NOE Spectroscopy and 3-D Structure

Early NOE studies on cationic Pd(ii) allyl complexes used reporter ligands [97, 98], i. e. simple bidentate nitrogen ligands, such as bipy or phenanthroline, whose 

An NOE study of the intermediate [Pd(ti -PhCHCHCHPh)(Binap)], 82, thought to be involved in the Pd-catalyzed allylic alkylation of a 1,3-diphenylpropene, revealed that two phenyl rings, one from the auxiliary, D, and one from the substrate, F, are forced to take up parallel positions, i.e, they are 7i-stacked, as shown in 83 [103]. Since the ti-stacking is repulsive, and thus selectively weakens one of the two Pd-C(allyl) bonds, the reaction becomes stereoselective. The D and F rings do not show inter-ligand NOEs. 

Daley and Bergens [124] have used this approach to characterize the 3-D structure of the intermediate [Ru(ii)(alkoxide)(CH3CN)(Binap)]BF4, 84, in their study of an enantioselective catalytic hydrogenation of a ketone. The ortho-protons of the equatorial and axial P-phenyl rings provide the reporter protons. 

These NOE studies teach us that many successful P (or N...etc) auxiliaries possess a relatively rigid and intrusive chiral pocket [105, 107]. The shape of this pocket is a function of the individual chelate ligand, i. e., there is no one successful shape. 

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