Posts Tagged ‘catalysis’
Sunday, June 18th, 2017
The iron complex shown below forms the basis for many catalysts.[cite]10.1002/anie.200502985[/cite] With iron, the catalytic behaviour very much depends on the spin-state of the molecule, which for the below can be either high (hextet) or medium (quartet) spin, with a possibility also of a low spin (doublet) state. Here I explore whether structural information in crystal structures can reflect such spin states.
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Tags:catalysis, Catalysts, Chemistry, energy, energy separations, energy span, Fe complex, Homogeneous catalysis, Kumada coupling, Organometallic chemistry, spin-state energy separations, Synergistic catalysis
Posted in crystal_structure_mining | 1 Comment »
Thursday, January 3rd, 2013
I return to this reaction one more time. Trying to explain why it is enantioselective for the epoxide product poses peculiar difficulties. Most of the substituents can adopt one of several conformations, and some exploration of this conformational space is needed.
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Tags:catalysis, conformational analysis, energy, epoxide product, free energy, Reaction Mechanism, similar systems, Tutorial material
Posted in Interesting chemistry | No Comments »
Monday, December 24th, 2012
tpap[cite]10.1055%2Fs-1994-25538[/cite], as it is affectionately known, is a ruthenium-based oxidant of primary alcohols to aldehydes discovered by Griffith and Ley. Whereas ruthenium tetroxide (RuO4) is a voracious oxidant[cite]10.1139/v76-304[/cite], its radical anion countered by a tetra-propylammonium cation is considered a more moderate animal[cite]10.1021/jo00038a009[/cite]. In this post, I want to try to use quantum mechanically derived energies as a pathfinder for exploring what might be going on (or a reality-check if you like).
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Tags:catalysis, energy, free energy, low energy elimination, metal, react freq, Reaction Mechanism, RuO4+ ethanol, triplet state energy, Tutorial material
Posted in Interesting chemistry | 2 Comments »
Wednesday, December 19th, 2012
The Sharpless epoxidation of an allylic alcohol had a big impact on synthetic chemistry when it was introduced in the 1980s, and led the way for the discovery (design?) of many new asymmetric catalytic systems. Each achieves its chiral magic by control of the geometry at the transition state for the reaction, and the stabilizations (or destabilizations) that occur at that geometry. These in turn can originate from factors such as stereoelectronic control or simply by the overall sum of many small attractions and repulsions we call dispersion interactions. Here I take an initial look at these for the binuclear transition state shown schematically below.
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Tags:asymmetric catalytic systems, Bob Hanson, catalysis, Julia Contreras-Garcia, little web site, NCI, non-covalent-analysis, Reaction Mechanism, terminal =CH
Posted in Interesting chemistry | No Comments »
Monday, December 17th, 2012
Part one on this topic showed how a quantum mechanical model employing just one titanium centre was not successful in predicting the stereochemical outcome of the Sharpless asymmetric epoxidation. Here in part 2, I investigate whether a binuclear model might have more success. The new model is constructed using two units of Ti(OiPr)4, which are likely to assemble into a dimer such as that shown below (in this crystal structure, some of the iPr groups are perfluorinated).
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Tags:catalysis, catalytic metal centre, Enantioselective, free energy difference, Reaction Mechanism, Sharpless epoxidation, t-butyl, This free energy difference, Tutorial material
Posted in Interesting chemistry | 2 Comments »
Sunday, December 9th, 2012
Sharpless epoxidation converts a prochiral allylic alcohol into the corresponding chiral epoxide with > 90% enantiomeric excess[cite]10.1021/jo00369a032[/cite],[cite]10.1021/jo00360a058[/cite]. Here is the first step in trying to explain how this magic is achieved.
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Tags:animation, asymmetric epoxidation, catalysis, Enantioselective, free energy, lower energy conformations, Reaction Mechanism, Tutorial material
Posted in Interesting chemistry | 1 Comment »
Sunday, April 22nd, 2012
Astronomers who discover an asteroid get to name it, mathematicians have theorems named after them. Synthetic chemists get to name molecules (Hector’s base and Meldrum’s acid spring to mind) and reactions between them. What do computational chemists get to name? Transition states! One of the most famous of recent years is the Houk-List.
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Tags:catalysis, condensation, energy, gas-phase optimised geometry, Historical, Houk-List, smallest steric exchange energy, so-called single-point solvation energy correction, steric exchange energy
Posted in Interesting chemistry | 4 Comments »
Saturday, October 30th, 2010
Reactions in cavities can adopt quite different characteristics from those in solvents. Thus first example of the catalysis of the Diels-Alder reaction inside an organic scaffold was reported by Endo, Koike, Sawaki, Hayashida, Masuda, and Aoyama[cite]10.1021/ja964198s[/cite], where the reaction shown below is speeded up very greatly in the presence of a crystalline lattice of the anthracene derivative shown below.
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Tags:animation, catalysis, free energy, free energy barrier, G/RT, Interesting chemistry, Organic scaffold, pericyclic
Posted in Interesting chemistry | 7 Comments »
Sunday, August 8th, 2010
Cavities promote reactions, and they can also trap the products of reactions. Such (supramolecular) chemistry is used to provide models for how enzymes work, but it also allows un-natural reactions to be undertaken. A famous example is the preparation of P4 (see blog post here), an otherwise highly reactive species which, when trapped in the cavity is now sufficiently protected from the ravages of oxygen for its X-ray structure to be determined. A colleague recently alerted me to a just-published article by Legrand, van der Lee and Barboiu (DOI: 10.1126/science.1188002) who report the use of cavities to trap and stabilize the notoriously (self)reactive 1,3-dimethylcyclobutadiene (3/4 in the scheme below). Again sequestration by the host allowed an x-ray determination of the captured species!
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Tags:author, Barboiu, catalysis, cavity, crystalline calixarene network, cyclobutadiene, Diels Alder, free energy, free energy barrier, gas phase calculation, host/guest, Interesting chemistry, Legrand, pericyclic, Prins, supramolecular, van der Lee, watoc11, X-ray
Posted in Interesting chemistry | 8 Comments »
Saturday, July 24th, 2010
Lactide is a small molecule made from lactic acid, which is itself available in large quantities by harvesting plants rather than drilling for oil. Lactide can be turned into polymers with remarkable properties, which in turn degrade down easily back to lactic acid. A perfect bio-renewable material!
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Tags:3g, animation, catalysis, chiroptical, dispersion energy corrections, E. L. Marshall, energy, gas phase, General, H. S. Rzepa, Interesting chemistry, Julia Contreras-Garcia, lowest energy pathway, oil, polymerisation, ring opening, V, V. C. Gibson
Posted in General, Interesting chemistry | 3 Comments »