132. A new method of preparation of aryl isothiocyanates

Abstract

Arylthioureas, when heated in chlorobenzene at 150" for 5-10 hr., undergo fission to give good yields of aryl isothiocyanates containing 1, 2, 3, and 4 aromatic rings. The mechanism of the reaction has been investigated. METHODS of preparation of aryl isothiocyanates 1 include (a) the use of thiocarbonyl chloride or its precursor thiocarbonyl tetrachloride (the latter reaction fails with naplithj*l compounds 3), (b) acid-induced fission of an NN'-diarylthiourea, iiivolving the loss of 1 mol. of amine, and (c) decomposition of an ammonium aryldithi~carbamate,~ the last method giving low yields for naphthyl and other compound^.^It has now been found that arylthioureas, when heated in a suitable solvent at 150°, undergo fission into ammonia and the aryl isothiocyanate. No isothiocyanate was obtained on attempted vacuum-distillation of 4-diphenylyl-or a-naphthyl-thiourea without a solvent.the methods of de Clermont 8 and Bertram failed for the latter substance. N-4-Diphenylylthiourea was obtainable by either method, or from N-4-diphenylylammoniuni thiocyanate. The related N-4-diphenylyl-S-methylisothiourea, ammonium N-4-diphenylyldithiocarbamate, and N-Pdiphenylylguanidine were prepared, the last by heating N-4-diphen ylyldiguanide.When N-4-diphenylylthiourea was heated in solvents of various b. p.s, chlorobenzene gave optimum yields of N-4-diphenylyl isothiocyanate ; 1 : 2-dichlorobenzene also aff ordecl the desired product, though in smaller yield, while nitrobenzene or 1 : 2 : 4trichlorobenzene gave no isothiocyanate. N-4-Diphenylyl isothiocyanate had m. p. 63.5--64-5", though Desai, Hunter, and Kureishy lo claim m. p. 119-120".Brewster and Horner 11 claim preparation of this isothiocyanate by 3 hours' boiling of the "'-diary1 thiourea with acetic anhydride. This procedure in our hands gave NN-diacetyl-4-aminodipheny1, m. p. 118.5-11 9", and NN'-diacetyl-NN'-bis-4-diphenylylurea, hydrolysed by alkali to 4-acetamidodiphenyl. Reaction of acetic anhydride with the isothiocyanate afforded the same products. Werner l2 reported the action of acetic anhydride on NN'-diphenylthiourea to give phenyl isothiocyanate in 96,37,7, and 0% yield after 5,30,45, and 60 minutes' boiling respectively, but did not identify the product of decomposition. This isothiocyanate is known l3 to give the N-acylaniline and diphenylurea when treated with acetic or formic acid, while benzoic anhydride gives NN-diben~oylaniline.1~ The mechanism of isothiocyanate formation was investigated by using a-naphthylthiourea. The rate of evolution of ammonia a t 150°, measured during 6 hr., gave the results shown in the Figure . A plot of log,, CAo/(Cao -CNH,) (where CAo = initial concentration of thiourea and CNH, = concentration of ammonia at time t ) was linear only for 1.5 hr., although the " half-time " of the reaction (65 min.) was independent of the magnitude of Cbo. The possibility of the reaction's occurring in three successive steps (a), (b), and (c), with the intermediate formation of a dithiobiuret, could be dismissed in view of the failure of ...

Thioureas

“…A new synthesis for isothiocyanates has been reported (10) which fits into this general category. Monoarylthioureas when heated at 150°C.…”

Condensation reactions of carbon disulfide

“…Piperazine, however, in the absence of added base, forms a bis-dithiocarbamate, isolated as the bis-piperazinium salt. Formation of the mono-dithiocarbamate of piperazine has also been claimed (6) in the absence of added base. Cystamine, however, forms a mono-dithiocarbamate zwitterion (I) in the presence of ammonia (4) It has +NH;,C H2CH,S2CH2CH2N HCSj -I been stated that addition of an aliphatic amine to a suspension of carbon disulfide in ammonium hydroxide gives rise to the ammonium salt of the dithiocarbamate (£>).…”

Thermolysis of 3-phenyl-5-arylamino-1,2,4-oxadiazole and thiadiazole derivatives

“…Another possible pathway for further reaction of the phenylaminothioamidyl radical involves decomposition into ammonia and phenyl isothiocyanate [14] through disproportionation. Scheme 2 also includes homolysis of the C-N bond by route (b) to give 3-phenyl-1,2,4-thiadiazolyl (iib) and anilino radical pairs.…”