Ethylene ligands, oxidative coupling

The example in Scheme 6.16.4 shows the association of the neutral ligand ethylene to the Ni-hydride catalyst applied in the SHOP process, which carries an o-diphenylphosphinebenzoic add ligand (Vogt, 2002). In addition, the first step in the Cr-metallacyde mechanism, the addition of two neutral ethylene molecules to the Cr prior to the oxidative coupling step, is a ligand assodation step. 

Oxidative coupling is the decisive step in the Cr-based ethylene oligomerization according to the metallacycle mechanism. As shown in Scheme 6.16.7, two ethylene units of the ligand sphere of the Crmetallacyclic intermediate. Assuming a cationic Cr-complex in the first place (with the MAO anion as very weakly coordinating counter-ion), the oxidation state of chromium increases in the process from +I to +III. 

Scheme 6.16.7 Oxidative coupling of ethylene ligands at a cationic chromium complex to form the respective Cr(l I l)-metallacycle that later inserts ethylene.

Karakhanov s group has also been exploring poly(ethylene oxide)- and poly(alkene oxide)-copolymer-bound catalysts [99-102]. A notable aspect of this work is the design of polyethers like 39 that contain jS-cyclodextrins and calyx[4]- and calyx[6]arenes. Such polyethers couple the molecular recognition associated with these macrocycles with the catalytic activity of acac, phosphine, dipyridyl, and catechol ligands. Metals complexed to such ligands have been used in reactions like hydroformylation, Wacker oxidations, and arene hy-droxylation. 

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