Steric effects oxazoline complexes

Bolm et al. (130) reported the asymmetric Baeyer-Villiger reaction catalyzed by Cu(II) complexes. Aerobic oxidation of racemic cyclic ketones in the presence of pivalaldehyde effects a kinetic resolution to afford lactones in moderate enan-tioselectivity. Aryloxide oxazolines are the most effective ligands among those examined. Sterically demanding substituents ortho to the phenoxide are necessary for high yields. Several neutral bis(oxazolines) provide poor selectivities and yields in this reaction. Cycloheptanones and cyclohexanones lacking an aryl group on the a carbon do not react under these conditions. 

The preparation of carbonyl-lr—NHC complexes (Scheme 3.1) and the study of their average CO-stretching frequencies [7], have provided some of the earliest experimental information on the electron-donor power of NHCs, quantified in terms of Tolman s electronic parameter [8]. The same method was later used to assess the electronic effects in a family of sterically demanding and rigid N-heterocyclic carbenes derived from bis-oxazolines [9]. The high electron-donor power of NHCs should favor oxidative addition involving the C—H bonds of their N-substituents, particularly because these substituents project towards the metal rather than away, as in phosphines. Indeed, NHCs have produced a number of unusual cyclometallation processes, some of which have led to electron-deficient... 

Next, Evans et al. [15] reported that Cu-based catalysts were superior in the Diels-Alder reaction of the oxazolidinone 9 with cyclopentadiene 8. ITie (5,5)-bis(oxazo-line)-Cu(II) and -Zn(II) complexes were very effective catalysts of the reaction. The optimum tert-butyl ligand 13-Cu(II) complex afforded (2S)-endo-ll with > 98 % ee. In contrast, the optimum catalyst system for the phenyl-substituted ligand 12-Zn complex afforded the enantiomeric (R) product, (2R)-endo-ll, with 92 % ee. The different direction of asymmetric induction was explained in terms of the geometry of cata-lyst-dienophile complexes at the corresponding metal centers. The bis(oxazoline)-Zn(II) complex-catalyzed reaction proceeded via the tetrahedral chiral Zn-dienophile complex VIII, in a manner similar to the bis(oxazoline)-Mg catalyst reported by Corey [13], whereas the reaction catalyzed by the cationic bis(oxazoline)-Cu complex proceeded via the square-planar Cu(II)-dienophile intermediate VII, so the diene preferred to approach from the opposite si face of the bound dienophile with s-cis configuration, avoiding steric repulsion by one of the tert-butyl substituents on the oxazoline rings. 

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