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18.2.16.4.1.4.1 Phosgenation of N-Silylamines

DOI: 10.1055/sos-SD-118-00330

Braverman, S.; Cherkinsky, M.Science of Synthesis Knowledge Updates, (20143263.

Phosgenation of N-silylated amines has some advantages over phosgenation of free amines in terms of high yields and mild reaction conditions and is particularly suitable for the preparation of acid-sensitive and/or thermally unstable isocyanates. The reaction proceeds via formation of unstable intermediate N-silylcarbaminoyl chlorides, which undergo intramolecular β-elimination at relatively low temperatures and afford the corresponding isocyanates in high yields. This procedure has been applied to the preparation of a wide range of isocyanates, including isocyanates of the furan, thiophene, and fluorophenyl and polyfluorophenyl series.[‌166‌] Solutions of N-trimethylsilyl derivatives of tetrahydrofurfurylamine, furfurylamine, substituted thiophen-2-amines, and polyfluoroanilines are first prepared by silylation of the starting amines with hexamethyldisilazane or chlorotrimethylsilane in toluene or diglyme. Next, treatment of the mixture with a 25% toluene solution of phosgene in a 10% excess at 5–20 °C provides the corresponding isocyanates 59 after isolation by vacuum distillation (Scheme 66). Similarly, aliphatic, carbocyclic, and arylalkyl isocyanates can be prepared from the corresponding N-silyl derivatives.[‌167‌] The yield of isocyanates decreases when going from aliphatic amines to benzyl­amines and rises as the length of the alkyl chain in (arylalkyl)amines is extended.

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