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

Abstract

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 °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 °C. Overall, the Mn pincer complex is an effective catalyst for the conversion of a variety of formamides and alcohols to carbamates at 150 °C with catalyst loadings as low as 1 mol %. The catalyst performance and substrate scope are comparable to those of the Fe analogue.

Manganese catalysed dehydrogenative synthesis of polyureas from diformamide and diamines

“…Interestingly, the reaction of complex 6 with aniline (toluene, 110 deg, 24 h) did not lead to the formation of any urea derivative suggestive of complex 6 to be an off-cycle intermediate. This is consistent with a recent report by Sanford and co-workers (that was published during the communication of this manuscript) 41 where the reaction of an analogue of complex 6 with cyclohexanol doesn't yield any carbamate derivative. They have suggested that the O-coordinated isocyanate rather than the N-coordinated one (like in complex 6) is the active species which might also be possible in our case.…”

Activated Mn‐MACHO Complexes Form Stable CO2 Adducts

“…Selected bond lengths (in Å) and angles (in °) for 1-CO 2 . : Mn1À N1: 2.0874( 9); Mn1À O2: 2.0968( 9); N1À C1: 1.5060( 14); C1À O1: 1.2183( 14); Mn1À P1: 2.3032( 5); Mn1À P2: 2.2982(5); O 1 À C 1 À O 2 : 130.99 (11).…”

“…Despite extensive investigations of Mn‐MACHO‐type complexes including [ Mn i PrH −Br ] (Figure 1c) as catalysts for CO 2 conversion, [4a,g–i,5] the formation of such adducts has not been reported so far for that class of compounds. Only the corresponding coordination of isoelectronic isocyanates to the M=N unit has been recently issued [11] . In this communication, we report the synthesis and molecular structure of the elusive CO 2 adduct for [Mn i Pr −CO 2 ] ( 1 ‐ CO 2 ) (Figure 1c).…”

“…Only the corresponding coordination of isoelectronic isocyanates to the M=N unit has been recently issued. [11] In this communication, we report the synthesis and molecular structure of the elusive CO 2 adduct for [Mn iPr À CO 2 ] (1-CO 2 ) (Figure 1c). We further explore the influence of the PR 2 group on the stability of the CO 2 adduct and the relative reactivity towards potential coreagents in CO 2 conversion of the adduct 1-CO 2 compared to the parent amido complex 1.…”

Metal-Free N-Formylation of Amines Using Carbohydrates as C1 Synthon via C–C Bond Cleavage

“…N-Formylation of amines is of great importance as these molecules are valuable intermediates in pharmaceutical compounds. 16,17 N-Formyl derivatives also serve as essential precursors for the synthesis of aryl amides, 18 isocyanides, [19][20][21][22][23][24] carbamates, [25][26][27][28] formamidines, 29 ureas, [30][31][32] nitriles, 33 and isocyanates. 34,35 Additionally, N-formyl derivatives has also been employed as critical agents in Vilsmeier formylation reactions, 36 or as protecting groups, 37,38 and these reactions have wide utility in pharmaceutical industry.…”