Electrophilic Alkane Activation

Marks has examined the reactivity of thorium metallacycles with hydrocarbons, where ring strain is used to provide the thermodynamic driving force for alkane activation in a reaction with methane (Eq. 17). Reaction with CD4 shows a dramatic kinetic isotope effect, with kH/kD=6, which is typical of the four-centered electrophilic transition state hydrocarbon activations [76]. The metallacy-cle is formed by the elimination of neopentane from the bis-neopentyl derivative [77]. Reaction with cyclopropane and tetramethylsilane gave the bis-cyclopropyl product Cp 2Th(c-propyl)2 and the bis-TMS product Cp 2Th(CH2SiMe3)2, respectively [78]. 

One final report of alkane activation has been reported by Moiseev. The mechanism of the reaction was not investigated, but this system might be classified as an electrophilic activation of methane, either of the Shilov type or of the concerted four-center type (Fig. lc) where X=triflate. Reaction of methane with cobalt(III)triflate in triflic acid solution leads to the formation of methyltriflate in nearly stoichiometric quantities (90% based on Co) (Eq. 18). Carbon dioxide was also observed, but not quantified. Addition of 02 led to catalysis (four turnovers) [79]. 

A wide variety of chemistry using electrophilic Pd11 derivatives has been investigated by Sen. This work will be reported as a separate chapter in this book. 

In addition to these exchange reactions, a number of alkane/alkane and al-kane/arene exchange reactions could be studied as equilibria (benzene, toluene, cyclopropane, methane, ethane, neopentane, cyclohexane). Determination of equilibrium constants allowed calculation of AG° values and estimation of relative metal-carbon bond energies. Wolczanski concluded that the differences between metal-carbon bond energies and the corresponding carbon-hydrogen bond energies were essentially the same [82]. 

Bergman has also reported an example of C-H addition to a zirconium-nitrogen double bond. The complex Cp2Zr(NHR)Me loses methane to generate an imido complex that can either be trapped with THF or reacted with benzene (Eq. 19). No reactions with alkanes were reported [86]. 

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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... 

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. 

The above mechanism, implying a bis-alkyl species, is the best way to explain (i) the observed selectivities and (ii) the primary and secondary products. It also takes into account important facts (i) the minor ](=SiO)2ZrH2] supported species (20-35% depending on the oxide support) is more electrophilic and activates C-H bonds of alkane faster than its homolog ](=SiO)3ZrH] (65-80%) [48] (ii) the higher selectivity in isobutene compared to that of methyl-branched pentenes is not consistent with a simple insertion of adsorbed propene into Zr-alkyl species because the concentration of Zr-propyl has to be greater than Zr-Me species (Scheme 3.12). 

The key of alkane transformation was assigned to the formation of CX3+-type cations that are electrophilic enough (probably due to a second complexation of A1X3), to abstract a hydride anion from linear and cycloalkanes. When these cations are generated in superacidic media, a protosolvation induces a superelectrophilic character, which was supported by Olah on the basis of high-level ab initio calculations 65 The generation of these cations was also used by various teams66,67 to initiate selective low temperature alkane activation. 

The intimate mechanism of the reaction deserves special attention not only because it was the first example of alkane activation by a metal complex. Activation of alkanes by platinum(II) complexes remains unique in many aspects. The reaction takes place in neutral water solution with conventional chloride ligands at the metal without special ways to form coordinatively unsaturated species (e.g., by irradiation). A number of works were directed towards the elucidation of the nature of the interaction between an alkane and a platinum(II) complex. The unique feature of platinum(II) complexes is to exhibit both nucleophilic and electrophilic properties. 

Among the earliest reports of alkane activation by a transition metal complex were the articles by Shilov in which Pt(n) served as a catalyst for methane oxidation and Pt(iv) served as a stoichiometric oxidant." The mechanism of C-H activation was termed electrophilic, as the cationic metal was postulated to interact with the electrons of the C-H bond which then lost a proton, forming a metal-carbon bond without a change in oxidation state. Oxidation of the complex by two electrons was then followed by nucleophilic attack at carbon, giving a functionalized hydrocarbon (Scheme 1). 

Electrophilic Activation by Electron-deficient Complexes. Alkane activation by Pt(ll) complexes was reported nearly 30 years ago. Methane was successfully converted to methyl chloride and methanol using equimolar amounts of Pt(lV) complexes as oxidants (13,14). A postulated methylplatinum(IV) intermediate is observed by NMR (eq. (6)) (13,14). 

