Conversion of Alkynes to Imidazolidin-2-ones: Insight into the Role of Magnesium Amido Complexes in the Reaction

A β-diketiminate-supported magnesium amido complex [LMgN(SiMe3)2·THF] (Mg-1, L = DepNacnac = (DepNCMe)2CH; Dep = 2,6-Et2-C6H3) was synthesized and characterized. Mg-1 efficiently catalyzes the hydroacetylenation of N,N′-diisopropylcarbodiimide (DIC) with terminal alkynes, yielding N,N′-substituted propiolamidines in moderate to good yields (up to 91%). Mechanistic studies identified key intermediates: the reaction proceeds via σ-bond metathesis, forming magnesium acetylide Mg-2, followed by carbodiimide insertion into the Mg–C bond to yield magnesium propiolamidinate Mg-3. Subsequently, these propiolamidines undergo noncatalytic addition with aryl isocyanates under mild conditions, leading to ureas that cyclize to afford imidazolidin-2-ones in a one-pot protocol. The reaction scope and a proposed mechanism for cyclization are presented, demonstrating the utility of this earth-abundant magnesium catalyst for synthesizing valuable heterocycles.