Hydrogenation of Imines Catalyzed by Trisphosphine‐Substituted Molybdenum and Tungsten Nitrosyl Hydrides and Co‐Catalytic Acid

Abstract

Hydride complexes Mo,W(CO)(NO)H(mer-etp(i)p) (iPr2PCH2CH2)2PPh=etp(i)p) (2 a,b(syn), syn and anti of NO and Ph(etp(i)p) orientions) were prepared and probed in imine hydrogenations together with co-catalytic [H(Et2O)2][B(C6F5)4] (140 °C, 60 bar H2). 2 a,b(syn) were obtained via reduction of syn/anti-Mo,W(NO)Cl3(mer-etp(i)p) and syn,anti-Mo,W(NO)(CO)Cl(mer-etp(i)p). [H(Et2O)2][B(C6F5)4] in THF converted the hydrides into THF complexes syn-[Mo,W(NO)(CO)(etp(i)p)(THF)][B(C6F5)4]. Combinations of the p-substituents of aryl imines p-R(1)C6H4CH=N-p-C6H4R(2) (R(1),R(2)=H,F,Cl,OMe,α-Np) were hydrogenated to amines (maximum initial TOFs of 1960 h(-1) (2 a(syn)) and 740 h(-1) (2 b(syn)) for N-(4-methoxybenzylidene)aniline). An 'ionic hydrogenation' mechanism based on linear Hammett plots (ρ=-10.5, p-substitution on the C-side and ρ=0.86, p-substitution on the N-side), iminium intermediates, linear P(H2) dependence, and DKIE=1.38 is proposed. Heterolytic splitting of H2 followed by 'proton before hydride' transfers are the steps in the ionic mechanism where H2 ligand addition is rate limiting.

Reversible Heterolytic Cleavage of the H–H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride

“…Heterolytic activation of the H–H bond by Mo complexes is rare; , Berke and co-workers reported heterolytic cleavage of H 2 across a MoN bond in [( i Pr 2 PCH 2 CH 2 ) 2 N)]­Mo­(NO)­(CO). , The product, [( i Pr 2 PCH 2 CH 2 ) 2 N H )]­Mo­( H )­(NO)­(CO), was identified by NMR spectroscopy, but it is unstable in the absence of H 2 . Molybdenum complexes reported by Long, Chang, and co-workers are electrocatalysts for the evolution of H 2 in aqueous solution. − Molybdenum or tungsten cationic complexes bearing phosphine or N-heterocyclic carbene ligands add H 2 to generate dihydride complexes. − In ionic hydrogenations with these complexes, a Mo–H bond is the proton donor, then the second Mo–H bond functions as a hydride donor .…”

“…Thus, the ability to understand and control the factors influencing rates of heterolytic cleavage of H 2 is important. Heterolytic activation of the H−H bond by Mo complexes is rare; 48,49 Berke and co-workers reported heterolytic cleavage of H 2 across a MoN bond in [( i Pr 2 PCH 2 CH 2 ) 2 N)]Mo(NO)-(CO). 48,49 The product, [( i Pr 2 PCH 2 CH 2 ) 2 NH)]Mo(H)(NO)-(CO), was identified by NMR spectroscopy, but it is unstable in the absence of H 2 .…”

“…Heterolytic activation of the H−H bond by Mo complexes is rare; 48,49 Berke and co-workers reported heterolytic cleavage of H 2 across a MoN bond in [( i Pr 2 PCH 2 CH 2 ) 2 N)]Mo(NO)-(CO). 48,49 The product, [( i Pr 2 PCH 2 CH 2 ) 2 NH)]Mo(H)(NO)-(CO), was identified by NMR spectroscopy, but it is unstable in the absence of H 2 . Molybdenum complexes reported by Long, Chang, and co-workers are electrocatalysts for the evolution of H 2 in aqueous solution.…”

Facile and Efficient Synthesis of Primary Amines via Reductive Amination over a Ni/Al2O3 Catalyst

“…Amines are significant intermediates for the synthesis of bulk chemicals and dyes as well as life science molecules. Among different kinds of amines, primary amines are the most useful key precursors or crucial intermediates for the production of pharmaceuticals, biomolecules, and fine chemicals. − Therefore, the development of efficient and universal routes toward primary amines production has attracted great attention, and several methods have been explored, mainly including hydrogenation of chemicals containing CN bonds, − the reduction of nitriles, , and reductive amination of carbonyl compounds. − Among which, the catalytic reductive amination of aldehydes and ketones with ammonia and molecular hydrogen as the nitrogen source and reducing agent, respectively, represents a vital important and robust method and deserves continuous increasing attention. − Furthermore, the aldehydes/ketones derived from biorefinery processes, such as furfural and 5-hydroxymethyfurfural, ,, may be alternatively used as starting materials for the sustainable production of primary amines.…”

Efficient and Mild Reductive Amination of Carbonyl Compounds Catalyzed by Dual-Function Palladium Nanoparticles

“…They are valuable building blocks for the synthesis of pharmaceuticals, agrochemicals, fine chemicals, polymers, dyestuffs, and plasticizing agents due to their straightforward valorization and structural diversity. − Therefore, developing efficient synthetic routes toward primary amines has attracted tremendous attention. Recent efforts mainly focused on the direct amination of alcohols, − reductive amination of carbonyl compounds, − elimination of carbonyl group of amides, hydrogenation of compounds containing CN bond(s), − reduction of nitro compounds, , and reductive hydrogenation of nitriles. , Among them, the reductive amination of carbonyl compounds has received considerable attention due to their ready availability in nature . Nevertheless, reductive amination using ammonia (NH 3 ) and molecular hydrogen (H 2 ), in current investigations, usually proceeded under harsh reaction conditions (NH 3 or H 2 of high-pressure and/or elevated temperatures), majority of which were carried out in organic solvents due to easy decomposition of imines (an intermediate derived from a carbonyl compound to final product amine) in H 2 O or under heated conditions.…”