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Carboximidates (or more generalimidates) are organic compounds, which can be thought of asesters formed between aimidic acid (R-C(=NR')OH) and analcohol, with the general formula R-C(=NR')OR".
They are also known asimino ethers, since they resembleimines (>C=N-) with an oxygen atom connected to the carbon atom of the C=N double bond.[1]
Imidates may be generated by a number of synthetic routes,[2] but are in general formed by thePinner reaction. This proceeds via the acid catalyzed attack of nitriles by alcohols.
Imidates produced in this manner are formed as their hydrochloride salts, which are sometimes referred to as Pinner salts. Carboximidates are also formed as intermediates in theMumm rearrangement and theOverman rearrangement.
Anamidate/imidate anion is formed upondeprotonation of anamide orimidic acid. Since amides and imidic acids aretautomers, they form the same anion upon deprotonation. The two names are thus synonyms describing the same anion, although arguably, imidate refers to the resonance contributor on the left, while amidate refers to the resonance contributor on the right. However, they are distinguished when they act as ligands for transition metals, withO-bound species referred to as imidates andN-bound species referred to as amidates. They can be considered aza-substituted analogues ofenolates with the formula R-N=C(O−)R.
Carboximidates are goodelectrophiles and undergo a range of addition reactions; withaliphatic imidates generally reacting faster thanaromatic imidates.[2] They can behydrolyzed to giveesters and by an analogous process react with amines (including ammonia) to formamidines. Aliphatic imidates react with an excess of alcohol under acid catalysis to formorthoesters RC(OR)3, aromatic imidates can also be converted but far less readily.
TheChapman rearrangement is the thermal conversion of arylN-arylbenzimidates to the corresponding amides, via intramolecular migration of an aryl group from oxygen to nitrogen.[4] It is named afterArthur William Chapman, who first described it,[5] and is conceptually similar to theNewman–Kwart rearrangement.
Carboximidates can act asprotecting group for alcohols.[6] For example, the base catalyzed reaction ofbenzyl alcohol upontrichloroacetonitrile yields atrichloroacetimidate. This species has orthogonal stability to acetate andTBS protections and may be cleaved by acid hydrolysis.[7]