The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform
Victor Snieckus - One of the best experts on this subject based on the ideXlab platform.
-
directed ortho metalation and anionic ortho fries rearrangement of polycyclic aromatic o Carbamates regioselective synthesis of substituted chrysenes
Journal of Organic Chemistry, 2018Co-Authors: Sindhu Kancherla, Victor Snieckus, Marianne Lorentzen, Kare B JorgensenAbstract:A general method for the regioselective synthesis of a series of ortho-substituted chrysenyl N,N-diethyl-O-Carbamates by the directed ortho-metalation (DoM) strategy is reported. The starting O-Carbamates were prepared from the corresponding chrysenols, available by oxidative photochemical cyclization or directed remote metalation tactics. Chrysen-1-yl and chrysene-3-yl ring site selectivity of directed ortho-metalation (DoM) and anionic ortho-Fries rearrangement (AoF) protocols, with s-BuLi/TMEDA, followed by electrophilic quench using a selection of electrophiles, were observed, leading to new chrysenyl derivatives. 5-Chrysenyl N,N-diethyl-O-Carbamate underwent instant AoF rearrangement even at −100 °C to furnish chrysenyl o-hydroxycarboxamide. Iterative DoM reactions were carried out to gain insight into the regioselectivity factors.
-
directed ortho metalation of o aryl n n dialkylCarbamates methodology anionic ortho fries rearrangement and lateral metalation
European Journal of Organic Chemistry, 2018Co-Authors: M Jalil A Miah, Mukund P Sibi, S Chattopadhyay, Oluwole B Familoni, Victor SnieckusAbstract:Described is the directed ortho-lithiation reaction of N,N-dialkyl O-aryl Carbamates, N,N-dialkyl O-naphthyl-1 and N,N-dialkyl O-naphthyl-2 Carbamates with sec-butyllithium/TMEDA followed by quenching with various electrophiles to afford a range of polysubstituted aromatic compounds (Tables 1 and 2, Schemes 2 and 3). Warming solutions of the ortho-lithiated O-Carbamates to room temperature without external quench with electrophiles leads to the formation of salicylamide and 1- and 2-hydroxynaphthamide derivatives by an anionic ortho-Fries rearrangements (Scheme 4). The relative stability and reactivity of different N,N-dialkyl O-Carbamates has been investigated. Lateral metalation of 2-tolyl O-Carbamate with LDA provides a synthesis of benzo[b]furan-2(3H)-ones (Scheme 2). Using reported results, a comparison of seven O-based directed metalation groups in reaction with several electrophiles is made. The described methodology is useful for the preparation of highly, 1,2,3-substituted, aromatic compounds
-
Directed ortho-Metalation and Anionic ortho-Fries Rearrangement of Polycyclic Aromatic O‑Carbamates: Regioselective Synthesis of Substituted Chrysenes
2018Co-Authors: Sindhu Kancherla, Victor Snieckus, Marianne Lorentzen, Kare B JorgensenAbstract:A general method for the regioselective synthesis of a series of ortho-substituted chrysenyl N,N-diethyl-O-Carbamates by the directed ortho-metalation (DoM) strategy is reported. The starting O-Carbamates were prepared from the corresponding chrysenols, available by oxidative photochemical cyclization or directed remote metalation tactics. Chrysen-1-yl and chrysene-3-yl ring site selectivity of directed ortho-metalation (DoM) and anionic ortho-Fries rearrangement (AoF) protocols, with s-BuLi/TMEDA, followed by electrophilic quench using a selection of electrophiles, were observed, leading to new chrysenyl derivatives. 5-Chrysenyl N,N-diethyl-O-Carbamate underwent instant AoF rearrangement even at −100 °C to furnish chrysenyl o-hydroxycarboxamide. Iterative DoM reactions were carried out to gain insight into the regioselectivity factors
-
suzuki miyaura cross coupling of aryl Carbamates and sulfamates experimental and computational studies
Journal of the American Chemical Society, 2011Co-Authors: Kyle W Quasdorf, Aurora Antoftfinch, Peng Liu, Amanda L Silberstein, Anna Komaromi, Tom Blackburn, Stephen D Ramgren, K N Houk, Victor Snieckus, Neil K GargAbstract:The first Suzuki−Miyaura cross-coupling reactions of the synthetically versatile aryl O-Carbamate and O-sulfamate groups are described. The transformations utilize the inexpensive, bench-stable catalyst NiCl2(PCy3)2 to furnish biaryls in good to excellent yields. A broad scope for this methodology has been demonstrated. Substrates with electron-donating and electron-withdrawing groups are tolerated, in addition to those that possess ortho substituents. Furthermore, heteroaryl substrates may be employed as coupling partners. A computational study providing the full catalytic cycles for these cross-coupling reactions is described. The oxidative addition with Carbamates or sulfamates occurs via a five-centered transition state, resulting in the exclusive cleavage of the aryl C−O bond. Water is found to stabilize the Ni-Carbamate catalyst resting state, which thus provides rationalization of the relative decreased rate of coupling of Carbamates. Several synthetic applications are presented to showcase the uti...
