Phosphorus NMR and Ab Initio Modelling of P-N Bond Rotamers of a Sterically Crowded Chiral β-P₄S₃ Diamide

Abstract:

Reaction of bicyclic β-P₄S₃I₂ with enantiomerically pure (R)-Hpthiq (1-phenyl-1,2,3,4-tetrahydroisoquinoline) and Et₃N gave a solution of a single diastereomer of the unusually stable diamide β-P₄S₃(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, ³¹P NMR multiplets for four rotamers could be observed and the spectra of three of them analysed fully. The large 2J(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-P₄S₃(pthiq)Cl confirmed the assignments of low-temperature rotamers of α-P₄S₃(pthiq)I reported previously. Changes in observed P-P coupling constants and ³¹P chemical shifts, on rotating a pthiq substituent, could then be compared between β-P₄S₃(pthiq)₂ and α-P₄S₃(pthiq)I, confirming both sets of assignments. The most abundant rotamer of β-P₄S₃(pthiq)₂ was not the one with the least sterically crowded sides of both pthiq substituents pointing towards the P₄S₃ cage, because of interaction between the two substituents. Only by using a DFT method could relative abundances of rotamers of β-P₄S₃(pthiq)₂ be predicted to be in the observed order. Use of racemic Hpthiq gave also the two diastereomers of β-P₄S₃(pthiq)₂ with Cs symmetry, for which the room temperature ³¹P{1H} NMR spectra were analysed fully.