Isoureas

Isoureas are a class of organic compounds that are constitutional isomers of ureas. The core feature is an isourea linkage, a C–N–N arrangement in which the carbonyl oxygen of urea is replaced by an imine-type nitrogen, giving a motif that can be represented as R1–NH–C(=NH)–NR2. The parent, unsubstituted isourea, is H2N–C(=NH)–NH2. Because of the C=N bond, isoureas are related to amidines and carbamimidates and are sometimes described as amidine-type derivatives without a carbonyl oxygen.

In practice, isoureas are less common and often exist only as fleeting intermediates or in specially stabilized, heavily substituted forms. Substitution on the nitrogen atoms or conjugation with electron-withdrawing groups can enhance stability. The exact structure and tautomeric form can depend on the substituents and the reaction conditions, with isourea alternating with other amidine-like arrangements under some circumstances.

Synthesis and interconversion methods reported in the literature include rearrangements of N,N'-disubstituted ureas under thermal or basic conditions, condensation routes that generate imidoyl or cyanamide intermediates followed by capture with amines, and related transformations involving isocyanates or cyanamides. Because of their general instability and the tendency to equilibrate with other nitrogen-containing motifs, isolable isoureas are typically limited to specific, well-protected or structurally constrained systems.

Applications of isoureas are mainly found in fundamental studies of isomerism in carbonyl chemistry and in the development of routes to amidine- or carbamimidate-type structures. They are of interest for understanding tautomeric equilibria, reactivity patterns of unusual C–N linkages, and as potential reactive intermediates in organic synthesis.