Neil K Garg - One of the best experts on this subject based on the ideXlab platform.
-
Suzuki–Miyaura Cross-Coupling of Aryl Carbamates and Sulfamates: Experimental and Computational Studies
2015Co-Authors: Neil K GargAbstract:The first Suzuki–Miyaura cross-coupling reactions of the synthetically versatile O-aryl Carbamate and O-sulfamate groups is described. The transformations utilize the inexpensive, bench-stable catalyst NiCl2(PCy3)2 to furnish biaryls in good to excellent yields. A broad scope for this methodology has been demonstrated. Substrates with electron-donating and electron-withdrawing groups (EDGs, EWGs) are tolerated, in addition to those that possess ortho substitutents. Furthermore, heteroaryl substrates may be employed as coupling partners. A computational study providing the full catalytic cycles for these cross-coupling reactions is described. The oxidative additions with Carbamates and sulfamates occur via a five-centered transition state, resulting in the exclusive cleavage of the Ar–O bond. Water is found to stabilize the Ni–Carbamate catalyst resting state, and thus provides rationalization of the relative decreased rate of coupling of Carbamates. Several synthetic applications are presented to showcase the utility of the methodology in the synthesis of polysubstituted aromatic compounds of natural product and bioactive molecule interest
-
nickel catalyzed amination of aryl Carbamates and sequential site selective cross couplings
Chemical Science, 2011Co-Authors: Tehetena Mesganaw, Peng Liu, Amanda L Silberstein, Stephen D Ramgren, Noah Fine F Nathel, Xin Hong, Neil K GargAbstract:We report the amination of aryl Carbamates using nickel-catalysis. The methodology is broad in scope with respect to both coupling partners and delivers aminated products in synthetically useful yields. Computational studies provide the full catalytic cycle of this transformation, and suggest that reductive elimination is the rate-determining step. Given that Carbamates are easy to prepare, robust, inert to Pd-catalysis, and useful for arene functionalization, these substrates are particularly attractive partners for use in synthesis. The sequential use of Carbamate functionalization/site-selective cross-coupling processes highlights the utility of this methodology.
-
suzuki miyaura cross coupling of aryl Carbamates and sulfamates experimental and computational studies
Journal of the American Chemical Society, 2011Co-Authors: Kyle W Quasdorf, Aurora Antoftfinch, Peng Liu, Amanda L Silberstein, Anna Komaromi, Tom Blackburn, Stephen D Ramgren, K N Houk, Victor Snieckus, Neil K GargAbstract:The first Suzuki−Miyaura cross-coupling reactions of the synthetically versatile aryl O-Carbamate and O-sulfamate groups are described. The transformations utilize the inexpensive, bench-stable catalyst NiCl2(PCy3)2 to furnish biaryls in good to excellent yields. A broad scope for this methodology has been demonstrated. Substrates with electron-donating and electron-withdrawing groups are tolerated, in addition to those that possess ortho substituents. Furthermore, heteroaryl substrates may be employed as coupling partners. A computational study providing the full catalytic cycles for these cross-coupling reactions is described. The oxidative addition with Carbamates or sulfamates occurs via a five-centered transition state, resulting in the exclusive cleavage of the aryl C−O bond. Water is found to stabilize the Ni-Carbamate catalyst resting state, which thus provides rationalization of the relative decreased rate of coupling of Carbamates. Several synthetic applications are presented to showcase the uti...
