Oxidative addition of alkanes

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

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... 

The next step, the oxidative addition of alkane to form the Ir(V) intermediate is quite sensitive to the methods and presents a challenging problem to determine which method gives the most accurate answer. The calculated activation (AE ) and reaction (AE°) energies show... 

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... 

Another effective way of staying clear of the thermodynamic barriers of C-H activation/substitution is the use of the c-bond metathesis reaction as the crucial elementary step. This mechanism avoids intermediacy of reactive metal species that undergo oxidative additions of alkanes, but instead the alkyl intermediate does a o-bond metathesis reaction with a new substrate molecule. Figure 19.13 illustrates the basic sequence [20],... 

The oxidative addition of alkanes to Rh1 and Ir1 species CpML is very facile. In the case of the tris(pyrazolyl)borate complex Tp Rh(CO)2 [Tp = HB(2,4-Me2pyr)3], the lifetimes of the intermediates were determined by ultrafast time-resolved infrared spectroscopy. In this case de-chelation of one of the pyrazolyl arms was found to precede the C—H oxidative addition step. The proposed intermediates for R—H addition and their lifetimes are shown in Fig. 21-1.127... 

Robert G. Bergman Wdham A. G. Graham Oxidative addition of alkanes by iridium complexes... 

Patricia L. Watson Tobin J. Marks Oxidative addition of alkanes by lanthanide and actinide compounds, respectively... 

Photolysis of Cp M(CO)2, M = Rh or Ir, and CpIr(CO)2 in methane matrices afforded the first direct observation of C-H activation from a photointermediate.Similarly, photolysis of Cp fr(CO)(H)2 in methane matrices yielded Cp Ir(CO)(H)(CH3). The rate of oxidative addition of alkanes to a CpRh(CO) photointermediate in the gas phase has been measured. 

The oxidative addition of alkanes with formation of alkyl hydride complexes was first demonstrated directly in studies using indium complexes [14], Thus the iridium dihydride derivative Cp lr(H)jPMe3 (Cp = pentamethylcyclopenta-dienyl) after irradiation in solution in cyclohexane or neopentane, produces the complexes Cp (PMe3)Ir(H)(C6Hn) and Cp (PMe3)Ir(H)CH2CMe3 in a satisfactory yield. Other saturated hydrocarbons and benzene also readily add to an iridium complex. 

Dehydrogenation of alkylcyclohexanes photocatalyzed by complex RhCl-(CO)(PMe3)z [43] affords mixtures of products (Scheme IV-22). The proposed mechanism of this reaction includes oxidative addition of alkane to the species RhCI(PMe3) and then elimination of hydrogen from the p-positionof alkyl chain of the alkylhydride derivative [43c,dj. 

While the activation of sp, sp, and heteroatom-substituted C—H bonds have become widely documented, the oxidative addition of alkane bonds remains restricted to only a few systems. Therefore the goal of understanding the chemistry of these complexes has attracted much study. The most studied systems involve the d fragment [Cp M(PMe3)], ( ) M = Rh, Ir. LCAO calculations based on nonlocal density functions reflected the more favorable reaction pathway for C—H bond activation by (9) than by [M(CO)4], (10), M = Ru, Complexes (9)... 

A typical reaction is the photochemical oxidative addition of alkanes to complexes such as Rh(Cp)(CO)2, which occurs via initial loss of CO followed by rr-coordination of the alkane before oxidative addition. 

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... 

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. 

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. ... 

In many other cases, oxidative additions of alkanes occur readily to transition-metal-alkyl complexes to generate hydride dialkyl intermediates that subsequently eliminate alkane and form a new metal-alkyl complex. For example, cations related to the alkyl hydrides of iridium formed by oxidative addition undergo reaction with alkanes at or below room temperature to generate new alkyl complexes (Equation 6.34). Cationic platinum complexes undergo similar reactions with substrates containing aromatic and aliphatic C-H bonds (Equation 6.35). " The C-H activation of the platinum complexes has been studied, in part, to understand and to develop systems related to the ones reported by Shilov that lead to H/D exchange, and oxidation and halogenation of alkanes. 

The high selectivity for reaction at primary C-H bonds is one of the most striking features of the oxidative addition of alkanes. This selectivity contrasts with that of hydrogen atom abstraction by radical species, which occur fastest at tertiary C-H bonds and slowest at primary C-H bonds. It also contrasts with the chemistry of metal-oxo or metal-carbene ° complexes, which tend to insert the 0x0 or carbene group into the weakest, sterically accessible C-H bond. This selectivity of oxidative addition for reaction at a primary C-H bond implies that alkanes can be converted selectively to products with functional groups in the terminal position. 

With few exceptions, transition metal complexes of terminal alkyl groups are more stable than those of isomeric secondary or tertiary alkyl groups. Consistent with this trend, the terminal alkylmetal hydride complexes of systems that undergo oxidative addition of C-H bonds are thermodynamically more stable than the branched alkylmetal hydride complexes. "- The oxidative addition of alkanes by the Cp Ir(PMe3) fragment is reversible, and this reversibility allowed for an evaluation of the thermodynamic stability of alkyl hydride complexes formed from a series of alkanes, as weU as from arenes. These data showed that the alkyl hydride complex formed from oxidative addition at the primary C-H bond leads to more stable products than those formed from... 

Examples of oxidative additions of alkanes to two metal centers are rare but can occur by a process that parallels the addition of Hj to two metal centers that was shown in Equations 6.10a-6.10c. Wayland showed that the Rh(II) complex containing tetramesityl porphyrin cleaves the C-H bond of methane by a termolecular process to generate a... 

The conversion of alkanes into more useful products is one of the most important practical problems in chemistry. The insertion of metals into C-H bonds was first discovered by Chatt and Davidson in 1965 [101] for low-valent ruthenium complexes. Following the discovery of the photochemically induced insertion of a transition metal into alkane C-H bonds [102, 103], a large number of organo-metallic complexes have been shown to activate C-H bonds in alkanes. A typical example is the photochemical oxidative addition of alkanes to complexes such as Rh(Cp)(CO)2- The reaction occurs via an initial loss of CO followed by a-... 

The reaction of electrophilic platinum cations developed to study the oxidative addition of alkanes to platinum center with diethyl ether has been shown to generate carbene complex 163 in high yield the product is thought to arise from the C-H activation of diethyl ether followed by an a-H elimination reaction (Equation (30)). 

---