Butane metathesis

This trend toward new nses of low-cost feeds for adding value to surfactants is continuing. Recently, BASF reported specific C13 detergent alcohols from hydroformylation of a C 2 olefin derived from dimerization of 3-hexene, a by-product of butane metathesis to propylene. - " Since 3-hexene appears to have no direct market value compared to ethylene, expectations are that the cost of the alcohol will be competitive with the ethylene-based alcohols. 

Figure 2.4 The ethane, propane, and butane metathesis product distribution from the reaction catalyzed by [TaH SiOj].

The product selectivities in propane metathesis can also be explained by using the same model in which [1,3]- and [1,2]-interactions determine the ratio of products. For instance, the butane/pentane ratios are 6.2 and 4.8 for [(= SiO)Ta(= CHfBu)(CH2tBu)2] and [(= SiO)2Ta - H], respectively (Table 5). A similar trend is observed for the isobutane/isopentane ratio, which are 4.1 and 3.0, respectively. The higher selectivity in butanes (the transfer of one carbon via metallacyclobutanes involving [l,3]-interactions) than that of pentanes (the transfer of two carbons via metallacyclobutanes involving [1,2]-interactions) is consistent with this model (Scheme 28). 

These results render support to the supposition that the metallocyclo-butane transition state which provides for metathesis is produced by an internal rearrangement of the complexed carbene and olefin substrates, as depicted in Eq. (5). 

The course of decomposition of confirmed or presumed metallocyclo-butane intermediates is important, but most results reported deal with stoichiometric rather than catalytic processes. Retention of the 3-carbon skeleton via pathways d or f in Eq. (26) occurs much more frequently than does cleavage to metathesis-related products. For example, thermolysis of phenyl-substituted platinocyclobutanes yields propenylben-zenes and phenyl-cyclopropane, but no styrene or ethylene (77). Similarly, the decomposition of tantalum carbene adducts (8) with olefins... 

In the presence of an acyclic alkane, 3 catalyzes at moderate temperature (25-200 °C) the metathesis reaction, leading to the formation of heavier and lower homolog alkanes by simultaneous breaking and formation of C-H and C-C bonds. For example, propane is transformed, even at 25 °C into a quasi-equimolar mixture of ethane and butanes (n- and iso-mixture) as well as methane and pentanes, in lower quantities. Lower and heavier homologs are also obtained starting from... 

These early results, along with a vast number of other data [45], establish that ring-opening metathesis polymerisation proceeds via a chain process [scheme (4) in Chapter 2] in which the structures of the active species fluctuate between metal alkylidenes (carbenes) and four-membered metallacycles (metallacyclo-butane)s, a concept that was first introduced by Herisson and Chauvin [46]. 

In addition, the (=Si-0)2Tani-H center catalyzed the metathesis of alkanes, an unprecedented process affording higher and lower homologs. For example, in the metathesis of propane to ethane and butanes, formation of four-centered intermediates was postulated (Scheme 51).708 In the first step, the C—11 bond is activated to yield (=Si-0)2Ta I-Prn and (=Si-0)2Tam-Pr ... 

Since the definitive experiments of Katz and Grubbs appeared in the literature, the non-pairwise mechanism has become the accepted pathway for metathesis. Subsequent investigations have supported this pathway.1718 Key intermediates in the non-pairwise mechanism are metal-carbene and metallacyclo-butane complexes. Both have been prepared and shown to catalyze metathesis, and more recently both species have actually been observed in the same reaction mixture and shown to interconvert during it, thus offering additional support for Mechanism 3.19 The next section will cover the discovery of discrete metal-carbene complexes that do serve as metathesis catalysts. 

Noteworthy also is the selectivity for higher alkanes. For example the metathesis of propane gives mainly butanes rather than pentanes, and butane rather than isobutane. This is in agreement with stereoelectronic factors that favor the transfer of one carbon on to, preferentially, primary alkyl surface species, leaving tertiary alkyl species unreactive (Scheme 10). The alkane selectivity also depends on the structure of the starting alkane (Figure 1). Overall, this reaction shows the potential of surface organometallic chemistry, and new, unprecedented reactions will be probably discovered in the near future. 

Determine the kinetic parameters for the metathesis reaction of the n-butyl radical with n-butane. 

Hydro-metathesis of propene under hydrogen atmosphere, in the presence of TaH/ KCC-1 catalyst, proceeds smoothly under dynamic reaction condition at 150 C for 65 h with 750 TON [83]. In addition to the expected hydrogenation product, propane, ethane, and butane were formed as major products, and methane, isobutane, and isopentanes formed as minor products in case of propene. Similarly in the case of 1-butene, propane and hexanes were formed as major products, and ethane, propene, pentanes, and heptanes were formed as minor products. In case the of butene, the catalyst was found to be stable even after 75 h and cumulative TON up to 1,150 achieved after 75 h of the reaction [83]. The most important issue with this catalyst is the stability and reusability of this Ta-H/KCC-1 catalyst and the high turnover numbers reached compared with the turnover numbers reported for the Ta-H/Si02 catalyst in alkane metathesis reaction. 

Recently, scandium-catalyzed hydromethylation of propene to iso-butane was achieved, but the reaction was very slow. The large isotope effect (CH4 vs CD4) is consistent with a n-bond metathesis mechanism (Scheme 4) (74). 

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