Tomohiko Ohwada - One of the best experts on this subject based on the ideXlab platform.
-
acid promoted chemoselective introduction of amide functionality onto aromatic compounds mediated by an isocyanate cation generated from Carbamate
Chemistry-an Asian Journal, 2014Co-Authors: Akinari Sumita, Hiroaki Kurouchi, Yuko Otani, Tomohiko OhwadaAbstract:: Carbamates have been used as precursors of isocyanates, but heating in the presence of strong acids is required because cleavage of the C-O bond in Carbamates is energy-demanding even in acid media. Direct amidation of aromatic compounds by isocyanate cations generated at room temperature from carbamoyl salicylates in trifluoromethanesulfonic acid (TfOH) was examined. Carbamates with ortho-salicylate as an ether group (carbamoyl salicylates) showed dramatically accelerated O-C bond dissociation in TfOH, which resulted in facile generation of the isocyanate cation. These chemoselective intermolecular aromatic amidation reactions proceeded even at room temperature and showed good compatibility with other electrophilic functionalities and high discrimination between N-monosubstituted Carbamate and N,N-disubstituted Carbamate. The reaction rates of secondary and tertiary amide formation were markedly different, and this difference was utilized to achieve successive (tandem) amidation reactions of molecules with an N-monosubstituted Carbamate and an N,N-disubstituted Carbamate with two kinds of aromatic compounds.
-
protonation switching to the least basic heteroatom of Carbamate through cationic hydrogen bonding promotes the formation of isocyanate cations
Chemistry: A European Journal, 2014Co-Authors: Hiroaki Kurouchi, Akinari Sumita, Yuko Otani, Tomohiko OhwadaAbstract:We found that phenethylCarbamates that bear ortho-salicylate as an ether group (carbamoyl salicylates) dramatically accelerate OC bond dissociation in strong acid to facilitate generation of isocyanate cation (N-protonated isocyanates), which undergo subsequent intramolecular aromatic electrophilic cyclization to give dihydroisoquinolones. To generate isocyanate cations from Carbamates in acidic media as electrophiles for aromatic substitution, protonation at the ether oxygen, the least basic heteroatom, is essential to promote CO bond cleavage. However, the carbonyl oxygen of Carbamates, the most basic site, is protonated exclusively in strong acids. We found that the protonation site can be shifted to an alternative basic atom by linking methyl salicylate to the ether oxygen of Carbamate. The methyl ester oxygen ortho to the phenolic (ether) oxygen of salicylate is as basic as the Carbamate carbonyl oxygen, and we found that monoprotonation at the methyl ester oxygen in strong acid resulted in the formation of an intramolecular cationic hydrogen bond (>CO(+) H⋅⋅⋅O<) with the phenolic ether oxygen. This facilitates OC bond dissociation of phenethylCarbamates, thereby promoting isocyanate cation formation. In contrast, superacid-mediated diprotonation at the methyl ester oxygen of the salicylate and the carbonyl oxygen of the Carbamate afforded a rather stable dication, which did not readily undergo CO bond dissociation. This is an unprecedented and unknown case in which the monocation has greater reactivity than the dication.
David H North - One of the best experts on this subject based on the ideXlab platform.
-
an evaluation of atmospheric pressure ionization techniques for the analysis of n methyl Carbamate pesticides by liquid chromatography mass spectrometry
Journal of the American Society for Mass Spectrometry, 1992Co-Authors: S Pleasance, Joseph F Anacleto, Ruth M Bailey, David H NorthAbstract:Atmospheric pressure ionization (API) techniques are evaluated for the mass spectral analysis of N-methyl Carbamate pesticides. Atmospheric pressure chemical ionization (APCI) using a heated nebulizer interface provided both protonated molecules and abundant, characteristic fragment ions. With ion spray (ISP; pneumatically assisted electrospray ionization), which utilizes a milder “ion evaporation” process, primarily protonated molecules were obtained, although fragment ions similar to those observed in APCI could be induced by variation of the API orifice voltage. Product ion spectra of ISP-derived protonated molecules, generated by tandem mass spectrometry using collision-induced dissociation, are also presented. The APCI and ISP spectra of the Carbamates are compared to those obtained with a thermospray interface and also to their electron ionization and methane CI spectra obtained with a particle beam interface. For all four interfaces, combined liquid chromatography mass spectrometry methods using conventional (4.6 mm i.d.) columns are described for the separation and detection of pesticide mixtures. These methods are applied to the confirmatory analysis of three representative Carbamate pesticides, spiked at the 0.1-ppm level in green peppers. For those Carbamates amenable to gas chromatography mass spectrometry, comparative results are presented.
