Lewis Acid-Lewis Base Mediated Metal-Free Hydrogen Activation and Catalytic Hydrogenation

Organocatalysis, the use of organic molecules as catalysts, is attracting increasing attention as one of the most modern and rapidly growing areas of organic chemistry, with countless research groups in both academia and the pharmaceutical industry around the world working on this subject. The literature review of this thesis mainly focuses on metal-free systems for hydrogen activation and organocatalytic reduction. Since these research topics are relatively new, the literature review also highlights the basic principles of the use of Lewis acid-Lewis base pairs, which do not react irreversibly with each other, as a trap for small molecules. The experimental section progresses from the first observation of the facile heterolytical cleavage of hydrogen gas by amines and B(C6F5)3 to highly active non-metal catalysts for both enantioselective and racemic hydrogenation of unsaturated nitrogen-containing compounds. Moreover, detailed studies of structure-reactivity relationships of these systems by X-ray, neutron diffraction, NMR methods and quantum chemical calculations were performed to gain further insight into the mechanism of hydrogen activation and hydrogenation by boron-nitrogen compounds.Organokatalyysi, eli pienten orgaanisten molekyylien käyttö katalyytteinä, herättää yhä enemmän kiinnostusta ja on yksi nopeimmin kasvavista modernin orgaanisen kemian tutkimusaloista. Tutkimusta tehdään kansainvälisesti lukuisissa ryhmissä, niin akateemisessa maailmassa kuin lääketeollisuudessa. Tämän väitöskirjan kirjallisuuskatsaus käsittelee metallittomia menetelmiä vedyn aktivoinnissa sekä organokatalyyttistä pelkistystä. Koska nämä tutkimusaiheet ovat suhteellisen uusia, kirjallisuustarkastelu sisältää myös pääperiaatteet reversiibelisti reagoivien Lewis happo- Lewis emäs-parien käytöstä pienten molekyylien loukkuina. Kokeellisessa osassa esitellään kaasumaisen vedyn heterolyyttinen pilkkominen amiineilla ja B(C6F5)3:lla, erittäin aktiivisia metallittomia katalyyttejä sekä enantioselektiivisten ja raseemisten tyydyttymättömien typpiyhdisteiden vedytys. Näiden menetelmien reaktiivisuuden riippuvuutta rakenteista tutkittiin yksityiskohtaisesti röntgendiffraktiolla, neutronidiffraktiolla, NMR-spektroskopialla ja kvanttikemiallisten laskujen avulla. Näin saatiin tarkempi kuva reaktiomekanismeista vedynaktivoinnissa ja pelkistyksissä boori-typpi yhdisteillä

H2 activation using the first 1:1:1 hetero-tri(aryl)borane

The novel 1:1:1 hetero-tri(aryl)borane (pentafluorophenyl){3,5-bis(trifluoromethyl)phenyl}(pentachlorophenyl)borane has been synthesised and structurally characterised. This has been show to act as the Lewis acidic component in FLPs for the heterolytic cleavage of H2 with three Lewis bases

A combined "electrochemical-frustrated Lewis pair" approach to hydrogen activation: surface catalytic effects at platinum electrodes

Herein, we extend our “combined electrochemical–frustrated Lewis pair” approach to include Pt electrode surfaces for the first time. We found that the voltammetric response of an electrochemical–frustrated Lewis pair (FLP) system involving the B(C6F5)3/[HB(C6F5)3]− redox couple exhibits a strong surface electrocatalytic effect at Pt electrodes. Using a combination of kinetic competition studies in the presence of a H atom scavenger, 6-bromohexene, and by changing the steric bulk of the Lewis acid borane catalyst from B(C6F5)3 to B(C6Cl5)3, the mechanism of electrochemical–FLP reactions on Pt surfaces was shown to be dominated by hydrogen-atom transfer (HAT) between Pt, [Pt[BOND]H] adatoms and transient [HB(C6F5)3]⋅ electrooxidation intermediates. These findings provide further insight into this new area of combining electrochemical and FLP reactions, and proffers additional avenues for exploration beyond energy generation, such as in electrosynthesis

An Electrochemical Study of Frustrated Lewis Pairs: A Metal-free Route to Hydrogen Oxidation

[Image: see text] Frustrated Lewis pairs have found many applications in the heterolytic activation of H(2) and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H(2) can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H(2) oxidation by 610 mV (117.7 kJ mol(–1)). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology

Metal-free electrocatalytic hydrogen oxidation using frustrated Lewis pairs and carbon-based Lewis acids

Whilst hydrogen is a potentially clean fuel for energy storage and utilisation technologies, its conversion to electricity comes at a high energetic cost. This demands the use of rare and expensive precious metal electrocatalysts. Electrochemical-frustrated Lewis pairs offer a metal-free, CO tolerant pathway to the electrocatalysis of hydrogen oxidation. They function by combining the hydrogen-activating ability of frustrated Lewis pairs (FLPs) with electrochemical oxidation of the resultant hydride. Here we present an electrochemical–FLP approach that utilises two different Lewis acids – a carbon-based N-methylacridinium cation that possesses excellent electrochemical attributes, and a borane that exhibits fast hydrogen cleavage kinetics and functions as a “hydride shuttle”. This synergistic interaction provides a system that is electrocatalytic with respect to the carbon-based Lewis acid, decreases the required potential for hydrogen oxidation by 1 V, and can be recycled multiple times