³¹P NMR Study of P-Chirogenic Phosphaphenanthrenes with Molecular Flexibility tuned by Amino Acid Substituents

Model phosphonamidates derived from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) with molecular flexibility tuned by amino acid substituents were prepared as pairs of diastereoisomers (DSIs) differing in configuration on phosphorus atom. X-ray diffraction (XRD) determined absolute configuration on phosphorus and revealed conformational flexibility of six-membered oxa-phospha-cycle. Quantum-chemical calculations combined with machine learning provided 2-4 representative conformers from each DSI present in solution. ³¹P chemical shift of RS DSIs was higher compared to the SS ones. Calculated ³¹P-¹H as well as ³¹P-¹³C J-couplings followed Karplus dependence of J-coupling values on dihedral angles between interacting nuclei, showing reasonable match with the literature data. The effect of molecular flexibility quantified by parameter nConf₂₀ on NMR parameters was significant, especially for residual dipolar couplings (RDCs), where Pearson correlation factor R decreased with increasing nConf₂₀ parameter.