36 research outputs found
ChemInform Abstract: The Cyclization of Parent and Cyclic Hexa-1,3-dien-5-ynes - A Combined Theoretical and Experimental Study.
No full text)‐1,5‐hexadiyne‐3‐enes: a systematic computational study
No full textThe effects of several substituents (-BH2, -BF2, -AlH2, -CH3, -C6H5, -CN, -COCH3, -CF3, -SiH3, -NH2, -NH3+, -NO2, -PH2, -OH, -OH2+, -SH, -F, -Cl, -Br) on the Bergman cyclization of (Z)-1,5-hexadiyne-3-ene (enediyne, 3) were investigated at the Becke-Lee-Yang Parr (BLYP) density functional (DFT) level employing a 6-31G basis set. Some of the substituents, (-NH3+, -NO2, -OH, -OH2+, -F, -Cl, -Br) are able to lower the barrier (up to a minimum of 16.9 kcal mol(-1) for difluoro-enediyne 7rr) and the reaction enthalpy (the cyclization is predicted to be exergonic for -OH2+ and -F) compared to the parent system giving rise to substituted 1,4-dehydrobenzenes at physiological temperatures. (C) 2001 John Wiley & Sons, Inc
Cyclization Reactions of Polyunsaturated Systems - Theory and Experiment
No full textDie vorliegende Arbeit bringt ein wenig Ordnung in die unüberschaubare Vielfalt der "Cyclisierungsreaktionen mehrfach ungesättigter Systeme". In einem ersten, theoretischen Teil wird mit Hilfe der Computerchemie gezeigt, daß eine große Gruppe von thermischen Umlagerungen und Cyclisierungen über sechsgliedrige Übergangszustände ablaufen oder zu cyclischen, sechsgliedrigen Produkten führen. Es kann gezeigt werden, daß diese Reaktionen alle zu einer großen Familie gehören, deren bekanntestes Mitglied wohl die Cope-Umlagerung von 1,5-Hexadien sein dürfte, weswegen diese Gruppe auch als Cope-Familie benannt wurde. Weiterhin kann gezeigt werden, daß die Cope-Familie in zwei Untergruppen unterteilt werden kann, und zwar in eine, in der Moleküle zusammengefaßt sind, die teilweise über konzertierte Prozesse, teilweise über Zwischenprodukte ineinander umlagern und in eine zweite, zu der die Verbindungen gehören, die zu teils offenschaligen, teils geschlossen-schaligen Produkten cyclisieren, ohne wieder zu öffnen. Für die erste Gruppe kann dabei die Zuordnung der Umlagerungspartner vorgenommen werden, sowie deutlich Abhängigkeiten der Reaktivitäten, sowie des Auftretens eines biradikalischen Zwischenproduktes von der Radikalstabilisierung durch Delokalisierung und Aromatisierung beobachtet werden. Für die zweite Gruppe, die nur sechs Mitglieder beinhaltet, können die Stammreaktionen im Detail beschrieben und teilweise mit experimentellen Daten verglichen werden. Die gemessenen Werte werden dabei durchweg gut bis sehr gut durch die berechneten reproduziert. Auf dieser Basis aufbauend werden teilweise neue Reaktionen der Stammsysteme vorhergesagt, teilweise die Reaktionsmechanismen geklärt. Der Effekt der Benzannelierung wird im Detail betrachtet und eine Erklärung für die im Vergleich zum Stammsystem immer höheren Barrieren und Reaktionsenthalpien gegeben. Außerdem wird gezeigt, daß durch die Inkorporation der Stammsysteme in ein carbocyclisches Ringsystem die Reaktivität durch Ringspannung verändert und dadurch gesteuert werden kann. In einem zweiten, praktischen Teil kann gezeigt werden, daß es neben den thermischen eine Vielfalt an kationisch induzierten Cyclisierungen an denselben Systemen geben kann, die zu ganz anderen Produkten führen. Es wurde eine sorgfältig ausgewählte Anzahl von mehrfach ungesättigten Systemen synthetisiert, dieser Cyclisierung unterworfen und anhand der teilweise überraschenden Ergebnisse eine Reaktivitätenreihenfolge der ungesättigten Molekülbausteine aufgestellt, die es ermöglichen sollte, die Ergebnisse noch nicht durchgeführter Cyclisierungen vorherzusagen. Auf beiden Gebieten, theoretisch sowie experimentell, wird im Rahmen dieser Arbeit der Grundstein für eine systematische Erfassung und Beschreibung der Reaktionen mehrfach ungesättigter Systeme gelegt.This work brings some order to the great variety of the "cyclizations of polyunsaturated systems". In a first theoretical part, it is shown by means of computational chemistry that a big