Cross-alkane metathesis

Cross-Alkane Metathesis In a parallel study on cross-alkane metathesis using supported, Ta polyhydrides [34], Schrock and coworkers recently reported the dual-catalytic, homogenous, cross-alkane metathesis of n-octane, and ethylbenzene. The authors [136] employed various W-monoaryloxide pyrrolide complexes in combination with several iridium pincer complexes. For example, employing Ir-2(H2) and W-1 with a ratio of n-octane/ethylbenzene (1 1.33, in v/v) produced the best productivity toward alkylbenzenes, with good selectivity over linear alkanes (Scheme 2.20). 

Products from the cross-alkane metathesis of octane/ethylbenzene at 180°C... 

Note also that (1) d° Ta alkyhdene complexes are alkane metathesis catalyst precursors (2) the cross-metathesis products in the metathesis of propane on Ta are similar to those obtained in the metathesis of propene on Re they differ only by 2 protons and (3) their ratio is similar to that observed for the initiation products in the metathesis of propane on [(=SiO)Ta(= CHfBu)(CH2fBu)2]. Therefore, the key step in alkane metathesis could probably involve the same key step as in olefin metathesis (Scheme 27) [ 101 ]. 

While alkane metathesis is noteworthy, it affords lower homologues and especially methane, which cannot be used easily as a building block for basic chemicals. The reverse reaction, however, which would incorporate methane, would be much more valuable. Nonetheless, the free energy of this reaction is positive, and it is 8.2 kj/mol at 150 °C, which corresponds to an equihbrium conversion of 13%. On the other hand, thermodynamic calculation predicts that the conversion can be increased to 98% for a methane/propane ratio of 1250. The temperature and the contact time are also important parameters (kinetic), and optimal experimental conditions for a reaction carried in a continuous flow tubiflar reactor are as follows 300 mg of [(= SiO)2Ta - H], 1250/1 methane/propane mixture. Flow =1.5 mL/min, P = 50 bars and T = 250 °C [105]. After 1000 min, the steady state is reached, and 1.88 moles of ethane are produced per mole of propane consmned, which corresponds to a selectivity of 96% selectivity in the cross-metathesis reaction (Fig. 4). The overall reaction provides a route to the direct transformation of methane into more valuable hydrocarbon materials. 

Additionally, grafting molecular entities on surfaces has already allowed to discover several reactions the low temperature hydrogenolysis of alkanes including the depolymerization of polyolefins, the alkane metathesis and the cross-metathesis of methane and alkanes. These two latter reactions can allow higher molecular weight alkanes to be built. 

Scheme 36 (a) H/D exchange, (b) alkane hydrogenolysis, (c) alkane metathesis, (d) cross-metathesis of toluene/ethane, (e) cross-metathesis of propane/methane. 

It is noteworthy that the double-bond isomerization step is faster than the overall elementary steps of alkane metathesis. Formation of lower alkanes is due to the tungsten hydride intermediate, favoring chain walking with double-bond migratirHi followed by fast cross metathesis with coordinated ethylene leading to lower alkenes in turn giving lower alkanes on hydrogenation. This intramolecular reaction pathway, without formation of the free olefin, probably is the difference between alkane and olefin metathesis. 

The cross metathesis between propane and methane to transform methane into higher alkanes has also been reported [35]. Mechanistic studies performed on a mixture of propane and C-labeled methane (1/1250) in a batch reactor afforded isotopomers of ethane (unlabeled, mono-, and dilabeled), confirming the incorporation of the labeled methane (>85%). Moreover, metathesis of the C-monolabeled ethane catalyzed by [TaH, /Si02] in a batch reactor was examined. The results revealed that the degenerative and productive alkane metathesis processes concomitantly occur the alkane product distribution afforded a 1/1 mixture of unlabeled and monolabeled methane [36]. Further... 

The use of a silica-supported, tantalum alkyhdene as a precursor for alkane metathesis was found to result in a stoichiometric, alkane cross metathesis in which an initial pendant alkyl-alkylidene group was transformed to produce the desired, active species [76, 90]. This reaction was later observed to work with additional, well-defined systems supported on alumina [58] and sihca-alumma [53]. As mentioned previously, this transformation does not occur when the surface organometallic precursor bears no alkyl group. Exposing these supported, metal neopentyl, neopentylidene, and neopentyhdyne complexes to alkane at 150 °C produced alkanes containing a neopentyl fragment (CH jiBu) via cross metathesis. Propane metathesis with these alkyl-alkyhdene surface complexes typically generates stoichiometric amounts of dimethylpropane, 2,2-dimethylbutane, and 2,2-dimethylpentane (Scheme 2.11). 

Methane-Propane Cross-Metathesis ( Alkane Methane-olysis )... 

Cross metathesis between two different alkanes represent one of the most difficult challenges in organic chemistry [53]. In 2001, Basset et al. first demonstrated the possibilities of sigma bond metathesis between two different alkanes [55]. In 2004, this same group has reported the cross metathesis between ethane and toluene [81] and methane and propane [82]. Silica-supported tantalum hydride catalyst [(=SiO)2TaH] [(=SiO)2TaH3] was employed for cross-metathesis reaction between toluene and ethane at 250°C. Under static condition, it produced mainly ethyl benzene and xylenes as major product along with propane and methane (Scheme 24). 

Another application for these silica-supported, tantalum polyhydrides [TaH /Si02l was the cross metathesis between ethane and toluene under partial pressures of these reactants in a 97.6 3.7 ratio, respectively, at 250 C (Scheme 2.4) [34]. The resulting aromatic products were typically found to be benzene (0.5%), ethylbenzene (15%), xylene (2.5%), and propylbenzene (3.6%). Moreover, the major products were also found to be the linear alkanes resulting from the competing self-metathesis of ethane, as ethane was used in large excess. Dynamic studies employing a continuous-flow reactor have shown that the formation of ethylbenzene competes at a slower rate than ethane metathesis. 

---