BINOL-3,3′-trifloneN,N-dimethyl phosphoramidites: Through-space 19F,31P spin-spin coupling with a remarkable dependency on temperature and solvent internal pressure

A combined computational and experimental study of the effects of solvent, temperature and stereochemistry on the magnitude of the through-space spin–spin coupling between ³¹P and ¹⁹F nuclei which are six-bonds apart is described. The reaction of 3-trifluoromethylsulfonyl-2,'2-dihydroxy-1,1'-binaphthalene (3-SO₂CF₃-BINOL) with hexamethylphosphorous triamide (P(NMe₂)₃) generates a pair of N,N-dimethylphosphoramidites which are diastereomeric due to their differing relative configurations at the stereogenic phosphorous centre and the axially chiral (atropisomeric) BINOL unit. Through-space NMR coupling of the ³¹P and ¹⁹F nuclei of the phosphoramidite and sulfone is detected in one diastereomer only. In the analogous N,N-dimethylphosphoramidite generated from 3,3'-(SO₂CF₃)₂-BINOL only one of the diastereotopic trifluoromethylsulfone moieties couples with the ³¹P of the phosphoramidite. In both cases, the magnitude of the coupling is strongly modulated (up to 400?%) by solvent and temperature. A detailed DFT analysis of the response of the coupling to the orientation of the CF₃ moiety with respect to the P-lone pair facilitates a confident assignment of the stereochemical identity of the pair of diastereomers. The analysis shows that the intriguing effects of environment on the magnitude of the coupling can be rationalised by a complex interplay of solvent internal pressure, molecular volume and thermal access to a wider conformational space. These phenomena suggest the possibility for the design of sensitive molecular probes for local environment that can be addressed via through-space NMR coupling.