A Convenient Method for the Synthesis of Sterically Hindered Aryl Isocyanates

Several methods have been reported for the syn­ thesis of isocyanates, most of them involving the use of phosgene: e.g. phosgene and primary amines [1] either in basic [2] or in acidic medium [1]; phosgene and diaryl urea [1]; phosgene and l-alkyl-3-arylsulphonyl urea [3]; or phosgene and sulfinyl amines [4], Isocyanates were further synthesized from olefins or alkyl halides by the use of isocyanic acid [5]. Methods involving the thermal decomposition of carbamic acid esters [6] or N-organo-N-silyl carbamic acid esters [7] have also been described. In the latest literature, isocyanates were reported to have been prepared by catalytic oxidation of isothiocyanates [8]. For further details, the reader is referred to re­ cent reviews [5, 9, 10]. According to patents [11], isocyanates can be ob­ tained by thermal decomposition of trisubstituted ureas in the presence of acids. However, the thermal decomposition of 1,3-disubstituted ureas was re­ ported to be inconvenient as a synthetic method due to the recombination of the produced isocyanates and amines [9, 12]. Only the thermal decomposition of ureas derived from weakly basic amines [1, 9, 13, 14] or of acylor sulfonylurea derivatives [10] was reported to be of synthetic value, producing the re­ quired isocyanate and the weakly basic amine, acid amide or sulfonamide, respectively. Furthermore, heating of the urea derived from a low boiling amine with a high boiling isocyanate yielded the more volatile isocyanate, which could be removed from the reaction mixture by distillation, and the urea derived from the high boiling isocyanate