Selective dimerisation of α-olefins using tungsten-based initiators

Abstract

The selective dimerisation of the alpha-olefins 1-pentene through to 1-nonene is reported using an in situ-generated catalyst derived from tungsten hexachloride, aniline, triethylamine and alkylaluminium halide. The influence of reagent identity and reaction stoichiometry, along with activator, solvent and alpha-olefin substrate choice are probed. The catalyst is found to be highly selective towards dimerisation, minimising the formation of undesired heavier oligomers. Notably, the selectivity within the dimer fraction is found to favour the formation of products with methyl branches. The selectivity towards individual olefin isomers has been determined and the system is found to also produce trace levels of dienes and alkanes. A kinetic study of the system reveals a second order dependence on substrate. Comparison of the products observed, with those expected for metallacyclic and Cossee-type mechanisms, suggests that the latter is in operation, something confirmed by the results of a C(2)H(4)/C(2)D(4) co-dimerisation experiment which showed full isotopic scrambling in the products. Thus a mechanistic proposal is made to account for the observed behaviour of the system, including the diene and alkane formation.

Exploration of Homogeneous Ethylene Dimerization Mediated by Tungsten Mono(imido) Complexes

“…both tungsten-based dimerization catalyst systems generated in situ and those employing discrete bis(imido) tungsten pre-catalysts, the characteristics of the organoimido ligand have a direct impact on catalytic performance. 24,35,37,38,39 Consequently, the steric and electronic nature of the various imido moieties used in the current study have been assessed and subsequently related to catalytic performance.…”

“…NEt3, prior to activation with an appropriate alkylaluminum chloride (W:ArNH2:NEt3:Al = 1:2:2:10). 35,36 Thus, these dimerization systems offer scope for tuning product distributions, with not only the nature of the aniline, but also the identity of the alkylaluminum reagent and solvent having been shown to impact significantly on catalytic activity and selectivity. [35][36] While the exact nature of the actual catalytically-active species in these WCl6-derived catalysts is unknown, it has been proposed that tungsten aryl imido (ArN 2-) species are implicated.…”

“…35,36 Thus, these dimerization systems offer scope for tuning product distributions, with not only the nature of the aniline, but also the identity of the alkylaluminum reagent and solvent having been shown to impact significantly on catalytic activity and selectivity. [35][36] While the exact nature of the actual catalytically-active species in these WCl6-derived catalysts is unknown, it has been proposed that tungsten aryl imido (ArN 2-) species are implicated. 24,37 This is supported by reports that, in combination with an appropriate alkylaluminum reagent, [35][36][37] discrete or immobilized imido complexes of the type [WCl4(NAr)] are active for ethylene and propylene dimerization, with related tungsten bis(imido) pre-catalysts [WCl2(NAr)2] also exhibiting selective ethylene dimerization behavior.…”

Role of Homogeneous Catalysis in Oligomerization of Olefins : Focus on Selected Examples Based on Group 4 to Group 10 Transition Metal Complexes

“…The in situ generated catalyst although ill-defined remains the most efficient system to promote olefin dimerization. A detailed catalytic study from Sasol led to selectivity above 99 %for dimerization of 1-pentene to 1nonene depending on the catalyst components and their stoichiometry [138). Having a closer look ta the products formed, the authors proposed that the Cossee-Arlman mechanism is effective, supported by a C 2 H 4 /C 2 D 4 co-dimerization experiment leading ta full isotopie scrambling.…”

Activated Niobium and Tantalum Imido Complexes: From Tuneable Polymerization to Selective Ethylene Dimerization Systems

“…The catalytic ethylene oligomerization performance of the niobium and tantalum complexes 1 to 4 was screened using 15 equivalents of EtAlCl 2 as activator at 60 °C, 40 barg ethylene pressure, and with chlorobenzene as solvent (Table ). The reaction conditions employed here mirror those previously optimised for related industrially‐competitive tungsten imido‐based dimerization systems, thus allowing direct comparison of the performance of the Nb and Ta imido complexes ,,. Utilisation of chlorobenzene as reaction medium for the catalytic testing was adopted since we have previously demonstrated that this was the optimal solvent for ethylene dimerisation mediated by tungsten imido pre‐catalysts and hence its choice facilitates a direct comparison of the performance of the group 5 and 6 catalyst systems; note, catalyst optimisation was not a focus of this current study ,.…”