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

Abstract

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 alpha effects on C-2 complexation shifts in the sequence OCH(3) > F > Br > NO(2). 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 (13)C and (1)H signals can be observed for all compounds indicating that enantiodifferentiation is easy in all classes of functionalities.

Stereoisomeric 2-butylphenylsulfoxides and their binding modes in the adduct formation with an enantiopure dirhodium tetracarboxylate complex

“…In order to synthesize all enantiomerically pure isomers of 1a-1e and 2a-2e, we started from the two commercially available enantiopure 2-butanols and prepared the respective 2-butylphenyl and their para-substituted derivatives 4a-4e [10]. Oxidation [12] of those thioethers resulted in mixtures of two diastereomeric, enantiopure sulfoxides in each case (x = a, b, c, d, and e; Scheme 2):…”

“…The syntheses of Rh* [4], the ethers 3 [8] and the thioethers 4 [10] have been described by us earlier. The sulfoxides 1a-1e and 2a-2e were prepared by H 2 O 2 oxidation of the thioethers 4a-4e, respectively [12].…”

“…Whereas ethers are hard ligands, thioethers are soft [11] and represent a different ligand category in the dirhodium experiment as compared to ethers [5]; i.e., thioethers can make use of an additional orbital interaction (HOMO-LUMO) for complexation. Indeed, this can be monitored by inspecting the dependence of complexation shifts at the aliphatic a carbon (C-2) and the ortho-carbon (C-2 0 ) on Hammetts inductive and resonance parameters, respectively [10]. For each carbon site and each parameter the correlations are opposite as compared to those of ethers, and this can be rationalized semi-quantitatively by effects of HOMO-LUMO energy changes on 13 C chemical shifts [10].…”

“…Indeed, this can be monitored by inspecting the dependence of complexation shifts at the aliphatic a carbon (C-2) and the ortho-carbon (C-2 0 ) on Hammetts inductive and resonance parameters, respectively [10]. For each carbon site and each parameter the correlations are opposite as compared to those of ethers, and this can be rationalized semi-quantitatively by effects of HOMO-LUMO energy changes on 13 C chemical shifts [10].…”

“…In order to gain further insight into the complexation mechanisms of chalcogen ligands, we extended this study to structurally analogous thioethers 4 (Scheme 2) [10]. Whereas ethers are hard ligands, thioethers are soft [11] and represent a different ligand category in the dirhodium experiment as compared to ethers [5]; i.e., thioethers can make use of an additional orbital interaction (HOMO-LUMO) for complexation.…”