[2 + 2] Cycloaddition reactions of unsymmetrically substituted carbodiimides

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Abstract
N‐Alkyl‐N′‐arylcarbodiimides add alkyl and aryl isocyanates to the N‐alkyl substituted CN double bond to yield 4‐arylimino‐1,3‐diazetidine‐2‐ones 3. In the case of bulky alkyl substituents the reaction still proceeds across the sterically hindered CN double bond. N‐Aryl‐N′‐[(4‐dimethylamino)phenyl]carbodiimides form [2 + 2] cycloadducts with aryl isocyanates preferentially across the CN double bond not attached to the electron‐rich aryl groups. However, steric crowding on this CN double bond directs the reaction to the adjacent double bond of the heterocumulative system. The rate of the [2 + 2] cycloaddition reaction of N'‐methyl‐N′‐phenylcarbodiimide with 4‐nitrophenylisocyanate is about 2.5 times faster in acetonitrile than in benzene.

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