13C NMR spectra of phosphole–P(IV) dimers; ABX and AA′X 13C-31P coupling in some derived structures

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Abstract
The easily obtained dimers of phosphole oxides, sulfides and methiodides give 13C NMR spectra where carbons within three (sometimes four) bonds of each 31P nucleus are doublets of doublets and thus constitute X in an AMX spectrum. Most of the spectra have been completely interpreted with the aid of spectral measurements at two magnetic fields. Saturation of the double bonds in dimers of methylphosphole–P(IV) derivatives causes the 31P nuclei to have very similar chemical shifts, with Δν not adequately different from 3J(PP) to give first‐order coupling. When both 31P nuclei couple with a given 13C, a second‐order (ABX) 31C NMR spectrum is obtained. The presence of the effect is revealed by running the 13C NMR spectra at high magnetic field; J(AX)+J(BX) is constant at all fields, but spacing between the lines of the multiplet varies. The spectrum of the oxide, with Δν/J=1.44 for the 31P spectrum at 36.43 MHz, approaches first order character at 75 MHz; the methiodide spectrum (Δν/J=4.55) is second order at 15 MHz but clearly first order at 50 MHz, and the sulfide (Δν/J=5.6) is nearly first order at 15 MHz. [2 + 2]‐Photochemical intramolecular cyclization of the dimer oxides provides cage‐like structures where the 31P nuclei are chemically equivalent, but magnetically non‐equivalent, making the 13C signals have the characteristics of X in an AA′X coupling pattern.

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