Towards Selective Ethylene Tetramerization

Abstract

Und die magische Zahl ist …︁ 4! Eine große Menge an oligomerfreiem 1‐Octen (99.9 %) wurde aus Ethylen bei der Bildung von Polyethylenwachs mithilfe eines Chromkatalysators erhalten (siehe GC‐MS‐Chromatogramm; die drei weiteren wesentlichen Signale stammen von Methanol (zum Abbrechen der Reaktion), Ethylacetat (Nadelreinigungsmittel) und Toluol (Solvens)).

A Chromium Ethylidene Complex as a Potent Catalyst for Selective Ethylene Trimerization

“…[1a] In the case of the chromium systems that are involved in the majority of these catalysts, the + I state has been conclusively linked to selective trimerization by both synthetic and spectroscopic work. [4][5][6] One of the characteristics of chromium, however, is the possibility of readily interconverting between + I, + II, and + III states as a part of a redox dynamism. [7] As each one of these oxidation states has its own distinctive catalytic behavior, [6a,b] stabilizing the desired state and slowing at the same time the dynamism is central to obtain selective behavior.…”

PNPN‐H in Comparison to other PNP, PNPN and NPNPN Ligands for the Chromium Catalyzed Selective Ethylene Oligomerization

“…One has to accept, that up to now, there are no results of mechanistic investigations, which support dinuclear complexes as the reason for the tetramerization . Only some experimental hints exist for such a change from mono‐ to dinuclear species which are not in line with the latest published results . Nevertheless, it seems so far that there is no general view on the mechanism possible because it always depends on the special character of the respective used ligand class.…”

“…This was only a concept, but later Gambarotta and co‐workers supported this by some very important experiments . In one paper, they described the tetramerization of ethylene to 1‐octene with high purity and little or no polymer byproducts, in which solvents and co‐catalysts had a dramatic impact on the selectivity.…”

“…Gambarotta and co‐workers summarized these results, “… as a working hypothesis we concur with the mechanistic concept recently proposed by Rosenthal and co‐workers that a dinuclear monovalent species, possibly without a Cr−Cr multiple bond, might be formed with the aid of a non‐anionic bidentate ligand and promote the formation of 1‐octene through a bimetallic reductive elimination.” and „If the seven‐membered ring is capable of expanding readily into the nine‐membered ring, it is hard to imagine why additional expansion should not occur equally fast.“ “Accordingly to their hypothesis, a dimetallic system with two low valent chromium centers that are not linked to each other may independently form two five‐membered metallacycles. Cooperative dimetallic reductive elimination affords 1‐octene“ …”

A Highly Selective Ethylene Tetramerization Catalyst

“…Cooperative dimetallic reductive elimination selectively affords 1‐octene. It is by following this fascinating hypothesis that we have recently for the first time observed the formation of 1‐octene that was uncontaminated by other olefins 3e. Albeit a step forward, this particular system also produced large amounts of waxes, possibly as a result of incomplete reduction of the precatalyst to the monovalent state.…”