Abstract
The absolute configuration and conformations of (−)-tert-butylphenylphosphinoamidate were determined using three different chiroptical spectroscopic methods, namely vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and optical rotatory dispersion (ORD). In each of the spectroscopic methods used, experimental data for the (−)-enantiomer of tert-butylphenylphosphinoamidate were measured in the solution phase. Using the concentration-dependent experimental infrared spectra, the existence of dimers in the solution was investigated, and the monomer–dimer equilibrium constant was determined. Concomitant quantum mechanical predictions of the VCD, ECD, and ORD for monomeric tert-butylphenylphosphinoamidate were carried out using density functional theory (DFT) calculations using the B3LYP functional and the 6-31G(d), 6-311G(2d,2p) and aug-cc-pVDZ basis sets. Similar predictions for dimeric tert-butylphenylphosphinoamidate were also obtained using the B3LYP/6-31G(d) method. A comparison of theoretically predicted data with the corresponding experimental data led to the elucidation of the absolute configuration as (−)-(R)-tert-butylphenylphosphinoamidate with one predominant conformation in the solution. This conclusion was independently supported by X-ray analysis of the complex with (+)-R-2,2′-dihydroxy-1,1′-binaphthol ((+)-R- BINOL).
Highlights
Over the past decade, a diverse assortment of chiral phosphorus reagents has been identified,[1−5] which subsequently resulted in substantial progress in application of these reagents in a variety of stoichiometric and catalytic versions of asymmetric and stereoselective synthesis
We report the determination of the absolute configuration and conformations of (−)-tert-butylphenylphosphinoamidate 1 (Figure 1) by three different chiroptical spectroscopic methods, namely
The electronic circular dichroism (ECD) spectra measured for (−)-1 in the CHCl3 solvent shows three weak positive bands with distinct maxima at 270, 264, and 257 nm, while the calculation with the aug-cc-pVDZ basis set indicates the longest wavelength transitions at 247 and 240 nm (Figure S1). It is well-known18,19a that the bands characteristic of the vibrational fine structure are associated with the π → π* electronic transition of phenyl chromophore, which is forbidden18,19a,20,21 but appears due to vibronic borrowing.19a,22b Such bands cannot be seen in the predicted spectra unless vibrational coupling is included in the theoretical predictions of the ECD
Summary
A diverse assortment of chiral phosphorus reagents has been identified,[1−5] which subsequently resulted in substantial progress in application of these reagents in a variety of stoichiometric and catalytic versions of asymmetric and stereoselective synthesis. Within the rich family of phosphorus compounds, phosphonic amides or phosphinamides were identified as useful reagents.[6−9] A series of phosphinamides was reported,3a,b which act as novel catalysts for the asymmetric reductions of ketones by borane. These studies indicated that phosphinamides act as Lewis bases that activate borane and effectively accelerate the reduction through the donation of electron density. Chan et al.[11] suggested the use of phosphinic amides to modulate the activity of the endothelin family of peptides, which is responsible for causing vasoconstriction This modulation potentially allows the treatment of endothelin-mediated disorders. The tautomers and conformers of 1 were investigated, and the predominant tautomer and its conformation were determined
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