T. Kondo, Kei Yamamoto, D. Takagi
Dec 10, 2010
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Influential Citations
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ChemCatChem
Abstract
The oligomerization of ethylene is of considerable scientific and industrial interest as a route to linear and branched alkenes. Linear alkenes, especially 1-hexene, are key intermediates in the production of plasticizers, detergents, and surfactants, as well as comonomers for the production of linear lowdensity polyethylene (LLDPE), whereas branched alkenes are of interest as high-octane additives to motor fuel. In the traditional transition metal complex-catalyzed oligomerization of ethylene, the formation of alkene mixtures (C4–C16 alkenes) [4] is unavoidable due to a mathematical distribution (Schulz–Flory or Poisson). Therefore, specific alkenes must be obtained by fractional distillation. The known ethylene trimerization catalysts are based mainly on chromium compounds with nitrogen, oxygen, and/or sulfur ligands, as well as mono(cyclopentadienyl)titanium(II) complexes. Sen and co-workers reported the first simple “ligandless” tantalum-based catalyst, and Mashima and co-workers recently reported a modified tantalum catalyst using 3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (BTCD) or its methyl derivative (MBTCD). Aside from Mashima’s catalyst, all of the trimerization catalysts reported to date have required large amounts of aluminum compounds such as methylaluminoxane (MAO) or other alkylating organometallic reagents such as (CH3)2Zn as a conventional catalyst activator. In sharp contrast, there are few examples of the trimerization of ethylene to branched C6 alkenes, isohexenes, and only two catalyst systems for this procedure, [NiBr2(PPh3)2]/ CH3AlCl2 [12] and [Pd(acac)2]/BF3·OEt2 (acac = acetylacetonate), [13] have been reported to date. We have previously developed several codimerizations and cotrimerizations of different alkenes with high atom efficiency. Among these, the unusual head-to-head dimerization of styrenes and linear codimerization of styrenes with ethylene proceeded smoothly with a zero-valent ruthenium catalyst, [Ru(h-cot)(h-dmfm)2] [15] (cot = 1,3,5-cyclooctatriene, dmfm = dimethyl fumarate), in the presence of primary alcohols to give (E)-1,4-diaryl-1-butenes and (E)-1-aryl-1-butenes, respectively, without aluminum compounds or alkylating organometallic reagents. We believe that these reactions proceed via ruthenacyclopentane intermediates derived from the oxidative cyclization of styrenes and/or styrenes with ethylene on an active ruthenium center. This result prompted us to develop a novel catalyst system for the selective trimerization of ethylene without the use of aluminum compounds or alkylating organometallic reagents. Firstly, the [Ru(h-cot)(h-dmfm)2]-catalyzed trimerization of ethylene (4.05 MPa) was examined in mesitylene at 110 8C for 24 h in the presence of 1-octanol to specifically give C6 alkenes (selectivity = 95 %, TON= 56), together with a small amount of butenes (selectivity = 5 %; Scheme 1). No alkenes larger than C8 were obtained at all.