Mechanism of ethene trimerization at an ansa-(arene)(cyclopentadienyl) titanium fragment

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 C-beta --> C-alpha' hydrogen transfer rather than by the more traditional beta-elimination/reductive elimination sequence. The pendant arene moiety "breathes" during the reaction, being more strongly bound at the Ti-II stage than at the Ti-IV 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.