Experimental verification of diverging mechanisms in the binding of ether, thioether, and sulfone ligands to a dirhodium tetracarboxylate

Complexation of the oxygen atom in 2-butylphenylethers and sulfur in 2-butylphenylthioethers to a rhodium atom in dirhodium tetracarboxylate Rh(II)2[(R)-(+)-MTPA]4 is compared. Oxygen atoms complex via electrostatic attraction exclusively leading to an increase in α effects on C-2 complexation shifts in the sequence OCH3 > F > Br > NO2. However, that trend is opposite in thioethers. This can be rationalized by an additional highest occupied molecular orbital (HOMO)–LUMO interaction and the response of this interaction upon complex formation shifts. Thereby, an experimental evidence was found for the existence of the HOMO–LUMO binding mechanism which has been proposed previously based on theoretical considerations and indirect spectroscopic evidence. Sulfones hardly bind to Rh(II)2[(R)-(+)-MTPA]4. Diastereomeric dispersion effects at 13C and 1H signals can be observed for all compounds indicating that enantiodifferentiation is easy in all classes of functionalities. Copyright © 2010 John Wiley & Sons, Ltd.