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

Vanadyl calix[6]arene complexes: synthesis, structural studies and ethylene homo-(co-)polymerization capability

Treatment of p-tert-butylcalix[6]areneH₆ (L⁶H₆) with in situ [LiVO(Ot-Bu)₄] afforded, after work-up, the dark green complex [Li(MeCN)₄][V₂(O)₂Li(MeCN)(L⁶H₂)₂]·8MeCN (1·8MeCN). On one occasion, the reaction led to the formation of a mixture of products, the bulk of which differing from 1 only in the amount of solvate, viz.2·9.67MeCN. The second minor, yellow product has the formula {[(VO₂)₂(L⁶H₂)(Li(MeCN)₂)₂]·2MeCN}n (3·2MeCN), and comprises a 1D polymeric structure with links through the L⁶H₂ ligand and Li₂O₂ units. When the reverse order of addition was employed such that lithium tert-butoxide (7.5 equivalents) was added to L⁶H₆, and subsequently treated with VOCl₃ (2 equiv.), the complex {[VO(THF)][VO(μ-O)]₂Li(THF)(Et₂O)][L⁶]}·2Et₂O·0.5THF (4·2Et₂O·0.5THF), which contains a trinuclear motif possessing a central, octahedral vanadyl centre linked via oxo bridges to two tetrahedral (C₃v) vanadyl centres, was isolated. The calix[6]arene in 4 is severely twisted and adopts a ‘down, down, down, down, out, out’ conformation. Use of excess lithium tert-butoxide led to a complex very similar to 4, differing only in the solvent of crystallization, namely 5·Et₂O·2THF. The ability of 1 and 5 to act as pre-catalysts for ethylene polymerization in the presence of a variety of co-catalysts and under various conditions has been investigated. Co-polymerization of ethylene with propylene and with 1-hexene have also been conducted; results are compared versus VO(OEt)Cl₂

Tweezers for Parahydrogen: A Metal-Free Probe of Nonequilibrium Nuclear Spin States of H₂ Molecules

To date, only metal-containing hydrogenation catalysts have been utilized for producing substantial NMR signal enhancements by means of parahydrogen-induced polarization (PHIP). Herein, we show that metal-free compounds known as molecular tweezers are useful in this respect. It is shown that <i>ansa</i>-aminoborane tweezers QCAT provided (20–30)-fold signal enhancements of parahydrogen-originating hydrogens in ¹H NMR spectra. Nuclear polarization transfer from the polarized hydrogens to ¹¹B nuclei leads to a 10-fold enhancement in the ¹¹B NMR spectrum. Moreover, our results indicate that dihydrogen activation by QCAT and CAT tweezers is carried out in a pairwise manner, and PHIP can be used for understanding the activation mechanism in metal-free catalytic systems in general