Abstract
A mild, efficient, and practical method for one-step synthesis of alkanoyl and aroyl isothiocyanates from carboxylic acids using a safe and inexpensive mixed reagent, trichloroisocyanuric acid/triphenyl- phosphine is described at room temperature. Availability of the reagents and easy workup of the reaction make this method attractive for organic chemists.
Highlights
By considering the activity of trichloroisocyanuric acid (TCCA) and N-chlorobenzotriazole (NCBT) towards triphenylphosphine, recently we reported direct preparation of acyl azides, esters, thioesters and sulfonyl azides from carboxylic acids and sulfonic acids under mild and neutral reaction conditions by using the above mentioned mixed reagents.[19,20,21]
In this article we report a more simplified one-step method for the conversion of carboxylic acids to acyl isothiocyanates (Scheme 1)
Complete conversion of benzoic acid was obtained in 40 min when the isothiocyanation reaction was examined in toluene by using 1 /0.3 /0.8 /2 molar ratio of TPP/TCCA/RCO2H/KSCN (Table 1, entry 20)
Summary
Acyl isothiocyanates have found wide application in the synthesis of various acyclic and heterocyclic compounds, including those possessing biological activity.[1,2,3]The methods of preparation of isothiocyanates are limited.[2] many methods have been described for the preparation of acyl isothiocyanates.[4,5,6,7,8,9,10,11,12,13,14] which are mainly based on the reaction of acyl halides with thiocyanate ion.[2,6] Acyl halides are not always easy to access or store. 2-(2,4-Dichlorophenoxy) Ethanoyl Isothiocyanate (Table 2, entry 19) Oil; IR(neat) ῡmax / cm–1: 3072, 2978, 2892, 1956 (N=C=S), 1731 (C=O), 1589, 1477, 1311, 1234, 1090, 910, 796, 718, 641; 1H NMR (400 MHz, CDCl3) δ /ppm: 2.35 (s, 2H, CH2), 8.15-8.17 (m, 3H, ArH); 13C NMR (100 MHz, CDCl3) δ/ ppm: 162.5 (C=O), 154.1 (N=C=S), 136.4, 131.8, 131.2, 128.5, 121.5, 115.2, 21.1; MS m / z: 264 (M++4, 5 %); 262 (M++2, 10 %), 260 (M+, 18 %); 174 (M+–CONCS, 10 %) 147 (PhCl2, 100 %).
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