Person: Hennessy, Elisabeth
Loading...
Email Address
AA Acceptance Date
Birth Date
Research Projects
Organizational Units
Job Title
Last Name
Hennessy
First Name
Elisabeth
Name
Hennessy, Elisabeth
4 results
Search Results
Now showing 1 - 4 of 4
Publication C-H Amination Catalysis from High-Spin Ferrous Complexes(2013-10-15) Hennessy, Elisabeth; Betley, Theodore A; Jacobsen, Eric; Ritter, TobiasThe C-H amination and olefin aziridination chemistry of iron supported by dipyrromethene ligands (RLAr, L=1,9-R2-5-aryldipyrromethene, R = Mes, 2,4,6-Ph3C6H2, tBu, Ad, 10-camphoryl, Ar = Mes, 2,4,6-Cl3C6H2) was explored. The weak-field, pyrrole-based dipyrrinato ligand was designed to generate an electrophilic, high-spin metal center capable of accessing high valent reactive intermediates in the presence of organic azides. Isolation of the reactive intermediate in combination with a series of mechanistic experiments suggest the N-group transfer chemistry proceeds through a rapid, single-electron pathway and maintains an overall S=2 electronic configuration throughout the catalytic cycle. We have established the catalysts' strong preference for allylic amination over aziridination with olefin containing substrates. Aziridination is limited to styrenyl substrates without allylic C-H bonds, while allylic amination has been demonstrated with both cyclic and linear aliphatic alkenes. Notably, the functionalization of &alpha-olefins to linear allylic amines occurs with outstanding regioselectivity.Publication Complex N-Heterocycle Synthesis via Iron-Catalyzed, Direct C-H Bond Amination(American Association for the Advancement of Science (AAAS), 2013) Hennessy, Elisabeth; Betley, TheodoreThe manipulation of traditionally unreactive functional groups is of paramount importance in modern chemical synthesis. We have developed an iron-dipyrrinato catalyst that leverages the reactivity of iron-borne metal-ligand multiple bonds to promote the direct amination of aliphatic C–H bonds. Exposure of organic azides to the iron dipyrrinato catalyst furnishes saturated, cyclic amine products (N-heterocycles) bearing complex core-substitution patterns. This study highlights the development of C–H bond functionalization chemistry for the formation of saturated, cyclic amine products and should find broad application in the context of both pharmaceuticals and natural product synthesis.Publication Catalytic C−H Bond Amination from High-Spin Iron Imido Complexes(American Chemical Society (ACS), 2011) King, Evan R.; Hennessy, Elisabeth; Betley, TheodoreDipyrromethene ligand scaffolds were synthesized bearing large aryl (2,4,6-Ph3C6H2, abbreviated Ar) or alkyl (tBu, adamantyl) flanking groups to afford three new disubstituted ligands (RL, 1,9-R2-5-mesityldipyrromethene, R = aryl, alkyl). While high-spin (S = 2), four-coordinate iron complexes of the type (RL)FeCl(solv) were obtained with the alkyl-substituted ligand varieties (for R = tBu, Ad and solv = THF, OEt2), use of the sterically encumbered aryl-substituted ligand precluded binding of solvent and cleanly afforded a high-spin (S = 2), three-coordinate complex of the type (ArL)FeCl. Reaction of (AdL)FeCl(OEt2) with alkyl azides resulted in the catalytic amination of C−H bonds or olefin aziridination at room temperature. Using a 5% catalyst loading, 12 turnovers were obtained for the amination of toluene as a substrate, while greater than 85% of alkyl azide was converted to the corresponding aziridine employing styrene as a substrate. A primary kinetic isotope effect of 12.8(5) was obtained for the reaction of (AdL)FeCl(OEt2) with adamantyl azide in an equimolar toluene/toluene-d8 mixture, consistent with the amination proceeding through a hydrogen atom abstraction, radical rebound type mechanism. Reaction of p-tBuC6H4N3 with (ArL)FeCl permitted isolation of a high-spin (S = 2) iron complex featuring a terminal imido ligand, (ArL)FeCl(N(p-tBuC6H4)), as determined by 1H NMR, X-ray crystallography, and 57Fe Mössbauer spectroscopy. The measured Fe−Nimide bond distance (1.768(2) Å) is the longest reported for Fe(imido) complexes in any geometry or spin state, and the disruption of the bond metrics within the imido aryl substituent suggests delocalization of a radical throughout the aryl ring. Zero-field 57Fe Mössbauer parameters obtained for (ArL)FeCl(N(p-tBuC6H4)) suggest a FeIII formulation and are nearly identical with those observed for a structurally similar, high-spin FeIII complex bearing the same dipyrromethene framework. Theoretical analyses of (ArL)FeCl(N(p-tBuC6H4)) suggest a formulation for this reactive species to be a high-spin FeIII center antiferromagnetically coupled to an imido-based radical (J = −673 cm−1). The terminal imido complex was effective for delivering the nitrene moiety to both C−H bond substrates (42% yield) as well as styrene (76% yield). Furthermore, a primary kinetic isotope effect of 24(3) was obtained for the reaction of (ArL)FeCl(N(p-tBuC6H4)) with an equimolar toluene/toluene-d8 mixture, consistent with the values obtained in the catalytic reaction. This commonality suggests the isolated high-spin FeIII imido radical is a viable intermediate in the catalytic reaction pathway. Given the breadth of iron imido complexes spanning several oxidation states (FeII−FeV) and several spin states (S = 0 → 3/2), we propose the unusual electronic structure of the described high-spin iron imido complexes contributes to the observed catalytic reactivity.Publication Iron-mediated intermolecular N-group transfer chemistry with olefinic substrates(Royal Society of Chemistry (RSC), 2014) Hennessy, Elisabeth; Liu, Richard Y.; Iovan, Diana Alexandra; Duncan, Ryan A.; Betley, TheodoreThe dipyrrinato iron catalyst reacts with organic azides to generate a reactive, high-spin imido radical intermediate, distinct from nitrenoid or imido species commonly observed with low-spin transition metal complexes. The unique electronic structure of the putative group-transfer intermediate dictates the chemoselectivity for intermolecular nitrene transfer. The mechanism of nitrene group transfer was probed via amination and aziridination of para-substituted toluene and styrene substrates, respectively. The Hammett analysis of both catalytic amination and aziridination reactions indicate the rate of nitrene transfer is enhanced with functional groups capable of delocalizing spin. Intermolecular amination reactions with olefinic substrates bearing allylic C–H bonds give rise to exclusive allylic amination with no apparent aziridination products. Amination of substrates containing terminal olefins give rise exclusively to allylic C–H bond abstraction, C–N recombination occurring at the terminal C with transposition of the double bond. A similar reaction is observed with cis-β-methylstyrene where exclusive amination of the allylic position is observed with isomerization of the olefin to the trans-configuration. The high levels of chemoselectivity are attributed to the high-spin electronic configuration of the reactive imido radical intermediate, while the steric demands of the ligand enforce regioselective amination at the terminal position of linear α-olefins.