Ethane/toluene, cross metathesis

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

Ethylbenzene can be prepared by cross-metathesis between toluene and ethane, catalyzed by (=SiO)2TaH, according to the equation [42] ... 

Figure 3.14 Toluene (a) and ethane (b) pressure effect on their TON during their crossed metathesis reaction at 250°C (=SiO)2TaH (3) (400mg) ethane/toluene (bar/Torr) A 10/28 B 25/28 C 10/310 D 25/540.

This reaction can be exploited to carry out the cross-metathesis of toluene and ethane to give ethylbenzene and xylenes (Scheme 36(d)) or that of methane and propane to yield two ethanes (Scheme 36(0)). ... 

Scheme 24 Cross metathesis of toluene and ethane with silica-supported tantalum hydride...

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. 

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