Degradation of CYANEX 301 in Contact with Nitric Acid Media

The nature of the degradation product obtained upon contacting
CYANEX 301 (bis­(2,4,4-trimethylpentyl)­dithiophosphinic acid) with
nitric acid has been elucidated and found to be a disulfide derivative.
The first step to the degradation of CYANEX 301 in toluene has been
studied using 31P­{1H} NMR after being contacted
with nitric acid media. The spectrum of the degradation product exhibits
a complex multiplet around δP = 80 ppm. A succession
of purifications of CYANEX 301 has resulted in single crystals of
the acidic form and the corresponding ammonium salt. Unlike the original
CYANEX 301, which consists of a complex diastereomeric mixture displaying
all possible combinations of chiral orientations at the 2-methyl positions,
the purified crystals were shown by single-crystal X-ray diffraction
to be racemates, containing 50:50 mixtures of the [R;R] and [S;S]
diastereomers. The comparison between the 31P {1H} NMR spectra of the degradation products resulting from the diastereomerically
pure CYANEX 301 and the original diastereomeric mixture has elucidated
the influence of the isomeric composition on the multiplicity of the 31P {1H} NMR peak. These NMR data indicate the initial
degradation leads to a disulfide-bridged condensation product displaying
multiple resonances due to phosphorus–phosphorus coupling,
which is caused by the inequivalence of the two P atoms as a result
of their different chirality. A total of nine different NMR resonances,
six of which display phosphorus–phosphorus coupling, could
be assigned, and the identity of the peaks corresponding to phosphorus
atoms coupled to each other was confirmed by 31P {1H} homodecoupled NMR analysis.