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This is the peer reviewed version of the following article: Angew. Chem. Int. Ed. 2019, 58, 12820 12838, which has been published in final form at https://doi.org/10.1002/anie.201810121. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving., [EN] N-Alkylamines are key intermediates in the synthesis of fine chemicals, dyes, and natural products, and hence are highly valuable building blocks in organic chemistry. Consequently, the development of greener and more efficient procedures for their production continues to attract the interest of both academic and industrial chemists. Reductive procedures such as reductive amination or N-alkylation through hydrogen autotransfer by employing carbonyl compounds or alcohols as alkylating agents have prevailed for the synthesis of amines. In the last few years, carboxylic/carbonic acid derivatives and CO2 have been introduced as alternative and convenient alkylating sources. The safety, easy accessibility, and high stability of these reagents makes the development of new reductive transformations with them as N-alkylating agents a useful alternative to existing procedures. In this Review, we summarize reported examples of one-pot reductive N-alkylation methods that use carboxylic/carbonic acid derivatives or CO2 as alkylating agents.

This is the peer reviewed version of the following article: Angew. Chem. Int. Ed. 2019, 58, 12820 12838, which has been published in final form at https://doi.org/10.1002/anie.201810121. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving., [EN] N-Alkylamines are key intermediates in the synthesis of fine chemicals, dyes, and natural products, and hence are highly valuable building blocks in organic chemistry. Consequently, the development of greener and more efficient procedures for their production continues to attract the interest of both academic and industrial chemists. Reductive procedures such as reductive amination or N-alkylation through hydrogen autotransfer by employing carbonyl compounds or alcohols as alkylating agents have prevailed for the synthesis of amines. In the last few years, carboxylic/carbonic acid derivatives and CO2 have been introduced as alternative and convenient alkylating sources. The safety, easy accessibility, and high stability of these reagents makes the development of new reductive transformations with them as N-alkylating agents a useful alternative to existing procedures. In this Review, we summarize reported examples of one-pot reductive N-alkylation methods that use carboxylic/carbonic acid derivatives or CO2 as alkylating agents.

[EN] Kinetic analysis of ruthenium-catalyzed reductive N-methylation of amines using dimethyl carbonate as C1 source and molecular hydrogen as reductant has been performed. Kinetic equations have been derived and kinetic modelling has been performed for experimental data generated previously at a constant hydrogen pressure as well as for additional experiments performed at different hydrogen pressures. The study has revealed interesting kinetic features related to an induction period strongly influenced by temperature. A kinetic model has been proposed based on advanced reaction mechanism featuring transformation between different type of catalytic species and inactivation of them during the reaction. Kinetic modelling was done for all data sets together showing excellent correspondence between calculations and experiments.

Hydrogen gas is a storable form of chemical energy that could complement intermittent renewable energy conversion. One of the main disadvantages of hydrogen gas arises from its low density, and therefore, efficient handling and storage methods are key factors that need to be addressed to realize a hydrogen-based economy. Storage systems based on liquids, in particular, formic acid and alcohols, are highly attractive hydrogen carriers as they can be made from CO2 or other renewable materials, they can be used in stationary power storage units such as hydrogen filling stations, and they can be used directly as transportation fuels. However, to bring about a paradigm change in our energy infrastructure, efficient catalytic processes that release the hydrogen from these molecules, as well as catalysts that regenerate these molecules from CO2 and hydrogen, are required. In this review, we describe the considerable progress that has been made in homogeneous catalysis for these critical reactions, namely, the hydrogenation of CO2 to formic acid and methanol and the reverse dehydrogenation reactions. The dehydrogenation of higher alcohols available from renewable feedstocks is also described. Key structural features of the catalysts are analyzed, as is the role of additives, which are required in many systems. Particular attention is paid to advances in sustainable catalytic processes, especially to additive-free processes and catalysts based on Earth-abundant metal ions. Mechanistic information is also presented, and it is hoped that this review not only provides an account of the state of the art in the field but also offers insights into how superior catalytic systems can be obtained in the future.