group of thermal rearrangements and cyclizations either proceed via six-membered cyclic transition states or lead to six-membered cyclicic products. It is shown that all these reactions belong to one familiy of which the "Cope"-rearrangement of 1,5-hexadiene ist probably the best known. This is why this family is called the "Cope"-family. Furthermore, it is shown that the Cope-family can be divided into two subgroups: the first one includes the molecules which rearrange into each other via concerted processes or biradicaloid intermediates and the second one includes the compounds which cyclize to open or closed shell products without re-opening. For the first group the rearrangement partners could be assigned and a distinct dependence of the reactivity and the appearance of biradicaloid intermediates on the radical stabilization by delocalization and aromatization could be observed. For the second group, which consists of only six members, the parent systems could be described in detail and partly compared to experimental data. The measured values were all very well reproduced by the computed results. On this basis, new reactions of some of the parent system were predicted and some of the reaction mechanisms were clarified. The effect of benzannelation was investigated in detail and an explanation is given for the higher barriers and reaction enthalpies compared to the parent system. Furthermore, proof is given that the reactivities of the parent systems can be altered and controlled by incorporation into a carbocyclic ring system. In a second, experimental part it is shown that besides the thermal cyclizations there is a great variety of cationic induced cyclizations of the same systems which yield completely different products. A carefully chosen number of polyunsaturated systems was synthesized and cyclized by cationic induction. On the basis of the sometimes surprising results a reactivity order of the unsaturated building blocks is established to be able to predict the results of not yet perfomed cyclizations. In both theoretical and experimental areas was built up a framework for the systematic description of reactions of polyunsaturated systems
Incorporating National Urban Alliance Strategies In A High School Biology Classroom Curriculum To Increase The Success Rate For Students Of Color
No full textThe purpose of this project was to research and develop curriculum that incorporated National Urban Alliance (NUA) strategies to increase the success rate for students. It documents one teacher’s experience with severe inequities in her classroom and how she began to eliminate the tracked classes in her high school. The developed project is focused on a high school cell biology unit with various NUA strategies built-in. The author explains in great detail how the curriculum was developed and how culturally relevant teaching is a necessity for all learners. She explains that with the use of appropriate strategies, all students can learn at a high level and with a great deal of rigor. The project includes 13 lessons and a slide presentation that is aligned with the Minnesota State Science Standards
Myers−Saito versus C<sup>2</sup>−C<sup>6</sup> (“Schmittel”) Cyclizations of Parent and Monocyclic Enyne−Allenes: Challenges to Chemistry and Computation
No full textThe Myers−Saito and the C2−C6 cyclization (“Schmittel” cyclization) of the parent enyne−allene
(Z)-1,2,4-heptatriene-6-yne were investigated with pure density functional theory (DFT) methods and were
compared to coupled cluster [CCSD(T)] and Brueckner doubles [BCCD(T)] high-level calculations. Both the
Becke−Lee−Yang−Parr (BLYP) and the Becke−Perdew−Wang (BPW91) DFT levels with the 6-31G(d)
basis sets are quite suitable to describe the cyclization barriers and are nearly as accurate as much more time-consuming high-level methods. As noted before for the Bergman-cyclization, the hybrid functional B3LYP
yields good geometries but is less suitable for energies due to Hartree−Fock mixing. Single-point energy
