A non-Karplus effect: evidence from phosphorus heterocycles and DFT calculations of the dependence of vicinal phosphorus-hydrogen NMR coupling constants on lone-pair conformation.

In contrast to literature reports of a Karplus-type curve that correlates (3)J(PH) with phosphorus-hydrogen dihedral angle, a recently reported glycine-derived 1,3,2-oxazaphospholidine (7c) has two hydrogen atoms on the ring with identical PNCH dihedral angles but measured coupling constants of ∼6 and 1.5 Hz. DFT calculations were in accord with these values and suggested that the smaller coupling constant is negative. Experimental evidence of the opposite signs of these coupling constants was obtained by analysis of the ABX NMR spectrum of the new glycine-derived N-p-toluenesulfonyl phosphorus heterocycle 6c. DFT calculations on 6c and on Me(2)NPCl(2) and t-BuPCl(2) were also in accord with NMR data and allowed confirmation of unusual features including a lone pair effect on (3)J(PH), the negative coupling constant, temperature-dependent chemical shifts due to rotation about the sulfonamide S-N bond, and vicinal phosphorus-hydrogen coupling constants over 40 Hz. Calculation of phosphorus-hydrogen coupling constants both as a function of PYCH dihedral angle θ (Y = O, N, C) and lone pair-PYC dihedral angle ω shows similar θ,ω surfaces for (3)J(PH) with a range of (3)J(PH) from -4.4 to +51 Hz and demonstrates the large non-Karplus effect of lone-pair conformation on vicinal phosphorus-hydrogen coupling constants.