The issue of selectivity has been the most challenging aspect of ethylene oligomerization since its discovery.[1] In the last three decades, trimerization systems with high selectivity have been discovered, [2] but tetramerization catalysts with high selectivity remain elusive. [3] Central to the future success of this endeavor is to understand the factors responsible for the selectivity of the catalytic cycle. [4] Trivalent chromium complexes are the catalyst precursors that are most commonly used for these transformations. In the presence of alkyl aluminum activators, these species are reduced to the divalent state (responsible for nonselective oligomerization and/or polymerization) or the monovalent state (responsible for selectivity).[5] To complicate the scenario, there is also the possibility for these monovalent and divalent species to undergo disproportionative redox processes that give inactive zero-valent chromium together with higher-valent species that are readily available for further cycles of reduction/reoxidation.[6] Because of the presence of such a redox dynamism in the catalytic cycle, the ancillary ligand system determines the selectivity by preferentially stabilizing one particular oxidation state. For example, when highly reactive monovalent species are provided with a sufficiently long lifetime, selective oligomerization is initiated by the so-called redox reaction/ring-expansion mechanism. [2,5,7] As mentioned above, selective ethylene tetramerization remains exceedingly rare. With a maximum of around 77 % in the case of the process developed by SK Energy,[3d] the selectivity is definitely good, but still far from the levels obtained with the trimerization systems. In addition, the unavoidable formation of polymers poses serious reactorfouling problems that complicate industrial application. The same redox reaction/ring expansion mechanism that accounts for the selectivity of the trimerization cycle has also been invoked for the rationalization of the tetramerization. [3b, 8] Following this mechanism, it is conceivable that high selectivity cannot be reached. The selectivity in this mechanism is determined by the rate of the reduction/elimination step compared to the rate of further ring expansion. If the sevenmembered ring is capable of expanding readily into the ninemembered ring, it is hard to imagine why additional expansion should not occur equally fast. [9] In the end, a distribution of oligomers is to be expected and 1-octene may be a dominant product.Rosenthal and co-workers [10] first emphasized this problem and postulated an alternative mechanism for the highly selective formation of 1-octene. According to their hypothesis, a dimetallic system with two low-valent chromium centers that are not linked with each other may independently form two five-membered metallacycles. 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 un...

Kleine Änderung mit großer Wirkung: Die Einführung einer zusätzlichen CH2‐Gruppe in die Brücke von Liganden mit zwei P/N‐Einheiten führt zu unterschiedlichen Selektivitäten der entsprechenden chrombasierten Katalysatoren. Während 1 zu einem Ethylentrimerisierungssystem führt, ergibt 2 ein Ethylentetramerisierungssystem, das 1‐Octen mit hoher Reinheit sowie wenige bis keine Polymer‐Nebenprodukte produziert (siehe Schema; DMAO=Me3Al‐armes Methylaluminoxan).