Cyclometallation 1,5-cyclooctadiene

A wide range of carbon, nitrogen, and oxygen nucleophiles react with allylic esters in the presence of iridium catalysts to form branched allylic substitution products. The bulk of the recent literature on iridium-catalyzed allylic substitution has focused on catalysts derived from [Ir(COD)Cl]2 and phosphoramidite ligands. These complexes catalyze the formation of enantiomerically enriched allylic amines, allylic ethers, and (3-branched y-8 unsaturated carbonyl compounds. The latest generation and most commonly used of these catalysts (Scheme 1) consists of a cyclometalated iridium-phosphoramidite core chelated by 1,5-cyclooctadiene. A fifth coordination site is occupied in catalyst precursors by an additional -phosphoramidite or ethylene. The phosphoramidite that is used to generate the metalacyclic core typically contains one BlNOLate and one bis-arylethylamino group on phosphorus. 

Mixed imidazolium triazolium salt 24 was readily metalated at the imidazolium heterocycle with either [Pd(OAc)2] or [Rh(OMe)(COD)]2 (COD = 1,5-cyclooctadiene), due to the higher acidity of the imidazolium C2-bound proton compared to the triazolium proton. Extended reaction times for palladation or the use of the acetate analogue of the rhodium precursor, [Rh(OAc)(COD)]2, induced cyclometalation and produced ehelate complexes... 

For ROMP of cyclooctadiene, the activity decreased in the order 55b > 55d > 55a, while complex 55c only reached low conversions, even after several days. These differences in reactivity may be due to an improved steric shielding of the metal center by the mesityl group in 55b. Moreover, phenyl groups may undergo cyclometalation, which would deactivate the catalyst. Kinetic studies suggested that an associative or dissociative initiation mechanism might be involved. These data were further supported by the calculated thermodynamic data for the activation parameters, which agreed very well with the experimental results. 

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