Alkane activation catalysis

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]. 

Several Alkane Activation and Water-Gas Shift Catalysts Applied Catalysis A General, 101 (1993) 317-338. 

We conclude that there is no evidence for WZ catalysts having superacidic properties or sites with the acidic character that would be necessary for initiation of catalysis by alkane protonation. In as much as WZ catalysts are some four orders of magnitude more active than zeolites for alkane isomerization,26 it is clear that there is no one-to-one correlation between acid strength of WZ and its catalytic activity. We therefore infer that although the acidity of WZ catalysts is important in alkane conversion catalysis, the reaction is most likely initiated by a reaction other than protonation of the alkane by the catalyst or a species formed from it. 

Keiski, R.L., Desponds, O., Chang, Y.F., and Somorjai, G.A. Kinetics of the water-gas-shift reaction over several alkane activation and water-gas-shift catalysts. Applied Catalysis. A, General, 1993, 101, 317. 

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]. 

Evidence for alkane activation has also been seen by the observation of H/D exchange between two alkanes, an alkane and an arene, or an alkane and THE Using CpRe(PPh3)2H2 as the photo catalyst, thousands of turnovers have been observed. While the intermediate responsible for this catalysis was not identified, it does not appear to be [CpRe(PPh3)H2] undergoing Rem/Rev oxidative addition/reductive elimination, since no deuterium incorporation was observed in the dihydride catalyst [99]. Several other metal hydrides are known to catalyze H/D exchange between alkanes and deuterated benzene, such as Ir(PMe3)2H5 [100],CpMo(dmpe)H3 [101], and Re[P(c-hexyl)3]2H7 [102]. 

Pincer (mer, tridentate) phosphines have proved resistant to degradation and useful in alkane dehydrogenation catalysis. Recently, Xu et al. [118] found that a dihydrido Ir complex containing a tridentate monoaifionic aryl bis(phosphino) (PCP) pincer, 31 (R = terf-butyl), is highly active catalyst for dehydrogenation... 

A key issue for synthetic chemists is the direct and selective functionalization of alkanes under mild conditions. A major problem in C-H bond activation by molecular catalysis is the lack of a suitable reaction medium, because most organic solvents are not inert under alkane activation conditions and therefore prevent the desired reactions. In this context, dense carbon dioxide seems to be a promising reaction medium as it is miscible with organics, including organometallics, and potentially stable under alkane activation conditions. Indeed, methane carbonylation and alkane dehydrogenation by molecular catalysis have been reported using dense carbon dioxide as the reaction medium (Scheme 67). " ... 

Homogeneous Catalysis by Transition-metal Complexes 4 Alkane Activation... 

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). 

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. ... 

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