Secondary phosphine oxides: tautomerism and chiral recognition monitored by multinuclear NMR spectroscopy of their Rh₂[(R)‐MTPA]₄ adducts

Abstract

Six secondary phosphine oxides and their tautomeric equilibria as free ligands and in the presence of an equimolar amount of the chiral dirhodium complex Rh* are described and discussed. Discrimination of enantiomers is easily possible by inspecting the (31)P NMR resonances; some (1)H and (13)C NMR resonances are useful as well. H/D exchange of the acidic protons in the phosphine oxides takes place with acetone-d(6), the solvent additive, after some hours but does not obscure the chiral recognition experiment. (103)Rh,(31)P coupling constants are discussed briefly. Decomposition of ligand molecules in 1:1-Rh*-adducts occurs slowly but completely.

Highly Selective and Solvent-Dependent Reduction of Nitrobenzene to N-Phenylhydroxylamine, Azoxybenzene, and Aniline Catalyzed by Phosphino-Modified Polymer Immobilized Ionic Liquid-Stabilized AuNPs

“…The gold loadings of precatalysts 2a and 2a.PEG were determined to be 0.037 mmol g -1 and 0.16 mmol g -1 , respectively, using ICP-OES. The solid state 31 P NMR spectra of 2a and 2a.PEG confirm the presence of a Au----P interaction which is clearly evident from the low field chemical shifts of  27.1 and 24.5 ppm, respectively; for comparison the 31 P NMR signals for polymers 1a and 1a.PEG appear at  -5 and -10.4 ppm, respectively (see ESI file for full details); these complexation shifts 39 are similar to previously reported values. 40 The 13 C CP/MAS NMR spectra of 1a and 1a.PEG and 2a and 2a.PEG contain a set of characteristic signals between  124-144 ppm associated with the imidazolium ring and the aromatic carbon atoms as well as signals in the range  11-50 ppm which correspond to the methyl groups attached to the imidazolium ring and the aliphatic carbon atoms of the polystyrene backbone (see ESI file for full details).…”

Rh2[MTPA]4, a dirhodium complex as NMR auxiliary for chiral recognition

“…It was shown that the tautomer A is generally favored in the free ligand whereas the equilibrium is strongly shifted toward the tautomer B in the presence of Rh * (1 : 1 molar ratio) including 103 Rh, 31 P coupling information in the 31 P NMR signal. This is a proof for the much larger binding energy in the latter case 23…”

Coordination chemistry and catalysis with secondary phosphine oxides

“…This was proved by deuterium exchange experiments and IR spectroscopic measurements in the 1960s. 15 In 2004, Pietrusiewicz, Duddeck and co-workers suggested 16 from NMR studies that the fast kinetics of the tautomeric equilibrium is due to the migration of an acidic proton. In this line, Hong and co-workers 17 proposed two reaction pathways for the transformation of SPOs to the corresponding PA (Scheme 2), based on DFT calculations.…”