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18.2.16.4.1.3.3 By Oxalyl Chloride

DOI: 10.1055/sos-SD-118-00330

Braverman, S.; Cherkinsky, M.Science of Synthesis Knowledge Updates, (20143262.

Oxalyl chloride can also be used as a phosgene substitute to prepare isocyanates. Usually, amino-substituted heterocycles undergo smooth carbonylation with this reagent (see Section 18.2.4.1.3.5). More recently, oxalyl chloride has been applied for the generation of aryl isocyanates from anilines.[‌163‌] Acylation of a variety of substituted aniline hydrochlo­rides with oxalyl chloride affords the intermediate oxamic chlorides. Employing the hydrochloride salt of aniline, the nucleophilicity of aniline is sufficiently attenuated to provide the desired oxamic chloride in good yield but formation of the bisoxamide byproduct is suppressed. Once the aniline hydrochloride substrate is fully consumed, the intermediate oxamic chloride is heated to 130 °C in a nonreactive solvent, such as chlorobenzene or dichlorobenzene, to effect a smooth thermal decomposition to the corresponding phenyl isocyanate (56) in 2–3 hours (Scheme 64). The reaction proceeds well with both electronegative and electropositive substituents on the phenyl ring. Due to the instability of the products, the in situ generated aryl-substituted isocyanates are isolated as the corresponding methyl carbamate derivatives.

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