The Hydrogenation Problem in Cobalt‐based Catalytic Hydroaminomethylation

Abstract

The hydroaminomethylation (HAM) reaction converts alkenes into N-alkylated amines and has been well studied for rhodiumand ruthenium-based catalytic systems. Cobalt-based catalytic systems are able to perform the essential hydroformylation reaction, but are also known to form very active hydrogenation catalysts, therefore we examined such a system for its potential use in the HAM reaction. Thus, we have quantum-chemically explored the hydrogenation activity of [HCo(CO) 3 ] in model reactions with ethene, methyleneamine, formaldehyde, and vinylamine using dispersion-corrected relativistic density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. Our computations reveal essentially identical overall barriers for the catalytic hydrogenation of ethene, formaldehyde, and vinylamine. This strongly suggests that a cobalt-based catalytic system will lack hydrogenation selectivity in experimental HAM reactions. Our HAM experiments with a cobalt-based catalytic system (consisting of Co 2 (CO) 8 as cobalt source and P(n-Bu) 3 as ligand) resulted in the formation of the desired N-alkylated amine. However, significant amounts of hydrogenated starting material as well as alcohol (hydrogenated aldehyde) were always formed. The use of cobalt-based catalysts in the HAM reaction to selectively form N-alkylated amines seems therefore not feasible. This confirms our computational prediction and highlights the usefulness of state-of-the-art DFT computations for guiding future experiments.

A Selective and General Cobalt‐Catalyzed Hydroaminomethylation of Olefins to Amines

“…Indeed, Bouwman, Bickelhaupt and co‐worker stated very recently: “The use of cobalt‐based catalysts in the HAM reaction (…) seems therefore not feasible”. [10] Obviously, there are considerable challenges to realize the development of general cobalt‐catalyzed hydroaminomethylations of alkenes (Figure 1 b ). Both experiments and theoretical studies have shown that the kinetic barrier for the reductive amination step under hydroformylation conditions is high for representative cobalt catalysts.…”

“…Both experiments and theoretical studies have shown that the kinetic barrier for the reductive amination step under hydroformylation conditions is high for representative cobalt catalysts. [ 3b , 10 , 11 ] In addition, competing alkene isomerizations mediated by Co‐H species are typically faster than hydroformylation or reductive amination. [ 1a , 6d , 11b ] Consequently, a mixture of isomeric olefins, aldehydes and alcohols is usually obtained.…”

A Cooperative Rh/Co‐Catalyzed Hydroaminomethylation Reaction for the Synthesis of Terpene Amines

“…With the aim of working under milder conditions (temperature, pressure, metal loading), we sought to explore the effect of cobalt in the HAM of terpenes. Despite recent reports by Bouwman, Bickelhaupt and co‐workers concluded from their quantum/chemical study that Co‐based catalysts had a low chemoselectivity favoring the hydrogenation of alkenes and aldehydes (at 150 °C and 75 bar total pressure), [15] one of the first experimental works on Co‐catalyzed HAM reaction was described under even harsher conditions (up to 260 °C and 479–755 atm of syngas, using a high loading of metallic cobalt) [16] . On the other hand, Beller's group has lately reported a highly regioselective Co/Xantphos‐derived catalytic system leading to linear amines, even using internal alkenes as substrate thanks to the Co‐promoted isomerization of alkenes, [17] albeit none of the reported works used terpenes.…”

Cobalt-Based Hydrotalcite: A Potential Non-Noble Metal-Based Heterogeneous Catalyst for Selective Hydrogenation of Aromatic Aldehydes

“…Several transition-metal-based catalysts derived from nickel, copper, and cobalt have exhibited promising activities for selective hydrogenation reactions. Nowadays, earth-abundant and environmentally benign catalysts based on cobalt compounds are highly attractive for the famous Fischer–Tropsch synthesis, hydroformylation (discovered by Otto Roelen), natural gas reforming, and other hydrogenation reactions. , Early studies have shown the potential application of cobalt catalysts in selective hydrogenation reactions such as hydroaminomethylation, hydrogenations of aliphatic and aromatic nitriles and nitro compounds, and asymmetric hydrogenation of α,β-unsaturated acids. − …”