Hydrogen gas is a storable form of chemical energy that could complement intermittent renewable energy conversion. One of the main disadvantages of hydrogen gas arises from its low density, and therefore, efficient handling and storage methods are key factors that need to be addressed to realize a hydrogen-based economy. Storage systems based on liquids, in particular, formic acid and alcohols, are highly attractive hydrogen carriers as they can be made from CO2 or other renewable materials, they can be used in stationary power storage units such as hydrogen filling stations, and they can be used directly as transportation fuels. However, to bring about a paradigm change in our energy infrastructure, efficient catalytic processes that release the hydrogen from these molecules, as well as catalysts that regenerate these molecules from CO2 and hydrogen, are required. In this review, we describe the considerable progress that has been made in homogeneous catalysis for these critical reactions, namely, the hydrogenation of CO2 to formic acid and methanol and the reverse dehydrogenation reactions. The dehydrogenation of higher alcohols available from renewable feedstocks is also described. Key structural features of the catalysts are analyzed, as is the role of additives, which are required in many systems. Particular attention is paid to advances in sustainable catalytic processes, especially to additive-free processes and catalysts based on Earth-abundant metal ions. Mechanistic information is also presented, and it is hoped that this review not only provides an account of the state of the art in the field but also offers insights into how superior catalytic systems can be obtained in the future.

Fe and Ru pincer-type catalysts are used for the racemization of benzylic alcohols. Racemization with the Fe catalyst was achieved within 30 minutes under mild reaction conditions, with a catalyst loading as low as 2 mol %. This reaction constitutes the first example of an iron-catalyzed racemization of an alcohol. The efficiency for racemization of the Fe catalyst and its Ru analogue was evaluated for a wide range of sec-benzylic alcohols. The commercially available Ru complex proved to be highly robust and even tolerated the presence of water in the reaction mixture.

This chapter discusses the most well-known click reaction the copper-catalyzed azide-alkyne cycloaddition (CuAAC), as well as ruthenium-catalyzed process (RuAAC). It also discusses a selection of click reactions that are orthogonal to CuAAC and RuAAC. However, these are not necessarily catalytic; they can be used in concert with their catalytic counterparts, allowing for the synthesis of a large variety of complex molecular structures. In CuAAC reactions, ligands protect Cu (I) from disproportionation and oxidation due to the presence of oxygen and other oxidizing species. Ligands may also activate the Cu atom for productive interaction with the alkyne and azide and promote the transition-state complex by substrate binding. Another efficient, transition-metal-mediated click reaction is the versatile Cu (I)-catalyzed nitroxide radical coupling (NRC) with an alkyl halide. Macro-molecular ligands derivatized with 2, 2, 6, 6-tetramethylpiperidine-oxyl (TEMPO) can be coupled with alkyl halides using a Cu bromide-ligand catalyst at elevated temperatures.

This chapter discusses the most well-known click reaction the copper-catalyzed azide-alkyne cycloaddition (CuAAC), as well as ruthenium-catalyzed process (RuAAC). It also discusses a selection of click reactions that are orthogonal to CuAAC and RuAAC. However, these are not necessarily catalytic; they can be used in concert with their catalytic counterparts, allowing for the synthesis of a large variety of complex molecular structures. In CuAAC reactions, ligands protect Cu (I) from disproportionation and oxidation due to the presence of oxygen and other oxidizing species. Ligands may also activate the Cu atom for productive interaction with the alkyne and azide and promote the transition-state complex by substrate binding. Another efficient, transition-metal-mediated click reaction is the versatile Cu (I)-catalyzed nitroxide radical coupling (NRC) with an alkyl halide. Macro-molecular ligands derivatized with 2, 2, 6, 6-tetramethylpiperidine-oxyl (TEMPO) can be coupled with alkyl halides using a Cu bromide-ligand catalyst at elevated temperatures.