evaluations with the much larger cc-pVTZ basis set do not necessarily improve the results; some even become
worse. The computed enthalpy of formation (ΔHf°) of the Myers−Saito product (107 ± 4 kcal mol-1) compares
relatively well to the experimental value (103 ± 3 kcal mol-1) where an upward correction within the error
bars seems indicated. Using isodesmic equations, the ΔHf° of the Schmittel product is predicted to be 129 ±
4 kcal mol-1. Since BLYP describes the barriers of the parent enyne−allene system quite well, it was utilized
to compute the cyclizations of monocyclic enyne−allenes (ring sizes = 7−10 carbons). The Myers−Saito
cyclization of the nine-membered ring is associated with the smallest reaction barrier and the highest
exothermicity. While ring strain effects are not able to favor the Schmittel products much over the Myers−Saito products, the eight-membered ring closures should give rise to a mixture of both products. The singlet−triplet separations of all cyclization products are very small (1 kcal mol-1); the hydrogen-abstracting ability of
such cyclic systems should therefore be rather high
High-energy proton imaging for biomedical applications
No full textThe charged particle community is looking for techniques exploiting proton interactions instead of X-ray absorption for creating images of human tissue. Due to multiple Coulomb scattering inside the measured object it has shown to be highly non-trivial to achieve sufficient spatial resolution. We present imaging of biological tissue with a proton microscope. This device relies on magnetic optics, distinguishing it from most published proton imaging methods. For these methods reducing the data acquisition time to a clinically acceptable level has turned out to be challenging. In a proton microscope, data acquisition and processing are much simpler. This device even allows imaging in real time. The primary medical application will be image guidance in proton radiosurgery. Proton images demonstrating the potential for this application are presented. Tomographic reconstructions are included to raise awareness of the possibility of high-resolution proton tomography using magneto-optics
ChemInform Abstract: The Cyclization of Parent and Cyclic Hexa‐1,3‐dien‐5‐ynes — A Combined Theoretical and Experimental Study.
No full textCan Fulvenes Form from Enediynes? A Systematic High-Level Computational Study on Parent and Benzannelated Enediyne and Enyne−Allene Cyclizations
No full textApart from the well-known Bergman, Myers−Saito, and Schmittel ring closure reactions of parent enediyne
(4) and enyne−allene (3), novel cyclization modes were identified using density functional (DFT) and coupled-cluster methods. The geometries obtained with several DFT functionals are quite similar; for consistency's
sake, we employed BLYP/6-31G* geometries; Brueckner double energy single points [BCCD(T)/cc-pVDZ]
on these geometries were used to determine the relative energies. The C−C5 cyclization of 4 leading to
fulvene biradical 8 is 40 kcal mol-1 endothermic, and the product lies 31 kcal mol-1 above 1,4-didehydrobenzene 7 because of the lack of aromatic stabilization. The heat of formation (ΔfH°) of 8 is predicted to be
172.0 ± 1.0 kcal mol-1. Yet another ring closure of 4 leading to dimethylenecyclobutene biradical 12 is 69
kcal mol-1 endothermic and is hardly of preparative interest. A new cyclization of 3 should lead to the seven-membered ring biradical 13, which is located 33 kcal mol-1 above 3 and 24 kcal mol-1 above the Schmittel
product 6. As the transition structure for both cyclizations differ by 11 kcal mol-1, 13 may form under suitable
conditions. All other possible modes of cyclization of 4 did not lead to stable products. Benzannelation has
a significant effect on the endothermicities of the Bergman and Myers−Saito cyclizations, which are 8−9
kcal mol-1 above the parent reactions due to reduced aromatization energy in the naphthalene derivatives.
The endothermicities of the other cyclization pathways are largely unaffected by benzannelation
