Evidence for the formation of isothiocyanate during sulphurisation of triphenyl phosphines using xanthane hydride

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6. 3-Amino-1,2,4-dithiazole-5-thione (1) (1 g, 6.7 mmol) was
dissolved in 400 ml of acetonitrile under a nitrogen
atmosphere at 30 C and a solution containing 1.75 g
(6.7 mmol) of triphenylphosphine in 100 ml of acetonitrile
was added in one portion. The reaction mixture was
stirred for 5 min and then 0.92 g (6.7 mmol) of 4-nitroaniline
in 20 ml acetonitrile was added. After 15 h, the
acetonitrile was removed and the solid residue was quickly
extracted with 3 · 40 ml of 3% aqueous potassium
hydroxide solution. After filtration, the insoluble material
which was practically pure triphenylphosphine sulfide
(1.85 g, 94%) was recovered. The filtrate was immediately
neutralised by concentrated HCl (pH = 3). Precipitated 1-
(4-nitrophenyl)dithiobiuret was filtered off and dried.
Yield 1.65 g (97%). Mp 168–170 C, 1H NMR (DMSOd6,
500 MHz) d 7.97 (AA0XX0, J 9.05, 2H, Ar–2H), 8.25
(AA0XX0, J 9.1, 2H, Ar–2H), 9.1 and 9.42 (2 · br s, 2H,
NH2), 11.01 (br s, 1H, NH), 13.22 (br s, 1H, NH). 13C
NMR (DMSO-d6, 125 MHz) d 123.3, 124.6, 143.7, 144.3,
177.3, 179.2. m/z (ESI) 255.0016 (M, C8H7N4O2S2
requires 255.0005).
The same procedure was carried with trimethyl phosphite
instead of triphenylphosphine. Yield of 1-(4-nitrophenyl)-
dithiobiuret was 1.35 g (80%).
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