Autocatalytic role of molecular hydrogen in copper-catalyzed transfer hydrogenation of ketones

Catalytic transfer hydrogenation of ketones and aldehydes is generally accepted to follow a dehydrogenation-hydrogenation mechanism on copper, which makes the increased hydrogenation rate and selectivity rather puzzling. Using first-principles microkinetics on a Cu(111) surface, we show that, rather than a dehydrogenation-hydrogenation mechanism, there is also direct proton transfer between the sacrificial alcohol and the reacting ketone. The ketone is hydrogenated to a stable alkoxy intermediate using surface hydrogen. This alkoxy intermediate is subsequently hydrogenated to the alcohol product via direct proton transfer from the sacrificial alcohol, also forming a sacrificial alkoxy intermediate. To close the catalytic cycle, the sacrificial alkoxy species dehydrogenates, forming its corresponding ketone. We also observed a surprising catalytic effect of molecular hydrogen, which can be explained by the rate-controlling step in transfer hydrogenation: the direct hydrogenation of the ketone to its alkoxy intermediate by surface hydrogen. Under all realistic reaction conditions, this step has the highest degree of rate control

The intramolecular dynamics of a rigid yet twisty 'Ferrocenyl' TetraPhosphine : served with some 31P-NMR delicacy

Polydentate ferrocenyl phosphines equipped with bulky functional groups are regarded as rigid ligands capable of stabilizing and/or activating a broad range of chemical compounds such as smaller complexes (being formed e.g. in transition metal-catalyzed cross coupling reactions), nanoparticles, or even larger surfaces. The fruitful rigidity of these fascinating molecular species originates from the internal steric constraints imposed by the substituents; hence, the rotational reorientation of the Cp rings around the vertical 5-fold symmetry axis is hampered, and a permanent polydentate phosphine ‘cage’ is created (see figure). Latter construction then provides a large variety of coordination modes for the actual substrate what is the core structural feature being responsible for the diverse applicability spectrum. If such ferrocenyl phosphine is investigated on a sufficiently long i.e. ‘NMR time scale’ however, its decelerated intramolecular motions might be discovered and quantitatively characterized. Indeed, selective 1D 31P-{1H} EXSY {Exchange Spectroscopy} pointed out the exchange of the chemically distinct phosphoruses (green and red spheres below) in the scrutinized Fc(P)4tBu ligand and thus successfully demonstrated the previously unknown rotation (i.e. antiparallel twisting) of the Cp rings around the Fe centre. Series of measurements performed at different temperatures enabled the evaluation of the respective thermodynamic parameters (ΔS#, ΔH#, ΔG#) for which the influence of the solvent was also studied – while the confrontation of the experimental and theoretical values computed by DFT methods completed the analysis of the motion. In fact, the four 31P-s of Fc(P)4tBu composes an AA’BB’ spin system giving rise to a puzzling second order 31P NMR spectrum. Although the respective J-couplings had already been presented reclining upon the output of in silico simulations, a side track of the current work covered the full deduction of the results by the means of a quantum mechanical approach. Besides, the internal ring rotations shed new light on the ’through space’ nature of the JAA’ coupling affecting the inner phosphoruses (red spheres). That is, the interaction showed unquenchable and endured higher ring rotation rates than its actual frequency value what highlighted the intricacy of the magnetization transfer phenomena between the two nuclei. Finally, exchange phenomena were revealed for the complexed state of the ligand as well. According to 2D 1H–1H EXSY spectra, in case of [Pd(II)Br2-Fc(P)4tBu] the familiar twisting of the cyclopentadienyl rings was complemented with the periodic transconnection of the [Pd(II)Br2-] moiety between the bidentate (-PPH2)2 sites – perfectly illustrating the possibility for multiple coordination ways offered by polydentate phosphines

Microsoft Access as a Support System in New Student Reception SMA Negeri 1 Curug Tanggerang

The rapid computer developments lately almost cover all areas of life one of them is a computer or computer USAge in educational institutions. The development of personal computers (PCs) are becoming more sophisticated and faster process but with a smaller form than the initial findings so that price is becoming more affordable and easily obtainable.Suggested advantage is that it can save time, effort and accuracy of the data more reliable, but still there are still perceived shortage is the lack of experts who understand the software that is used is when there hamabatn obstacles and interference

A diverse view of science to catalyse change:Valuing diversity leads to scientific excellence, the progress of science and, most importantly, it is simply the right thing to do. we must value diversity not only in words, but also in actions

No abstract available.publishe

A diverse view of science to catalyse change

Valuing diversity leads to scientific excellence, the progress of science and, most importantly, it is simply the right thing to do. We must value diversity not only in words, but also in actions

Design and synthesis of ruthenium indenylidene-based catalysts for olefin metathesis

As part of a European wide effort to develop metathesis catalysts for use in fine chemical and pharmaceutical compound synthesis, this study focuses on the design and synthesis of ruthenium based catalysts for olefin metathesis. The aim, of this work was simple: to develop new, more active, more stable, easy to synthesise and commercially viable Ruthenium based catalysts, as well trying to rationalize the effect of structural changes on reactivity. Two different approaches were explored in order to develop more active catalysts bearing N-heterocyclic carbene (NHC) ligands: changing the leaving group and the effect of the NHC moiety in indenylidene type complexes. Over 12 new catalysts were developed and their activity compared to that of commercially available catalysts. Overall, the new complexes exhibited superior reactivity compared to previously reported catalysts in several benchmark transformations. However, olefin metathesis is a very substrate specific reaction, and rather than finding one catalyst that is superior to all, a catalogue of catalysts suitable for specific transformations was developed. In addition, the effect of structural changes on substrate activity was investigated in the ring closing metathesis of 1,8-nonadienes. The reaction profiling showcased the presence of a gem-difluoro group as an accelerating group in this incarnation of the olefin metathesis reaction and leads to ring formation over polymerization. In order to rationalize the effect of structural changes on catalyst activity, kinetic studies dealing with the initiation mechanism of ruthenium-indenylidene complexes were examined and compared with that of benzylidene counterparts. It was discovered that not all indenylidene complexes followed the same mechanism, highlighting the importance of steric and electronic properties of so-called spectator ligands, and that there is no single mechanism for the ruthenium-based olefin metathesis reaction. These results highlight the importance of systematic development of catalysts and that as scientists we should not take for granted