DOI: 10.1002/asia.201301263 Highly Selective Dimerization and Trimerization of Isobutene to Linearly Linked Products by Using Nickel Catalysts Arno Behr,* [a] Nils Rentmeister, [a] Thomas Seidensticker, [a] Jonas Vosberg, [a] Stephan Peitz, [b] and Dietrich Maschmeyer [b] Introduction The dimerization of olefins by transition-metal complexes is of considerable industrial interest. [1] For example, olefins, such as propene and 1-butene, are dimerized on a large scale to give mainly branched products that are used as feedstocks for gasoline blending or alcohol production. [2] Highly branched products are favored in fuels, but higher linearities of products are more desirable as precursors for plasticizers. Although several catalysts are known to gener- ate branched dimers, only a few catalysts are capable of pro- ducing linear dimers from 1-alkenes. [3] For example, the linear dimerization of 1-alkenes can be performed by using nickel acetylacetonate complexes. Keim et al. showed that the use of the one-component catalyst (4- cyclooctene-1-yl)(1,1,1,5,5,5-hexafluoro-2,4-acetylacetona- to)nickel (1) led to high product linearity (e.g., S C 8 , linear = 75– 80 %) in the dimerization of 1-butene. [4] Analogous alumi- num-activated nickel systems were also reported in the liter- ature. [5] Highly active iron and cobalt complexes with bis- ACHTUNGTRENNUNG(imino)pyridyl ligands were successfully used by Brookhart et al. and Gibson et al. [6] The more selective cobalt systems generate linear dimers from different 1-alkenes (e.g., S C 8 , linear = 98 % for 1-butene). [7] However, bis-(imino)pyridyl systems seem to suffer from intolerance towards highly branched substrates like isobutene. Beside the catalyst system, the choice of the substrate plays a key role for the product distribution in oligomeriza- tion reactions. Although ethene can be applied in most cases, the use of propene and higher 1-alkenes often leads to lower activity and lower linearity. Although 1-alkenes reveal lower activity in oligomerization reactions compared with ethane, higher dimer selectivity can be achieved. 2,2- Dialkylated 1-alkene isobutene differs significantly from monoalkylated 1-alkenes and usually shows no reactivity for a linear linkage. Due to the two methyl substituents, the double bond of isobutene possesses higher electron density and shows stron- ger polarization compared with other aliphatic 1-alkenes. In consequence, the reactivity of isobutene is strongly depen- dent on the ready formation of a carbocation by either Lewis or Brønsted acids. In acid-catalyzed oligomerization reactions, the electrophilic center of the tert-butyl cation is attacked by the nucleophilic C1 position of isobutene. Through this pathway, highly branched oligomers of isobu- tene are accessible. [8] Due to the carbocation activity of iso- butene, many transformations, such as the conversion to methyl tert-butyl ether, isooctene, polyisobutene, and butyl rubber, have been industrially established. [9] Owing to the reaction pathway, the resulting products are always highly branched. An alternative route to create a new kind of very valuable linearly linked product is desirable, especially for the pro- duction of plasticizers in which higher linearity results in lower viscosity and better performance at lower tempera- tures. [10] A comparison of the classic carbocation pathway and the desired pathway for the dimerization of isobutene is shown in Scheme 1. Abstract: The unique linear linkage of isobutene to generate highly valuable C 8 precursors for plasticizers is feasible by using special nickel catalysts. (4-Cy- clooctene-1-yl)(1,1,1,5,5,5-hexafluoro- 2,4-acetylacetonato)nickel and alumi- num-alkyl-activated nickel acetylaceto- nates produce isobutene dimers with high selectivities of up to 95 %. More- over, selectivity for the head-to-head products (2,5-dimethylhexenes) is re- markably high at up to 99 %. Addition- ally, novel C 12 isobutene trimers are also formed with a very high selectivity of up to 99 % for the linear linkage. The trimer structure (2,5,8-trimethylno- nenes) reflects the stepwise characteris- tic of the reaction mechanism. Path- ways of insertion and activation and the deactivation processes of the cata- lyst are discussed in detail. Keywords: dimerization · head-to- head linkages · homogeneous catal- ysis · isobutene · nickel [a] Prof. A. Behr, N. Rentmeister, T. Seidensticker, J. Vosberg Lehrstuhl für Technische Chemie Technische Universität Dortmund Emil-Figge-Str. 66, 44227 Dortmund (Germany) Fax: (+ 49) 231-755-2311 E-mail : behr@bci.tu-dortmund.de [b] Dr. S. Peitz, Dr. D. Maschmeyer Evonik Industries Paul-Baumann-Str. 1, 45772 Marl (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301263. Chem. Asian J. 2014, 9, 596 – 601  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 596 FULL PAPER