subject: Organic
0-5 of 5
Automerizations and skeleton rearrangements of polycyclic aromatic hydrocarbons
description- – Reversible benzene ring contractions are suggested to be the common mechanistic feature of automerization reactions, skeleton transformations and interconversions of polycyclic aromatic hydrocarbons. Experiments conducted with $/sp[13]$C-labeled PAH bring new insights into the acid catalyzed automerizations of (1-$/sp[13]$C) biphenyl and (1-$/sp[13]$C) anthracene and deepens the understanding of the 1,2-switch of carbon atoms in the thermal automerizations of (1-$/sp[13]$C) anthracene, (6-$/sp[13]$C) benzo (c) phenanthrene and (1-$/sp[13]$C) and (4-$/sp[13]$C) phenanthrenes at high temperatures (900-1100$/sp/circ$C). Eight bay region PAH (chrysene, benz (a) anthracene, phenanthrene, picene, perylene, benzo (ghi) perylene, benzo (e) pyrene, benz (a,c) anthracene) were discovered to undergo an intriguing transformation when heated at 1100$/sp/circ$C in flash pyrolyses. The novel information extracted from the experimental data can be summarized as follow:loss of two hydrogens to increase the C/H ratio, the creation of five-membered rings from a purely benzenoid compound, an increase in the total number of rings, permanent destruction of a benzene ring and generation of a fullerene subunit. Tests of the proposed mechanistic pathways were achieved by independent generation of the proposed intermediates and by $/sp[13]$C-labeling experiments. Thermal formation of five-membered rings was used to synthesize a series of interesting new nonalternant hydrocarbons (cyclopenta (cd) fluoranthene, dicyclopenta (cd,fg), (cd,jk) and (cd,mn) pyrene, cyclopenta (cd) coronene and benzo (ghi) cyclopenta (cd) perylene, all of which are products that are formed in pyrolyses of other PAH and/or in ethylene flames. In the search for an authentic example of a Stone-Wales rearrangement i.e., benzo (d) pyracylene to cyclopenta (cd) fluoranthene, the first example of migration of an ethynyl group from one benzene ring to another along the edge of an anthracene ring was discovered. All the transformations presented could play a significant role in the chemistry of hydrocarbons in flames, in the mechanism of formation of carcinogens in smoke and in the production and/or interconversions of fullerenes.
- – 1996-01-01
Design and development of new, catalytic carbon-carbon bond forming reactions using zirconium and nickel
description- – Key mechanistic features of the Zr-catalyzed carbomagnesation are illustrated below. Addition of EtMgCl to I leads to the formation of the derived zirconate II which undergoes site-selective cleavage, due to chelation of magnesium with the neighboring Lewis basic alkoxide (III to IV). Subsequent $/beta$-hydride abstraction followed by reductive elimination regenerates the catalyst (V) and provides the alkylmagnesium VI which is then trapped by an electrophile (e.g., O$/sb2$) to afford VII. Catalytic asymmetric synthesis. 2,5-Dihydrofuran in equation 1 undergoes highly enantioselective ethylmagnesation (97% enantiopurity). Catalytic kinetic resolution. As exemplified below, subjection of a racemic mixture of pyrans to asymmetric carbometallation conditions affords efficient and facile kinetic resolution. Similar results have been obtained for pyrans with a variety of substitution patterns. Ni-catalyzed alkylations. We have been investigating a phosphine directed Ni-catalyzed addition of Grignard reagents to allylic ethers. The Lewis basic phosphine is necessary for reactivity and selectivity (eq 3). An example shown below is representative.* ftn*Please refer to the dissertation for diagrams.
- – 1996-01-01
Zirconium-catalyzed carbomagnesation reaction development and application in synthesis
description- – Chapter I. The chemistry and catalytic activity of $/rm Cp/sb2ZrCl/sb2$ is reviewed, particularly in connection with its interaction with Grignard reagents. The most recent advances in carbon-carbon bond formation by the addition of alkyl magnesium halide reagents to olefins in the presence of $/rm Cp/sb2ZrCl/sb2$ as a catalyst is discussed. Additionally, the use of zirconium based catalysts to facilitate intramolecular diene cyclization is presented. Chapter 2. The development of the zirconium-catalyzed carbomagnesation of unactivated olefins into a diastereoselective reaction is achieved. High levels of diastereoselectivity were obtained for allylic and homoallylic alcohols and ethers. The mechanism of this reaction was initially studied using norbornenol as a mechanistic probe. The rigidity of this system enabled us to draw significant mechanistic conclusions. The mechanism of the zirconium-catalyzed carbomagnesation of acyclic substrates was then studied. The identity of the active catalyst, mechanism of alkylation and origin of selectivity in zirconacyclopentane cleavage were thoroughly investigated. Chapter 3. The first synthesis of the aglycon portion of the antifungal agent sch 38516 was accomplished applying our newly developed diastereo- and enantioselective carbomagnesation reactions, in addition to a variety of other new catalytic reactions. The synthesis was accomplished in 13 steps, nine of which employed metal catalysis. Two new tandem catalytic reactions were discovered and a novel Molybdenum catalyzed macrocyclic metathesis was performed.
- – 1996-01-01
Intramolecular cyclobutadiene cycloadditions and ruthenium-catalyzed selective ring-opening metathesis
description- – <?Pub Inc>Chapter 1. A brief survey of ruthenium-catalyzed transformations is included. Chapter 2. Described in this chapter is the development of the first intramolecular cycloaddition of cyclobutadiene and an olefin. These reactions result in cyclobutene formation which were further functionalized through thermolysis to generate substituted 1,3-cyclohexadienes.* Chapter 3. Selective ring-opening metathesis of cyclobutenes is demonstrated as a direct route to selectively substituted 1,5-dienes which are further transformed stereospecifically to 1,5-cyclooctadiene systems.* Chapter 4. During investigation of selective cyclobutene ring-opening metathesis, intramolecular olefin coordination to ruthenium was observed and confirmed through crystal structure analysis. Chapter 5. A novel usage of Grubbs ruthenium catalyst is investigated. This transformation is characterized as a Kharasch addition of chloroform to mono- and 1,1-disubstituted olefins.* *Please refer to dissertation for diagrams.
- – 1999-01-01
The Golgi-disrupting agents ilimaquinone and norrisolide: Determination of biological interactions through synthesis
description- – Chapter 1. Case studies of the natural products brefeldin A, discodermolide, and nakijiquinone C are reviewed. Total synthesis, analog preparation, and biological activity studies are discussed that have given insight into structure-activity relationships.* Chapter 2. Previous work on the total synthesis of ilimaquinone and analogs in our laboratories is reviewed, as well as determination of the biologically active analogs. Synthetic biologically active analogs of ilimaquinone, including affinity, photoaffinity, and fluorescent analogs, support the biological target of ilimaquinone identified as S-adenosylhomocysteine hydrolase. Fluorescent labeling of the Golgi apparatus shows that the methylating agent S-adenosylmethionine reverses the biological effects of ilimaquinone.* Chapter 3. Synthesis of the hydrindane core of norrisolide is accomplished in four steps. The side chain of norrisolide is prepared rapidly via development of a cyclopropanation/thermal rearrangement strategy, and altering the cyclopropanation catalyst gives good enantioselectivities. A Shapiro reaction followed by addition of the hydrindane vinyllithium to the Weinreb amide of the sidechain accomplishes the key coupling step. A few more functional group manipulations have led to the diastereomer of norrisolide, and work is in progress to use a similar strategy to prepare norrisolide itself.* *Please refer to dissertation for diagrams.
- – 2004-01-01
0-5 of 5