Browsing by Subject "Ring formation (Chemistry)"
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Item Studies on cycloaddition reactions of ketenes: further investigations of pseudopericyclic reaction mechanisms(Texas Tech University, 2004-05) Zhou, ChunCycloaddition reactions of ketenes are widely used in organic synthesis. The cycloaddhions of kelene with ethylene, formaldimine and formaldehyde were studied theoretically using B3LYP/6-31G*, MP2/6-31G* and MCSCF/6-31G* methods. A pseudopericyclic mechanism was proposed for the reactions between ketene with ethylene and formaldehyde. A pseudopericyclic transition state was also located for the reaction of kelene with formaldimine even though h has a higher energy barrier than a conrotalory eleclrocyclization transition state, which is a concerted in the gas phase and a stepwise in solvent confirmed by IRC calculations. The cycloadditions of formaldimine with conjugated ketenes were studied theoretically al the B3LYP/6-3IG* level. The concerted and stepwise [4 + 2], [2 + 2] cycloaddition reactions were examined systematically. For the reaction of formaldimine with vinylkelene, the stepwise [2 + 2] and the concerted [4 + 2] are all pericyclic cycloaddhion reactions and have similar energy barriers. For the cycloaddition reactions between formaldimine with imidoylketene and formylketene, the stepwise [4 + 2] pathways are the lowest energy barrier ones. The concerted [4 + 2] and the second step of the stepwise [4 + 2], eleclrocyclization steps are pseudopericyclic with dramatically low barriers. In addition, the choice of formaldimine led the 1, 5- and I, 3-hydrogen shift reaction possible in some zwitterions intermediate, therefore transition stales of them were located. These transition states have low energy barriers and planar geometries. Thus these reactions are best interpreted as pseudopericyclic as well. The least developed conjugated ketenes, imidoylkelenes were studied more experimentally and theoretically. For the first lime, a novel reaction condition was developed to generate a variety of substituted imidoylkelenes. The first biomolecular reactions observed are dimerizations of imidoylkelenes, which were studied experimentally and theoretically. A series of trapping reactions with other reagents were studied as well. In addition, a new method to generate oxokelenes from p-kelo carboxylic acids were investigated and some experimental evidence for the pseudopericyclic reaction mechanism of the thermal chelelropic reactions were found by analyzing the Xray structures of ground states of pyrrolediones, furandiones and their derivatives. Finally, sequential transition states were located in the formation of a semibullvalene and the valley-ridge inflection point was located on its potential energy surface as well.Item Theoretical and experimental studies on the reactivities of conjugated ketenes(Texas Tech University, 1998-08) Ham, SihyunThe reactivity of imidoylketene was examined using ab initio molecular orbital theory. MP4(SDQ)/6-31G*//MP2/6-31G* calculations on the conformations of imidoylketene as well as transition states for several of its reactions show parallels between the reactivity of imidoylketene and its oxygen analog formylketene. All reactions proceed via concerted, planar (or nearly so) transition structures regardless of the number of electrons involved. Calculated activation energies are remarkably lower than those for a pericyclic process, as expected from the case of formylketene. The reactions are interpreted in light of their pseudopericyclic orbital topology. N-Propylacetacetimidoylketene was produced by the solution pyrolysis of t-butyl N-propyl 3-amino-2-butenoate. Selectivities of acetimdoylketene toward various polar reagents were measured for the first time in a series of competitive trapping reactions. Significant steric and electronic discriminations of this ketone were observed, suggesting further synthetically useful applications. These experimental reactivity trends indirectly provide support for the planar, pseudopericyclic transition structures predicted by ab initio calculations. The mechanism of the reactions of nitrosoketene to form cyclic nitrones (which leads stereoselective synthesis of a-amino acids) was investigated using ab initio molecular orbital theory (MP4(SDQ)/6-31G*//MP2/6-31G* + ZPE). The direct [3+2] cycloadditions of nitrosoketene with ketones are calculated to be favored over the alternative [4+2] pathway via concerted, asynchronous, pseudopericyclic transition states. The detailed conformations and the reactivity of nitrosoketene toward sterically and electronically different ketones render useful information of the synthetic route for the biologically important reactions. Transition structures for a series of eight cheletropic decarbonylations were optimized at the MP4(SDQ)/D95**//MP2/6-3lG* + ZPE level. Dramatic differences in activation energies and in exothermicities are discussed in terms of the molecular orbital topology. A fundamental question regarding pseudopericyclic orbital overlap is addressed, specifically, how many and what type of orbital orthogonahties in the reaction sites are needed for a reaction to be pseudopericyclic. Generalizations regarding the characteristics of the pseudopericyclic reactions are made to provide a better understanding of the "allowedness" and "favoredness" of the orbital topologies.Item Transition metal and organo-catalyzed cyclizations, cycloadditions and couplings(2004) Cauble, David Frederic; Krische, Michael J.Item Transition metal-mediated cyclizations and synthesis of annonaceous acetogenin analogs(2006-08) Gorman, Jeffrey Scott Thomas, 1976-; Pagenkopf, Brian L.Item Tridentate nitrogen ligands derived from 2,6-bis-hydrazinopyridine (BHP) : preparation and study of the 2,6-bis-hydrasonopyridines, 2, 6-bis-pyrazolylpytidines, and 2,6-bis-indazolylpyridines.(2009-06-02T17:55:17Z) Duncan, Nathan C.; Garner, Charles M. (Charles Manley), 1957-; Chemistry and Biochemistry.; Baylor University. Dept. of Chemistry and Biochemistry.The development of ligands for asymmetric catalysis has been a focal point in our research group. Tridentate nitrogen ligands have been used in a variety of asymmetric catalytic reactions. Of these, the 2,6-bis-pyrazolylpyridine class of ligands has found only limited use, due to difficulties in the normal synthetic route that limit the synthesis of ligands with bulky chiral groups attached to the pyrazole rings. The chiral derivatives that have been used in catalysis have shown modest to poor results, due in part to limitations in the synthesis of these ligands that prevent the development of bulky, chiral ligands. A new synthetic route has been developed using 2,6-bis-hydrazinopyridine (BHP). This route allows for a facile, one-pot synthesis of 2,6-bis-pyrazolylpyridines that is not limited by the bulkiness of the groups that become attached to the pyrazole ring. The primary focus of this research has been the development of new chiral 2,6-bis-pyrazolylpyridines and an investigation into the limitations of steric bulk on chelation ability in this class of ligands. For this purpose, several novel chiral ligands have been synthesized. The effect of sterics and electronics on the regioselectivity of the formation of the pyrazole ring was also studied in order to develop more regioselective routes to this class of ligands. The new bulky pyrazolylpyridine ligands that have been synthesized using this route have also been tested for the ability to sucessfully coordinate a transition metal. This study lead to a better understanding of the limitations the size of the substitutents attached to pyrazole ring have on the ligands’ ability to chelate metals. While BHP was developed primarily for the synthesis of the 2,6-bis-pyrazolylpyridine class of ligands, its use has now been expanded to the synthesis of other classes of tridentate-nitrogen ligands, many of which would be diffiuclt or impossible to synthesize through any other route. Using this new methodolgy, the syntheses of two novel classes of ligands, each with unique properties, the 2,6-bis-hydrazonopyridines and 2,6-bis-indazolylpyridines, have now been accomplished. The previously unknown 2,6-bis-indazolylpyridine class of ligands is offers the possibility to synthesize more robust catalysts than is possible using the bis-pyrazole ligands because of the electronic nature of the indazole ring.