Mechanistic Study of the N-Formylation of Amines with Carbon Dioxide and Hydrosilanes

N-formylation of amines with CO2 and hydrosilane reducing agents proceeds via fast and complex chemical equilibria, which hinder easy analysis of the reaction pathways. In situ reaction monitoring and kinetic studies reveal that three proposed pathways, via direct- and amine assisted formoxysilane formation (pathways 1 and 2, respectively) and via a silylcarbamate intermediate (pathway 3), are possible depending on the reaction conditions and the substrates. While pathway 1 is favored for non-nucleophilic amines in the absence of a catalyst, a base catalyst results in noninnocent behavior of the amine in the CO2 reduction step toward the formoxysilane intermediate. The reaction pathway is altered by strongly nucleophilic amines, which form stable adducts with CO2. Silylcarbamate intermediates form, which can be directly reduced to the N-formylated products by excess hydrosilane. Nevertheless, without excess hydrosilane, the silylcarbamate is an additional intermediate en route to formoxysilanes along pathway 2. Exchange NMR spectroscopy (EXSY) revealed extensive substituent exchange around the hydrosilane silicon center, which confirms its activation during the reaction and supports the proposed reaction mechanisms. Numerous side reactions were also identified, which help to establish the reaction equilibria in the N-formylation reactions.

Carbon dioxide for hydrogen storage via formic acid derivatives and methanol

Katerina Stefania Sordakis

Securing our energy future while minimizing the associated environmental impacts is a challenging endeavor and the fundamental aspect of sustainable development. The choice of energy resource and the reduction of greenhouse gas emissions, primarily carbon dioxide (CO2), are key parameters in this process. Hydrogen gas (H2) is an energy carrier with the potential of "green" production and utilization, in addition to having attractive inherent fuel properties. However, complications during its storage arise from its intrinsically light nature. Chemical H2 fixation within liquid formic acid (FA) and its derivatives is therefore a promising approach. In the present dissertation, reversible H2 storage cycles based on homogeneous FA/formate dehydrogenation and CO2/bicarbonate hydrogenation, as well as the related CO2 transformation to methanol are investigated. The first chapter provides an overview of challenges underlying energy sustainability, prospects for integration of H2 with our energy infrastructure, the employment of FA as a liquid organic hydrogen carrier and state of the art homogeneous catalysts for selective FA dehydrogenation and CO2 hydrogenation. The combination of these reactions to form a closed CO2 loop is the basic concept behind a reversible H2 storage system as it is envisaged by our group. In the second chapter, FA dehydrogenation/formation in the presence of triethylamine is discussed. The results are summarized as equilibrium positions within this couple, which can be controlled by adjustment of the operating pressure and temperature. The basic additive promotes H2 fixation but also allows for on-demand H2 release from a triethylammonium formate substrate. The third chapter addresses H2 storage in basic media, i.e. aqueous bicarbonate hydrogenation, with a Ru(III) catalyst precursor and a number of water-soluble phosphine ligands. The latter are introduced to tailor the exhibited activity by affecting the steric and electronic parameters of the catalyst. All phosphines provide high bicarbonate conversions, albeit with low reaction rates. Chapter four summarizes the optimization and mechanistic studies of the reversible formate dehydrogenation reaction, in the presence of a Ru(II)-mTPPTS catalyst. Cesium is chosen as the cation for the formate/bicarbonate salts because it leads to their high solubility in the aqueous solvent and therefore an increase in the hydrogen storage capacity. The involved equilibria are determined and a reaction mechanism for the formate dehydrogenation step is proposed, based on NMR studies. Advantages of the presented approach include the utilization of water as the solvent, the absence of CO2 throughout the reactions and the successful in situ recycling of the environmentally benign hydrogen carrier. In Chapter five the feasibility of producing both FA and methanol directly from CO2, at room temperature, in a "one-pot" reaction using aqueous solvent is demonstrated. Formic acid formation occurs in water without additives or organic solvents with a homogeneous iridium catalyst. Formic acid also undergoes disproportionation into methanol, with the same complex. Under optimized conditions, FA conversions of 98% and methanol selectivities of 96% are achieved in the disproportionation reaction. These reactions are relevant to the field of sustainable H2 storage, but might also provide an alternative approach to the commercial fossil-fuel-dependent production of formic acid and methanol.

