in alkane or arene solvents, depending on the desired product ". This complex shows general C-H activation behavior with sp, sp, and sp hybridized C-H bonds . ... 

CpReL3 (L = PMe3) is also photoreactive with alkanes by loss of The product is CpReL2(R)H (R = methyl, 1-hexyl, cyclopentyl and cyclopropyl). The more sterically hindered substrate, cyclohexane, did not give an alkyl hydride cyclometalation of the coordinated PMe3 or oxidative addition of the C—H bond of the free L takes place instead. This more bulky system has a lower tolerance for steric bulk in the alkane. 

For many years, only intramolecular C-H additions were observed because this type of reaction is favored kinetically and thermodynamically. Intermolecular additions of arenes were later observed, and arenes are more reactive than alkanes toward oxidative addition to all single-metal centers. In 1982, the isolation of an alkyl hydride complex from the oxidative addition of an alkane was first reported. - Since that time, many complexes have been reported that undergo oxidative additions of alkanes. Many of these complexes do not provide stable alkyl-hydride products, but these complexes can be induced in some cases to undergo productive transformations. The following sections describe the development of intramolecular and intermolecular oxidative addition of the C-H bonds of al%l groups, aryl groups, alkanes, and arenes. 

Some of the first reactions of soluble metal complexes with methane occurred by a-bond metathesis. Like the first examples of oxidative addition of alkyl C-H bonds, the first examples of a-bond metathesis with alkyl C-H bonds were intramolecular. Yet, the lute-tium- and yttrium-methyl complexes, Cp MMe (M = Lu and Y) were shown by Watson to react intermolecularly with C-labeled methane to form the labeled methyl complexes and unlabeled methane at 70 °C (Equation 6.51). Related scandium compounds have now been shown to undergo similar reactions with alkanes, and a thoracyclobutane... 

The mechanism of dehydrogenation involves the fundamental steps discussed in previous chapters of this book. The reaction mechanism includes oxidative addition of the C-H bond of the alkane, followed by p-hydrogen elimination of the resulting alkyl complex to generate the alkene product. Dissociation of the alkene and either reductive elimination of Hj or transfer of the hydrogen to a hydrogen acceptor regenerates the species that adds the alkane (Scheme 18.8). 

Reactions Involving sp -CH Activation. The insertion of ruthenium complexes into alkane C—H bonds is quite limited most synthetic routes require an adjacent nitrile group to first coordinate to the metal center. Oxidative addition of the C—H bond to the ruthenium center gives the hydrido ruthenium intermediate. An aldehyde or an a.jS-unsaturated carbonyl acts as the electrophile, which after reductive elimination from the metal affords the corresponding alcohol. This reaction is typically catalyzed by either CpRuCl(PPh3)2 or RuH2(PPh3)4 (58). 

Alkanes are notably unreactive compounds and are among the most challenging substrates for activation. After the discovery of the cyclometallation reaction (the oxidative addition of a C—H bond of a ligand to a metal complex e.g., step i in Fig. 12.4) in the early 1960s, several attempts were made to add alkanes to low-valent metals. All of these met with failure, and interest in the subject waned until Shilov - reported his observations on the ability... 

In 1982-1983, three research groups (R. Bergman, W. Graham and W. Jones) independently reported the intermolecular oxidative addition of a C-H bond of alkanes on iP and Rh to give the corresponding metal-alkyl-hydride complexes. 

In Chap. 3, we have examined the addition of an alkane C-H bond to a 16-electron metal center M giving a transient 18-electron metal-alkane intermediate M(RH) that ultimately gives oxidative addition of the C-H bond to M(R)(H). In this organomet-allic reaction, activation of the C-H bond occurs at the less substituted carbon for steric reasons. For instance, a linear alkane yields an w-alkyl-metal species via C-H activation at a terminal methyl group. The selectivity is thus primary > secondary > tertiary. This opens the valuable possibility of generating linear alkyl-functionalized products if such an oxidative addition mechanism can be made catalytic ... 

Several reaction pathways for reaction 1 are possible. A clear reaction mechanism has not been elucidated. Although it is premature to discuss the details of the reaction pathway for this silylation reaction, one possible pathway for the chelation-assisted silylation of C-H bonds is shown in Scheme 2. The catalytic reaction is initiated by oxidative addition of hydrosilane to A. Intermediate B reacts with an olefin to give C. Then, addition of a C-H bond to C leads to intermediate D. Dissociation of alkane from D provides Ru(silyl)(aryl) intermediate E. Reductive elimination making a C-Si bond gives the silylation product and the active catalyst species A is regenerated. Another pathway, addition of a C-H bond to A before addition of hydrosilane to A is also possible. At present, these two pathways cannot be distinguished. 

A thoughtful reader would have noticed that, while plenty of methods are available for the reductive transformation of functionalized moieties into the parent saturated fragments, we have not referred to the reverse synthetic transformations, namely oxidative transformations of the C-H bond in hydrocarbons. This is not a fortuitous omission. The point is that the introduction of functional substituents in an alkane fragment (in a real sequence, not in the course of retrosynthetic analysis) is a problem of formidable complexity. The nature of the difficulty is not the lack of appropriate reactions - they do exist, like the classical homolytic processes, chlorination, nitration, or oxidation. However, as is typical for organic molecules, there are many C-H bonds capable of participating in these reactions in an indiscriminate fashion and the result is a problem of selective functionalization at a chosen site of the saturated hydrocarbon. At the same time, it is comparatively easy to introduce, selectively, an additional functionality at the saturated center, provided some function is already present in the molecule. Examples of this type of non-isohypsic (oxidative) transformation are given by the allylic oxidation of alkenes by Se02 into respective a,/3-unsaturated aldehydes, or a-bromination of ketones or carboxylic acids, as well as allylic bromination of alkenes with NBS (Scheme 2.64). 

One other example of alkane oxidative addition to a higher oxidation state late transition metal has been reported by Goldberg. Reaction of the trispyra-zolylborate complex K[r 2-Tp PtMe2] with B(C6F5)3 leads to the abstraction of a methyl anion and the formation of a transient species that adds to the C-H bonds of benzene, pentane, or cyclohexane (Eq. 15). This result provides the first example of the intermolecular addition of a C-H bond to a Ptn species to give a stable PtIV product [71]. Earlier work by Templeton had demonstrated that the trispyrazolylborateplatinumdialkylhydride product would be stable [72]. 

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