Phosphorus NMR and ab initio modelling of P-N bond rotamers of a sterically crowded chiral β-P 4 S 3 diamide

Academic Article

Abstract

  • Reaction of bicyclic β-P 4 S 3 I 2 with enantiomerically pure (R)-Hpthiq (1-phenyl-1,2,3,4-tetrahydroisoquinoline) and Et 3 N gave a solution of a single diastereomer of the unusually stable diamide β-P 4 S 3 (pthiq) 2 , accounting for 83 % of the phosphorus content. Despite the steric bulk of the substituents, each amide group of this could adopt either of two rotameric positions about their P-N bonds, so that, at 183 K, 31 P NMR multiplets for four rotamers could be observed and the spectra of three of them analysed fully. The large 2 J(P-P-P) coupling became greater (253, 292, 304 Hz) with decreasing abundance of the individual rotamers. The rotamers were modelled at the ab initio RHF/3-21G* level, and relative NMR chemical shifts predicted by the GIAO method using a locally dense basis set, allowing the observed spectra to be assigned to structures. Calculations at the same level for the model compound α-P 4 S 3 (pthiq)Cl confirmed the assignments of low-temperature rotamers of α-P 4 S 3 (pthiq)I reported previously. Changes in observed P-P coupling constants and 31 P chemical shifts, on rotating a pthiq substituent, could then be compared between β-P 4 S 3 (pthiq) 2 and α-P 4 S 3 (pthiq)I, confirming both sets of assignments. The most abundant rotamer of β-P 4 S 3 (pthiq) 2 was not the one with the least sterically crowded sides of both pthiq substituents pointing towards the P 4 S 3 cage, because of interaction between the two substituents. Only by using a DFT method could relative abundances of rotamers of β-P 4 S 3 (pthiq) 2 be predicted to be in the observed order. Use of racemic Hpthiq gave also the two diastereomers of β-P 4 S 3 (pthiq) 2 with C s symmetry, for which the room temperature 31 P{ 1 H} NMR spectra were analysed fully. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.
  • Authors

    Digital Object Identifier (doi)

    Author List

  • Tattershall BW; Knight JG; Andrews MJ
  • Start Page

  • 1442
  • End Page

  • 1450
  • Volume

  • 633
  • Issue

  • 9