First direct discrimination of chiral phosphine selenide (P=Se) derivatives by multinuclear magnetic resonance spectroscopy in the presence of a chiral dirhodium complex

Abstract

Enantiomeric ratios of compounds with P=Se functionalities (phosphine selenides) can easily be determined by (1)H, (13)C, (31)P, and (77)Se NMR spectroscopic inspection of their diastereomeric complexes with (R)-Rh(2)(MTPA)(4) (MTPA-H identical with methoxytrifluoromethylphenylacetic acid; Mosher's acid). This is the first facile and rapid spectroscopic method for chiral recognition in this class of compounds. Whereas most complexation shifts Delta(delta) are moderate or even negligible, significant signal dispersions Delta(nu) can be observed. Some rationalization for the adduct formation mode is presented. NMR spectral characteristics of the free P=Se compounds 1-5 are described.

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

“…8 Very recently, we reported that phosphines (R 1 R 2 R 3 P) form long-lived adducts with Rh* at room temperature so that 103 Rh, 31 P couplings give rise to 31 P signal splitting. 9,10 This is in sharp contrast to all other ligands studied previously, including phosphine chalcogenides, 6,11 where the individual adducts may be observable only at low temperatures. 12 In the present study, we report on the extension of our studies to secondary phosphine oxides 1-6 (Scheme 1) which are regarded as convenient precursors to synthesize a variety of valuable organophosphorus compounds as well as ligands per se.…”

“…The CDCl 3 solutions always contain a small portion of acetone-d 6 for the sake of better solubility of Rh*.…”

“…All adduct samples were prepared by mixing 48.6 mg Rh* (0.04 mMol) and the appropriate molar equivalent of the ligand in 0.75 ml CDCl 3 , and 7.5 ml acetone-d 6 were added to increase the solubility of Rh*.…”

“…6 J(P,H-5) f 1.9 (5); 1 J(P,C-2) = 62.7 (4), 63.6 (5), 62.7 (6), 1 J(P,C-1V) = 101.7 [37.0/36.2] (1), 104.6 (2), 99.9 (3), 99.5 (4), 96,1 (5), 98.0 (6), 1 J(P,C-1W) = 101.6 (2), 100.8 (113.9/113.3) [108.9/108.9] (3), 2 J(P,C-2V) = 11.5 [12.1/ 11.9] (1), 11.7 (2), 11.5 (3), 10.7 (4), 10.7 (5), 10.7 (6), 2 J(P,C-1W) = 8.4 (4), 2.7 (5), 2 J(P,C-2W) = 3.6 [2.1/<2.9] (2), 9.8 (6.7/6.9) [9.8/9.9] (3), 2 J(P,C-6W) = 7.1 [11.7/11.9] (2), 2 J(P,C-8aW) = 8. 4 J(P,C-4V) = 2.9 (2.9/3.1) [2.6/2.8] (1), 3.1 (2), 2.9 (3), 2.7 (4), 2.9 (5), 2.9 (6), 4 6 J(P,C-6W) = 0.6 (5). Table 1 contains most 31 P NMR data; complete sets of 1 H and 13 C chemical shifts of ligands 1-6 and couplings involving 31 P are listed in the Experimental section.…”

“…4 During the last few years it was shown that enantiomers of chiral compounds with soft-base functionalities can be discriminated easily by their NMR signals in the presence of an equimolar amount of the chiral auxiliary dirhodium complex Rh 2 [(R)-MTPA] 4 (Rh*, MTPA-H u methoxytrifluoromethylphenylacetic acid; Mosher's acid). 5,6 Due to the formation of diastereomeric adducts most of the ligand's NMR signals are duplicated (diastereotopism), and the distance of the two signals in each pair is called dispersion Dn, in Hz. The ''dirhodium method'' is complementary to the application of the classical chiral lanthanide shift reagents.…”