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18.2.4.2.1.1 Variation 1: Reactions with Heteroatom Nucleophiles

DOI: 10.1055/sos-SD-018-00070

Braverman, S.; Cherkinsky, M.; Birsa, M. L.Science of Synthesis, (200518149.

Reactions of isocyanates with nucleophiles usually follow Scheme 135. The mechanism of this reaction has been investigated thoroughly.[‌547‌] Nucleophilic reagents rapidly attack the electrophilic carbon of the isocyanate group; aromatic isocyanates react more rapidly than aliphatic. Steric as well as electronic effects influence the rate of reaction, which is increased by electron-withdrawing substituents in the aromatic ring and reduced by electron-donating groups. Aliphatic isocyanates react in the following order: primary > secondary > tertiary. Steric hindrance in the attacking nucleophile similarly influences the reaction rate. Alcohols react in the following order: primary > secondary > tertiary. The reactivity of an attacking nucleophile is proportional to its nucleophilicity: aliphatic amines > aromatic amines > alcohols > benzenethiols > phenols. The reactivity of the slower reacting phenols and benzenethiols can be increased considerably by using the corresponding anions. Isocyanates react with water to give a carbamic acid, which undergoes decarboxylation to form carbon dioxide and amine. The latter immediately reacts with more isocyanate to yield a disubstituted urea. With amines, isocyanates react very fast and quantitatively to produce disubstituted ureas. The reaction of isocyanates with alcohols and phenols to form carbamates is reversible and catalyzed by tertiary amines. Carboxylic acids react with isocyanates to give mixed anhydrides, which decarboxylate to amides at elevated temperature. Most of these transformations are subject to catalysis by bases, acids, Lewis acids, and certain metal compounds, of which tertiary amines and tin carboxylates are used on large scale in industrial production of polyurethanes from diisocyanates and polyalcohols.

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