New phosphorus sulfide cyanides and amides: Assigning phosphorus NMR spectra of polycyclic compounds using low-level ab initio calculations of shieldings

Oxidation of P4S3 with ICN gives exo,exo- and endo,exo-beta -P4S3I2, -beta -P4S3(CN)(2), and -beta -P4S3I(CN) as initial products. Ab initio GIAO calc ulations of NMR shieldings at the RHF/3-21G* level are sufficient to assign the spectrum of endo,exo-beta -P4S3(CN)(2) and to confirm the identity of the observed endo,exo-beta -P4S3I(CN), which has iodine rather than cyanide in the hindered endo position. Reaction of enantiomerically pure (S)-1-phe nyltetrahydroisoquinoline with exo,exo-beta -P4S3I2 gives an exo,exo-diamid e in which the C-s symmetry of the beta -P4S3 cage is lost. The two amido g roups, planar at nitrogen, are each capable of two orientations, giving fou r P-N bond rotamers at 183 K. The P-31 NMR spectra of three of these have b een fitted, and assigned to particular rotamers by ab initio shielding calc ulations.