Alkanes activation

It is well known that C-H bonds in coordinated ligands can be cleaved by metals. This cyclometalation reaction has been studied for many 

A question that has not been answered until very recently is how this reaction could be effected intermolecularly to cleave CH bonds in alkanes. 

An early report by Shilov et al that PtCU in CH3COOD homogeneously effected H/D exchange attracted much attention. The reaction may go via electrophilic attack of the metal on the alkane, but the details are not yet clear. 

Cycloheptane and cyclooctane gave r/ -cycloheptadienyl and cyclooc-tadiene complexes, respectively, but other alkanes, e.g., methylcyclopen-tane, adamantane, or bicyclooctane, give as yet uncharacterized products which probably contain the dehydrogenated alkane. 

Cyclometalation might be expected to have occurred in preference to alkane activation in these PPhs complexes. Recent work on other systems has shown, however, that alkane activation can occur even in PPha complexes. 

Kinetic studies on Eq. 12.28 suggest that the reaction goes by nucleophilic attack of the hydride on CO2 to give the transient ion pair shown this then collapses to give the final product. [Rh(dpe)2H] forms the stable salt [Rh(dpe)2][HC02].  

Chemistry of this type is probably involved in the catalytic reduction of CO2 with H2 to give HCOOH. Although this is uphill thermodynamically (AG° = +8 kcal/mol) the reaction becomes favorable under gas pressure and in the presence of base to deprotonate the formic acid formed. The best catalyst to date is [Rh(cod)Cl]2/Ph2P(CH2)4pPh2, which gives 45 turnovers per hour at room temp, at 40 atm pressure.  

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 

C—H Oxidative Addition The key step is the C—H bond cleavage, for which there are as yet only a small number of pathways possible. These probably all go through an alkane complex, 12.7, followed by oxidative addition (12.8 in Eq. 12.29a), proton loss (12.9 in Eq. 12.29b) or C—H homolysis. 

In some cases, the alkene, once formed, can dissociate and is not further dehydrogenated. - This makes the alkane alkene conversion potentially catalytic, but the reaction is thermodynamically uphill below 300°C, so we need to drive the reaction. If t-BuCH=CH2 is present, it can do so by acting as hydrogen acceptor (Eq. 12.31). 

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Class II dependence for the activation of a chemical bond as a function of surface metal atom coordinative unsaturation is typically found for chemical bonds of a character, such as the CH or C-C bond in an alkane. Activation of such bonds usually occurs atop of a metal atom. The transition-state configuration for methane on a Ru surface illustrates this (Figure 1.13). 

The general technique of the metal vapor experiments described below was to co-condense the vapors of the transition metal with those of the chosen hydrocarbon or hydrocarbon mixtures. In this paper we briefly outline the technique of metal atom synthesis and then show how it can be applied to alkane activation reactions. 

Klabunde (JJL) was the first to describe alkane activation by metal atoms. He showed that cocondensation of nickel atoms and pentane yielded a solid material which contained Ni,C and H. 

Alkane activation by metal atoms by osmium atoms, 273-274 by rhenium atoms, 265-271 by tungsten atoms, 270-272 description, 265 Alkane elimination reactions, processes, 20,22... 

Operando DRIFTS measurements suggest that bridged hydroxyl groups are in extensive interaction with hexane molecules during the reaction even at 553 K. However adsorbed alkene or surface alkoxide could not be detected. These findings questions, whether the Haag-Dessau mechanism [4] gives true description of the alkane activation process over zeolite catalysts. 

Synthetic organic chemistry applications employing alkane C-H functionalizations are now well established. For example, alkanes can be oxidized to alkyl halides and alcohols by the Shilov system employing electrophilic platinum salts. Much of the Pt(ll)/Pt(rv) alkane activation chemistry discussed earlier has been based on Shilov chemistry. The mechanism has been investigated and is thought to involve the formation of a platinum(ll) alkyl complex, possibly via a (T-complex. The Pt(ll) complex is oxidized to Pt(iv) by electron transfer, and nucleophilic attack on the Pt(iv) intermediate yields the alkyl chloride or alcohol as well as regenerates the Pt(n) catalyst. This process is catalytic in Pt(ll), although a stoichiometric Pt(rv) oxidant is often required (Scheme 6).27,27l 2711... 

Thus, a highly reactive species is needed to make this type of bond activation reaction feasible under mild conditions. In addition, to be useful, the C-H bond activation must occur with both high chemo- and regiose-lectivity. Over the past several decades, it has been shown that transition metal complexes are able to carry out alkane activation reactions (1-5). Many of these metal-mediated reactions operate under mild to moderate conditions and exhibit the desirable chemoselectivity and regioselectiv-ity. Thus, using transition metal complexes, alkane activation can be preferred over product activation, and the terminal positions of alkanes, which actually contain the stronger C-H bonds, can be selectively activated. The fact that a hydrocarbon C-H bond has been broken in a... 

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

A very different neutrally charged complex for alkane activation has been reported recently and is shown in Scheme 34(A). The compound is a hydridoplatinum(II) complex bearing an anionic ligand based on the familiar nacnac-type, but with a pendant olefin moiety (97).This complex is extremely soluble in arenes and alkanes and activates C-H bonds in both types of hydrocarbons. This is indicated by deuterium incorporation from deuterated hydrocarbon into the substituents on the arene of the ligand and into the Pt hydride position (A A-d27, Scheme 34). The open site needed for hydrocarbon coordination at Pt(II) is created by olefin insertion instead of anion or solvent substitution (97). 

In the preceding discussion, we have covered recent discoveries which shed light on the first two steps of Pt(II) catalyzed alkane functionalization (Section III Scheme 4) alkane activation by Pt(II) and oxidation of the resulting platinum(II) alkyl. The final step is the release of the... 

Several groups have proposed related species as being intermediates in alkane activation reactions (81). 

Lewis acid sites have empty orbitals able to accept electron density from the occupied orbitals of a Lewis base, in parallel with back-donation from the catalyst to the empty anti-bonding orbitals of the base [33]. This interaction leads to the formation of an activated acid-base adduct. In the case of alkanes activation may proceed by hydride abstraction [38]. Y and Beta are good examples of zeolites with Lewis acidity, often quite significant for catalysis [39, 40]. 

Until now, for most of the systems described here it has been accepted that alkane activation occurred through oxidative addition to the 14-electron intermediate complexes. Yet, Belli and Jensen [26] showed, for the first time, evidence for an alternative reaction path for the catalytic dehydrogenation of COA with complex [lrClH2(P Pr3)2] (22) which invoked an Ir(V) species. Catalytic and labeling experiments led these authors to propose an active mechanism (Scheme 13.12), on the basis of which they concluded that the dehydrogenation of COA by compound 22 did not involve an intermediate 14-electron complex [17-21], but rather the association of COA to an intermediate alkyl-hydride complex (Scheme 13.12). 

The most relevant catalytic reactions approached by SOMC are olefin polymerization (and depolymerization), alkane activation (including a new reaction, discovered thanks to SOMC-alkane metathesis), alkene metathesis and epoxidation. All these reactions are discussed in this chapter. 

There are many bi- and polynuclear iron(III) complexes where bridging oxide is supported by carboxylates. Combinations of Fe—O—Fe plus carboxylate bridges occur in model compounds for proteins and in biomineralization. Electronic spectra of [Fe(/u-0)(//-02CR)2Fe] complexes have been reviewed briefly. [Fe2(//-0)(M-02CMe)2(bipy)2Cl2] is both an alkane activation catalyst and a bio-model. ... 

SOME IMPORTANT SINGLE-REACTION ALKANE ACTIVATION... 

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