Origin of ¹³C complexation shifts in the adduct formation of 2‐butyl phenyl ethers with a dirhodium tetracarboxylate complex

Abstract

Complexation of the oxygen atom in 2-butyl phenyl ethers to a rhodium atom of the dirhodium tetracarboxylate Rh(II) 2[(R)-(+)-MTPA]4(Rh*, MTPA-H = methoxytrifluoromethylphenylacetic acid identical with Mosher's acid) deshields an sp3-hybridized 13C nucleus directly bonded to the ether oxygen; apparently, the inductive effect of the oxygen is enhanced when it is complexed to the rhodium atom. On the other hand, deshielding complexation shifts of aromatic ipso-carbons (alpha-positioned) are minute but ortho- and para-carbon signals are influenced by the resonance effect of oxygen. This effect can be modulated by further substituents at the benzene ring. In turn, this modulation of the resonance correlates linearly ith the magnitude of the inductive effect exerted on the aliphatic alpha-carbon atoms. Diastereomeric dispersion effects at 13C signals can be observed for most compounds, indicating that enantiodifferentiation is possible in this class of ethers.

Chiral recognition of ethers by NMR spectroscopy

“…100 This finding and the good correlation between the C-2 0 /6 0 complexation induced shifts and the -parameters of X 102 (Scheme 25) confirm the aforementioned resonance interaction model of donor ability modulation.…”

“…99 2-Butyl phenyl ethers bind to the dirhodium complex 52, too, as can be monitored from their complexations shifts Dd in the standard dirhodium experiment (Table 4). 100 As expected, positive Dd-values at the directly attached aliphatic carbon C-2 indicate adduct formation but the donor ability of the ether oxygen atom is modulated by the parasubstituent. This is not surprising when the HOMOs of those molecules are inspected (Scheme 24); note that in unsaturated molecules, the HOMO-LUMO energy has a major influence on the paramagnetic contribution of the nuclear 13 C shielding constant which, in turn, dominates the total shielding.…”

Comparing various chiral dirhodium tetracarboxylates in the dirhodium method

“…[1][2][3][4][5] However, in contrast to Rh1, the complex Rh2 fails with weak ligands (e.g., 3 and 4). Therefore, Rh2 cannot be regarded a generally applicable auxiliary because before the experiment one has to consider and estimate the binding properties of the ligand molecule to be studied.…”

“…In addition, moderate 1 H and 13 C deshielding occurs if those nuclei are close to the binding site in terms of intervening bonds. 5 This parameter, called complexation shift Dd, is an excellent indicator as to which Lewis-basic atom forms the bond to one of the rhodium atoms in the complex. If, however, a hydrogen atom is situated above or below an aromatic ring of the MTPA residues, it is shielded, i.e., its signal is shifted to lower frequencies and d-values, due to the anisotropy (ring-current) effect.…”

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

“…The assignment strategies were analogous to those reported for the ethers 1a-1e. [7] 13 C and 1 H chemical shifts are listed in Table 1. 13 C and 1 H complexation shifts δ and diamagnetic dispersion effects ν provoked by adduct formation in the presence of an equimolar amount of Rh * are collected in Table 2; NMR data of 1e, which have not been reported before, [7] are given in the Section on Experimental.…”