The overall mechanism shown in Scheme 2, outlined by Shilov at an early stage in the research, has been essentially vahdated by all subsequent work. The reaction begins with activation of a C-H bond at a Pt(II) center. (There are examples of arene, but not alkane, activation by Pt(lV) these probably involve classical electrophihc routes via 7i-complexes and Wheland intermediates [10].) This fact seems incontrovertible since Pt(ll) by itself catalyzes H/D exchange the detailed mechanism of the activation is much less obvious, as discussed in Sect. 3. The resulting RPt(II) complex is extremely sensitive to electrophilic cleavage - no [RPt Cl c(H20)3 J species can be observed in the presence of any protmi source - so using Pt(II) alone, no alkane conversion beyond isotopic exchange would be feasible. However, [Pt Cle] effects oxidation to RPt(IV), which is virtually completely inert to protonolysis but quite susceptible to nucleophilic... 

Note that the above electrophilic carbonylation reaction catalyzed by very electron-deficient Rh has its counterpart in the chemistry of electron-rich Rh catalysis of the carbonylation which takes place subsequent to the oxidative addition of the Ph-H bond. This oxidative addition process with a Rh complex bearing PPhs ligands is more facile than the alkane activation described in section 2.5 that requires the more electron-releasing PMe3 ligands. ... 

Reaction 2.3.5.4 is an example of alkane activation by electrophilic substitution. A proton in methane is substituted by a stronger electrophile, the cationic metal complex. We will discuss this reaction and its relevance for selective oxidation of methane in greater detail in Section 8.5.5. 

This issue of ground state inhibition is observed in many catalytic alkane functionalization systems that operate by the CH activation reaction. Thus, this is the fundamental reason for the inhibition observed in the electrophilic CH activation-based Hg(II)/H2S04 and (bpym)Pt(II)/H2S04 systems that limit the maximum concentration of methanol to 1M. Theoretical and experimental studies show that the inhibition of the (bpym)Pt(II) system results from ground state stabilization from preferential coordination of water or methanol to the electrophilic Pt(II) catalyst (Fig. 7.22). This type of ground state stabihza-tiori is also the basis for the inhibition observed in the Hg(ll)/H2S04 system. 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

Zirconium, titanium, and hafnium hydrides can activate the C-H bonds of several alkanes at low temperatures (even at room temperature) because they are very electrophilic and reactive. Moreover, the surface complex is immobilized by the strong metal-silica bonds, and this immobilization can prevent the coupling reactions leading to the deactivation of the complex. 

For the C-H activation sequence, the different possibilities to be considered are shown in Scheme 5 (a) direct oxidative addition to square-planar Pt(II) to form a six-coordinate Pt(IV) intermediate and (b, c) mechanisms involving a Pt(II) alkane complex intermediate. In (b) the alkane complex is deprotonated (which is referred to as the electrophilic mechanism) while in (c) oxidative addition occurs to form a five-coordinate Pt(IV) species which is subsequently deprotonated to form the Pt(II) alkyl product. 

What purpose does the alkane binding to the Pt(II) center serve For the electrophilic pathway (Scheme 5, b), this is immediately apparent, a-Alkane complexes should be considerably more acidic than free alkanes, such that deprotonation may become a viable C-H activation pathway. While the acidic character of alkane complexes has not been directly observed, it can be inferred from the measured acidity of analogous agos-tic complexes (36) and from the acidity of the a-complexes of dihydrogen (37), both of which can be regarded models for alkane complexes (see Section III.E). 

The selective oxidation and, more generally, the activation of the C-H bond in alkanes is a topic of continuous interest. Most methods are based on the use of strong electrophiles, but photocatalytic methods offer an interesting alternative in view of the mild conditions, which may increase selectivity. These include electron or hydrogen transfer to excited organic sensitizers, such as aryl nitriles or ketones, to metal complexes or POMs. The use of a solid photocatalyst, such as the suspension of a metal oxide, is an attractive possibility in view of the simplified work up. Oxidation of the... 

Much of our research has involved the use of dicationic electrophiles in reactions with very weak nucleophiles, such as non-activated arenes and alkanes. By comparison to similar monocationic electrophiles, we have been able to show the extent of electrophilic activation by adjacent cationic centers. For example, carbocations show an increased reactivity with a nearby cationic charge (eqs 3-4).9 When 1,1-diphenyletheneis reacted with superacidic CF3SO3H... 

Ethers may be prepared by (1) dehydration of alcohols and (11) Williamson synthesis. The boiling points of ethers resemble those of alkanes while their solubility Is comparable to those of alcohols having same molecular mass. The C-O bond In ethers can be cleaved by hydrogen halides. In electrophilic substitution, the alkoxy group activates the aromatic ring and directs the Incoming group to ortho and para positions. 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

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