Conformational analysis of p-X-anilino dioxaphosphinanes. Substituent effects on 31P and 15N NMR signals and on negative hyperconjugation (n–σ*)

The conformational analysis of anancomeric cis-ax and cis-eq 2-p-X-anilino-2-thio-4,6-dimethyl-1,3,2λ5-dioxaphosphinanes (X=OCH3, C6H11, H, Cl, CN and NO2) is informed. In accordance with 3JHH, 3JHP, 4JHP, and 3JCP coupling constants the preferred conformation in solution is a chair in both series of compounds. Structural parameters obtained through X-ray diffraction studies of the series of cis-ax and cis-eq diastereomers 1–6, suggest that the stabilization of the axial and equatorial diastereomers in chair conformation rely on stereoelectronic nπO–σ*P–N and nπN–σ*P–O interactions, respectively. Theoretical Kohn–Sham DFT calculations support the participation of the cited stereoelectronic interactions not only to understand the conformational behavior of these systems but also to give an explanation of the observed substituent-induced chemical shift (SCS) on 31P and 15N NMR signals.