Metathesis of Acyclic Alkanes

A study of the stoichiometric cyclopentane reaction over Ta-H has revealed that tantalum hydride very easily achvates cyclopentane, forming the corresponding cyclopentyl derivative. However, the latter is very quickly transformed into a cyclo-pentadienyl compound, as shown by NMR and EXAFS studies. This cyclopenta-dienyl derivative presents no achvity in alkane hydrogenolysis  

Indeed, cyclopentane formation during all cyclic alkane hydrogenolysis and cyclopentane transformation into a cyclopentadienyl derivative, inert in hydrogenolysis, explains the rapid deactivation process of this catalyst in the presence of cyclic alkanes. 

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The silica-supported Ta hydride (=SiO)2Ta-H (93a) reacts with PMc3 to give (=SiO)2Ta(H)(PMe3) (93b). In comparison to (93a), the phosphine hydrid (93b) is inactive toward metathesis of acyclic alkanes, but affords the oligomerization of ethylene into 1-butene and higher oligomers. 

Fig. 3 Product selectivity in the metathesis of various acyclic alkanes...

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... 

Alkane metathesis was first reported in 1997 [84]. Acyclic alkanes, with the exception of methane, in contact with a silica supported tantalum hydride ](=SiO)2TaH] were transformed into their lower and higher homologues (for instance, ethane was transformed into methane and propane). Later, the reverse reaction was also reported [85]. Taking into accountthe high electrophilic character ofa tantalum(III) species, two mechanistic hypotheses were then envisaged (i) successive oxidative addition/reductive elimination steps and (ii) o-bond metathesis. Further work has shown that aLkyhdene hydrides are critical intermediates, and that carbon-carbon... 

Zirconium tribenzyl complex 212 incorporating diisopropyltriazacyclonone, a type of mono-anionic, tridentate diamino-amido [/V,iV ,./V] ligand, was obtained via alkane elimination (Scheme 42) alternatively, this complex can also be prepared via either salt metathesis or amine elimination approaches.179 The crystal structure of 212 reveals a monomeric form on heating at 80 °C in benzene solution for 24 h, this complex undergoes elimination of 1 equiv. of toluene, affording complex 213 bearing a dianionic, acyclic, diamido-amino [/V-,iV,./V-] moiety. 

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