Hydrogenation of Secondary Amides using Phosphane Oxide and Frustrated Lewis Pair Catalysis

Counterintuitive chemoselectivity in the reduction of carbonyl compounds

The reactivity of carbonyl functional groups largely depends on the substituents on the carbon atom. Reversal of the commonly accepted order of reactivity of different carbonyl compounds requires novel synthetic approaches. Achieving selective reduction will enable the transformation of carbon resources such as plastic waste, carbon dioxide and biomass into valuable chemicals. In this Review, we explore the reduction of less reactive carbonyl groups in the presence of those typically considered more reactive. We discuss reductions, including the controlled reduction of ureas, amides and esters to aldehydes, as well as chemoselective reductions of carbonyl groups, including the reduction of ureas over carbamates, amides and esters; the reduction of amides over esters, ketones and aldehydes; and the reduction of ketones over aldehydes.

Structure-Reactivity Relationships in Borane-Based FLP-Catalyzed Hydrogenations, Dehydrogenations, and Cycloisomerizations

ConspectusThe activation of molecular hydrogen by main-group element catalysts is an extremely important approach to metal-free hydrogenations. These so-called frustrated Lewis pairs advanced within a short period of time to become an alternative to transition metal catalysis. However, deep understanding of the structure-reactivity relationship is far less developed compared to that of transition metal complexes, although it is paramount for advancing frustrated Lewis pair chemistry.In this Account, we provide detailed insight into how Lewis acidity and Lewis basicity correlate to reactivity. The reactivity of frustrated Lewis pairs will be systematically discussed in context with selected reactions. The influence of major electronic modifications of the Lewis pairs is correlated with the ability to activate molecular hydrogen, to channel reaction kinetics and reaction pathways, or to achieve C(sp³)-H activations.First, we will describe how we entered this emerging field of research after quickly realizing that information was lacking on how the reactivity changes with modification of the frustrated Lewis pair. This led us to the development of a qualitative and quantitative structure-reactivity relationship in metal-free imine hydrogenations. The imine hydrogenation was utilized as the model reaction to experimentally determine the activation parameters of the FLP-mediated hydrogen activation for the first time. This kinetic study revealed autoinduced catalytic profiles when Lewis acids weaker than tris(pentafluorophenyl)borane were applied, opening up to study the Lewis base dependency within one system. With this knowledge of the interplay between Lewis acid strength and Lewis basicity, we developed methods for the hydrogenation of densely functionalized nitroolefins, acrylates, and malonates. Here, the reduced Lewis acidity needed to be counterbalanced by a suitable Lewis base to ensure efficient hydrogen activation. The opposite measure was necessary for the hydrogenation of unactivated olefins. For these, comparably less electron-releasing phosphanes were required to generate strong Brønsted acids by hydrogen activation. These systems displayed highly reversible hydrogen activation even at temperatures as low as -60 °C. A systematic study of these systems enabled the development of acceptorless dehydrocouplings of amines with silanes and dehydrogenations of aza-heterocycles by C(sp³)-H activations. Furthermore, the C(sp³)-H and π-activation was utilized to achieve cycloisomerizations by carbon-carbon and carbon-nitrogen bond formations. Lastly, new frustrated Lewis pair systems featuring weak Lewis bases as active components in the hydrogen activation were developed for the reductive deoxygenation of phosphane oxides and carboxylic acid amides.

Towards the Development of Frustrated Lewis Pair (FLP) Catalyzed Hydrogenations of Tertiary and Secondary Carboxylic Amides

The development of the frustrated Lewis pair catalyzed hydrogenation of tertiary and secondary amides is reviewed. Detailed insight into our strategies in order to overcome challenges during the reaction development process is provided. Furthermore, the developed chemistry is extended to the hydrogenation of polyamides and of trifluoroacetamides for the convenient introduction of trifluoroethyl groups into organic molecules.

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen

The articulation of the notion of "frustrated Lewis pairs" (FLPs) emerged from the discovery that H₂ can be reversibly activated by combinations of sterically encumbered main group Lewis acids and bases. This has prompted numerous studies focused on various perturbations of the Lewis acid/base combinations and the applications to organic reductions. This Perspective focuses on the new directions and developments that are emerging from this FLP chemistry involving hydrogen. Three areas are discussed including new applications and approaches to FLP reductions, the reductions of small molecules, and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chemistry, this simple concept of FLPs is being applied across the discipline.

Hydrogenation of Secondary Amides using Phosphane Oxide and Frustrated Lewis Pair Catalysis

The metal‐free catalytic hydrogenation of secondary carboxylic acid amides is developed. The reduction is realized by two new catalytic reactions. First, the amide is converted into the imidoyl chloride by triphosgene (CO(OCCl₃)₂) using novel phosphorus(V) catalysts. Second, the in situ generated imidoyl chlorides are hydrogenated in high yields by an FLP‐catalyst. Mechanistic and quantum mechanical calculations support an autoinduced catalytic cycle for the hydrogenation with chloride acting as unusual Lewis base for FLP‐mediated H₂‐activation.