Metal Identity Effects in the Pincer Complex-Catalyzed Dehydrogenative Coupling of Formamides with Alcohols to Form Carbamates

This manuscript describes the impact of metal (ruthenium versus iron versus manganese) on the PNP pincer complex-catalyzed dehydrogenative coupling of formamides with alcohols to yield carbamates. Studies are conducted to compare the structure and reactivity of key organometallic intermediates as a function of metal. These reveal that all three pincer catalysts undergo fast reaction with isocyanates (the initial organic products of formamide dehydrogenation) to form off-cycle metallacyclic species. However, the structures and reactivities of these intermediates vary as a function of metal. The N-bound isomer of the metallacycle is formed with Fe (literature) and Ru (this work), while both O-bound (kinetic) and N-bound (thermodynamic) metallacycles are observed with Mn (this work). At 100 degrees C, the O-bound Mn cycloadduct reacts rapidly with cyclohexanol to release the pincer catalyst along with the corresponding carbamate. In contrast, all three of the N-bound metallacycles show dramatically lower reactivity toward catalyst regeneration under analogous conditions. The reactivity of the pincer complexes toward formamide dehydrogenation also varies as a function of metal, with Ru and Fe >> Mn at 100 degrees C. Overall, the Mn pincer complex is an effective catalyst for the conversion of a variety of formamides and alcohols to carbamates at 150 degrees C with catalyst loadings as low as 1 mol %. The catalyst performance and substrate scope are comparable to those of the Fe analogue.

Automated summary

This manuscript investigates the impact of different metals (ruthenium, iron, and manganese) on the PNP pincer complex-catalyzed dehydrogenative coupling reaction. The study reveals that the structures and reactivities of the intermediates vary depending on the metal used. It is found that the manganese pincer complex is an effective catalyst for the conversion of formamides and alcohols to carbamates under specific conditions.