EPFL2016

N-formylation and N-methylation of amines using metal-free N-heterocyclic carbene catalysts and CO2 as carbon source

Felix Daniel Bobbink, Shoubhik Das, Paul Joseph Dyson

N-formylation and N-methylation of amines are important reactions that are used to produce a wide range of key intermediates and compounds. This protocol describes the environmentally benign N-formylation and N-methylation of primary and secondary amines using carbon dioxide (CO2) as the carbon source, hydrosilanes as reductants and N-heterocyclic carbenes (NHCs) as catalysts. Using CO2 as a reagent has the advantage of low cost and negligible toxicity. However, the catalyst is air-sensitive and must be generated fresh before use; consequently, the techniques used to prepare and manipulate the catalyst are described. The synthetic approach described in this protocol does not use any toxic reagents; using the appropriate catalyst, N-formylated or N-methylated products can be obtained with high selectivity. The overall time for catalyst preparation and for conducting several catalytic reactions in parallel is 15-48 h, depending on the nature of the substrates.

Nature Publishing Group2017

Development of heterogeneous catalytic systems for the transformation of carbon dioxide to methanol under mild conditions.

Aswin Gopakumar

Rising level of carbon dioxide (CO2) is a significant contributor to global warming and poses a major concern in modern society and environment. Anthropogenic activities such as fossil fuel combustion, cement production and deforestation are the primary causes of CO2 emissions. Creating sustainable pathways by converting abundantly available CO2 into fuel could help in mitigating the ever-growing global energy demand. Owing to the high intrinsic thermodynamic stability of CO2 molecule, the chemical reduction mechanism requires an appropriate catalyst system to reduce the high activation energy barrier. Additionally, the prospect of utilising CO2 as C1 synthon to convert the amines to afford value-added formamides is even more appealing. In the present dissertation, the development of efficient industrial friendly heterogeneous catalytic routes for CO2 fixation on amines by chemical reduction method to formamides, as well as their subsequent conversion to fuel, methanol under mild conditions are investigated. The first chapter describes an overview about the current trends in global climate change, about the challenges in CO2 capture and storage, the employment of CO2 as a source of carbon to synthesise industrially essential formamide products by catalytic reduction. Subsequently, the reduction pathway of formamides to afford methanol is also elucidated. Additionally, the prospect of using formic acid (a known CO2 and H2 carrier) to formylate amines and the consequent reduction to methanol is also given. The combination of these reactions presents a viable CO2 reduction system using amines. In the second chapter, N-formylation and N-methylation of amines using CO2 as the carbon source catalysed by palladium nanoparticles (Pd NPs) under mild conditions are discussed. The concluding results point out the high efficiency and high selectivity of Pd NPs in catalysing the amines to fix CO2 to afford formamides at room temperature. The Pd NPs exhibited excellent catalytic activity and selectivity on par with the reported catalysts. The overall selectivity and conversion of the products could be altered by adjusting the CO2 pressure, catalyst loading, temperature and solvent. The catalyst could be recycled for multiple reactions without significant loss of activity. The third chapter describes a âGeo-inspiredâ catalyst system with the use of an iron-rich natural mineral-Gibeon meteorite, as the catalyst for N-formylation and N-methylation of amines using CO2 as the carbon source at room temperature. Similar to the second chapter, the Gibeon meteorite exhibited excellent catalytic activity and selectivity on par with the reported catalysts. The overall selectivity and conversion could be altered by adjusting the CO2 pressure, catalyst loading, temperature and solvent. Comparative studies with the similar iron-containing alloys were also carried out. A plausible mechanism is proposed with the Gibeon meteoriteâs surface bound hydroxide ions. Moreover, the catalyst could be easily recovered and reused for multiple reactions as well without significant loss of activity. The fourth chapter encompasses the investigation of a metal-free catalytic system using polymerisable ionic liquid- (4-Vinylbenzyl)trimethylammonium chloride catalysed N-formylation and N-methylation of amines using CO2 as the carbon source under mild conditions. Similar to second and third chapters, the monomer and the polymer exhibited excellent catalytic activity (comparable