Alkanes intermolecular oxidative addition

The gas-phase reactions of the cationic Irm complexes follow a previously unreported mechanism for their observed a-bond metathesis reactions. Previous discussions had considered a two-step mechanism involving intermolecular oxidative addition of either [Cp Ir(PMe3)(CH3)]+ or [CpIr(PMe3)(CH3)]+ to the C-H bond of an alkane or arene producing an Irv intermediate, followed by reductive elimination of methane, or a concerted a-bond metathesis reaction sim-... 

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. 

Bergman et al. [12] reported one of the first studies of C—H bond activation with a transition metal system capable of intermolecular oxidative addition. The reaction involved the photolysis of [Ir(ri-Cp )(PMe3)(H)J in different hydrocarbon solvents. Figure 25.5 shows that the reaction likely proceeds via loss to form a very reactive IF 16 electron metal intermediate. The C—H activation proceeds via a 3-centered transition state leads to an Ir hydrido-alkyl complex in a high yield at room temperature. The process is well described as an oxidative addition of the alkane. 

In 1982. Janowicz and Bergman al Berkeley -" and Hoyano and Graham at Albertat- reported (he first stable alcane intermolecular oxidative addition products. The Berkeley group photolyzed (q -MejCsMMejPlIrHj with the loss of H2. while the Alberta group photolyzed (ii -Me,C,)lr(CO)2 with the loss of CO to give highly reactive iridium imermediates which cleave C—H bonds in alkanes ... 

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. 

Although intermolecular oxidative addition of alkanes is diffienlt, intramolecular oxidative addition has been known for a mneh longer time, and is indeed favored by the entropic assistanee. 

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... 

These findings have stimulated enormously the search for intermolecular activation of C-H bonds, in particular those of unsubstituted arenes and alkanes. In 1982 Bergman [2] and Graham [3] reported on the reaction of well-defined complexes with alkanes and arenes in a controlled manner. It was realised that the oxidative addition of alkanes to electron-rich metal complexes could be thermodynamically forbidden as the loss of a ligand and rupture of the C-H bond might be as much as 480 kl.mol, and the gain in M-H and M-C... 

Equation (1) depicts an early example of an intermolecular addition of an alkane C-H bond to a low valent transition metal complex [12], Mechanistic investigations provided strong evidence that these reactions occur via concerted oxidative addition wherein the metal activates the C-H bond directly by formation of the dative bond, followed by formation of an alkylmetal hydride as the product (Boxl). Considering the overall low reactivity of alkanes, transition metals were able to make the C-H bonds more reactive or activate them via a new process. Many in the modern organometallic community equated C-H bond activation with the concerted oxidative addition mechanism [10b,c]. 

Rhodium-mediated intermolecular C— H insertion is thought to proceed via oxidative addition of an intermediate rhodium carbene into the alkane C—H bond. Evidence that the rhodium and its ligands are directly associated with the product-determining transition state has been put forward by Callot, who ob-... 

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]. 

The theoretical calculations using density functional theory showed that the intermolecular C-H activation of alkanes by the complex CpIr(PMc3)(CH3) (described by Bergman, vide supra) is a lower-energy process and that both inter- and (nfronolecular C-H activation proceed only through an oxidative-addition mechanism (Scheme VI.8) [64] (compare Scheme VI.7). 

Isotopic labeling of alkane substrates for the investigation of EIEs in intermolecular exchange reactions such as oxidative addition reactions, cr-bond metathesis reactions, and 1,2-addition reactions has also been explored by various... 

Prototype Examples of Intermolecular Activation of Fluorinated Alkanes 1.26.1.5.1 Oxidative addition... 

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 oxidative addition of disilanes occurs to palladium complexes of isonitrile ligands and platinum complexes of trialkylphosphine ligands as part of tiie catalytic silylation of alkynes and aryl halides. The addition of stannylboranes to Pd(0) complexes has also been reported,and the addition of diboron compounds to many metal systems, such as Pt(0) complexes (Equation 6.67), is now common. These reactions all occur with metal complexes that do not undergo intermolecular reactions with alkane C-H bonds, let alone C-C bonds. Thus, the Lewis acidic character of these reagents must accelerate the coordination of substrate and cleavage of the E-E bonds. 

Now, one must wonder if there is any limit to the ability of metals to bond in a stable way to other o bonds, including those in non-reactive molecules like alkanes. In fact, evidence for an intermediate methane complex has been found at low temperature in the reductive elimination of methane from a cationic rhenium methyl hydride [34]. The ab initio theoretical study of the intermolecular process of oxidative addition of a methane C - H bond has led to the location of transition states where the bond is partially broken [35]. The same results have been fond for intramolecular oxidative additions which are related to agostic interactions. In fact, agostic interaction itself is a kind of non-oxidative coordination [15,36]. For unsaturated substrates like ethylene, the activation of a C - H bond seems to follow an intermolecular path, without any previous coordination of the double bond. A feasible explanation consists here of the fact that metal orbitals suitable for ethylene coordination are the same as those which are responsible for oxidative addition, thus making the processes competitive [37]. 

Both oxidative addition and a-bond metathesis are core subjects of organo-transition metal chemistry and catalysis. They also provide the unique regioselective intermolecular alkane activation modes at the terminal (less substituted) C atom contrary to radical-type and cationic C-H activation (see Chap. 19). 

Surprisingly, the study of this system indicated that the kinetic selectivity for aliphatic CH bond oxidative addition favours intermolecular reaction over intramolecular when neat hydrocarbon is the solvent there is, however, a moderate thermodynamic preference for the intramolecular reaction. Therefore, thermodynamic but not kinetic terms would favour the unimolecular cyclometallation reactions and it is possible that some systems may activate alkane CH bonds intermolecularly but that the products may not have been observed because of its thermal instability. 

More importantly, this silver system catalyzes the intermolecular amination of hydrocarbons, as shown in Table 6.3. In addition to animating weaker benzylic C-H bonds, stronger aliphatic C-H bonds such as those in cyclohexane were also reactive. Although yields with more inert hydrocarbons were modest with the bathophenan-throline system, the discovery of the first silver-catalyzed intermolecular amination opens opportunities for further developments. This reaction favored tertiary cyclic sp3 C-H bonds over secondary cyclic sp3 C-H bonds, and showed limited success with simple linear alkanes. No conversion was observed with any aromatic C-H bonds. The compound NsNH2 was tested as the nitrene precursor with different oxidants. The use of PhI(OAc)2 as oxidant gave the expected amination product with a lower yield, while persulfate and peroxides showed no reactivity. 

Several examples of intermolecular C-H bond functionalization have appeared during the past decade. In addition to the oxidations reported above in Shilov-type systems, and the dehydrogenation of alkanes to make alkenes, catalytic systems have been developed to introduce functional groups into hydrocarbons. 

Pb(C2H5)4 is employed as a chain-starter for the alkylation of alkanes or cycloalkanes by alkenes or cycloalkenes [697], and in the liquid-phase oxidation of alkylaromatic hydrocarbons to give hydroperoxides [892]. Pb(C2Hs)4 catalyzes the photochemical addition of HBr [661, 681] or of H2S or mercaptans to alkenes, such as propene [661, 679, 681], and it catalyzes the intermolecular condensation of arylalkanes in sunlight or on heating [684]. 

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