Mechanism of Ethene Trimerization at an ansa-(Arene)(cyclopentadienyl) Titanium Fragment

Abstract

A theoretical study of ethene trimerization at a cationic (C6H5CH2C5H4)Ti fragment generally supports the metallacycle mechanism proposed earlier for this reaction. However, the crucial formation of the 1-hexene complex from a titanacycloheptane intermediate occurs by direct Cβ → Cα ‘ hydrogen transfer rather than by the more traditional β-elimination/reductive elimination sequence. The pendant arene moiety “breathes” during the reaction, being more strongly bound at the TiII stage than at the TiIV stage of the reaction. Its main role is to make the olefin complex formation more endothermic, thus increasing the barriers for formation of titanacyclopentane and titanacycloheptane intermediates. For the “naked” (C5H5)Ti system, which lacks this effect, further ring growth wins over hexene formation. But even for the bridged (C6H5CH2C5H4)Ti system, we find that the various reactions are very delicately balanced.

(nBuCp)2ZrCl2‐catalyzed ethylene‐4M1P copolymerization: Copolymer backbone structure, melt behavior, and crystallization

“…See Scheme 2 which has been extracted from a review of the relevant literature. [68][69][70][71] The mechanism of ethylene trimerization, as shown in Scheme 2, follows a metallacyclic route. Accordingly, two ethylene molecules are added to the metal, followed by insertion of another ethylene unit, which yields a higher order metallacycle (zirconacycloheptane).…”

“…The resulting 1-hexene gets incorporated into the growing polyethylene chain through 1, 2 insertion and synthesizes the branched polyethylene. [68][69][70][71] The literature mostly reports a typical tandem process that involves two different transition metal precatalysts with one cocatalyst [68][69][70][71] as well as one zirconocene precatalyst with two different cocatalysts. 72 2 shows that a tandem copolymerization can also be achieved using a single precatalyst-cocatalyst ion pair.…”

Distinguishing Chain Growth Mechanisms in Metal-catalyzed Olefin Oligomerization and Polymerization Systems: C2H4/C2D4 Co-oligomerization/Polymerization Experiments Using Chromium, Iron, and Cobalt Catalysts

“…12,13 Subsequent release of the product proceeds either via a sequential β-H eliminationreductive elimination process or alternatively through a concerted [3,ω]-hydrogen shift. 17 With certain chromium catalysts higher molecular weight oligomers can be obtained via this mechanism. 18,19,20 Scheme 1.…”

Recent Research Progress in Preparation of Ethylene Oligomers with Chromium-Based Catalytic Systems

“…The selective trimerization of ethylene to 1-hexene is a well-known technology [44]. However, the corresponding selective tetramerization reaction of ethylene to 1-octene was unknown until recently.…”

“…In this mechanism, the seven-membered chromacyclopentane would expand to a nine-membered one. This is thought unlikely to occur [45] because the ninemembered ring is the least-favored one and should be disfavored relative to the seven-membered ring [7,44].…”