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Phosphorus NMR and Ab Initio Modelling of P–N Bond Rotamers of a Sterically Crowded Chiral β ‐P 4 S 3 Diamide
Author(s) -
Tattershall Bruce W.,
Knight Julian G.,
Andrews Mark J.
Publication year - 2007
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.200700161
Subject(s) - conformational isomerism , steric effects , chemistry , diastereomer , ab initio , substituent , chemical shift , nmr spectra database , crystallography , amide , stereochemistry , coupling constant , nuclear magnetic resonance spectroscopy , ab initio quantum chemistry methods , computational chemistry , spectral line , molecule , organic chemistry , physics , astronomy , particle physics
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.