Graeme Puxty - One of the best experts on this subject based on the ideXlab platform.
-
toward rational design of amine solutions for pcc applications the kinetics of the reaction of co2 aq with cyclic and secondary amines in aqueous solution
Environmental Science & Technology, 2012Co-Authors: William Conway, Debra Fernandes, Geoffrey A. Lawrance, Graeme Puxty, Xiaoguang Wang, Robert T Burns, Marcel MaederAbstract:The kinetics of the fast reversible Carbamate formation reaction of CO2(aq) with a series of substituted cyclic secondary amines as well as the noncyclic secondary amine diethanolamine (DEA) has been investigated using the stopped-flow spectrophotometric technique at 25.0 °C. The kinetics of the slow parallel reversible reaction between HCO3– and amine has also been determined for a number of the amines by 1H NMR spectroscopy at 25.0 °C. The rate of the reversible reactions and the equilibrium constants for the formation of carbamic acid/Carbamate from the reactions of CO2 and HCO3– with the amines are reported. In terms of the forward reaction of CO2(aq) with amine, the order with increasing rate constants is as follows: diethanolamine (DEA) < morpholine (MORP) ∼ thiomorpholine (TMORP) < N-methylpiperazine (N-MPIPZ) < 4-piperidinemethanol (4-PIPDM) ∼ piperidine (PIPD) < pyrrolidine (PYR). Both 2-piperidinemethanol (2-PIPDM) and 2-piperidineethanol (2-PIPDE) do not form Carbamates. For the Carbamate formi...
-
Investigations of primary and secondary amine Carbamate stability by 1H NMR spectroscopy for post combustion capture of carbon dioxide
The Journal of Chemical Thermodynamics, 2012Co-Authors: Debra Fernandes, Robert C. Burns, Geoffrey A. Lawrance, William Conway, Marcel Maeder, Graeme PuxtyAbstract:Carbamate formation is one of the major chemical reactions that can occur in solution in the capture of CO2 by amine-based solvents, and Carbamate formation makes a significant enthalpy contribution to the absorption-desorption of CO2 that occurs in the absorber/stripper columns of the PCC process. Consequently, the formation of Carbamates of selected series of primary and secondary amines over the temperature range (288 to 318) K has been investigated by equilibrium 1H NMR studies, and the stability constants (K9) for the equilibrium: RNH2+HCO3-⇄K9RNHCOO-+H2O are reported. van’t Hoff analyses have resulted in standard molar enthalpies, ΔHmo, and entropies, ΔSmo, of Carbamate formation. A ΔHmo-ΔSmo plot generates a linear correlation for Carbamate formation (providing a mean standard molar free energy, ΔGmo, for Carbamate formation of about −7 kJ · mol−1), and this relationship helps provide a guide to the selection of an amine(s) solvent for CO2 capture, in terms of enthalpy considerations. A linear ΔHmo-ΔSmo plot also occurs for Carbamate protonation. The formation of the Carbamates has been correlated with systematic changes in composition and structure, and steric effects have been identified by comparing molecular geometries obtained using density functional B3LYP/6-311++G(d,p) calculations. Trends in steric effects have been identified in the series of compounds monoethanolamine (MEA), 1-amino-2-propanol, 2-amino-1-propanol (AP) and 2-amino-2-methyl-1-propanol (AMP). In the case of 2-piperidinemethanol, 2-piperidineethanol and 3-piperidinemethanol, strong intramolecular hydrogen bonding is shown to be the likely cause for lack of Carbamate formation, and in the ring systems of pyrrolidine, morpholine, piperidine and thiomorpholine trends in Carbamate formation (as given by K9) have been correlated with the internal ring angle at the amine nitrogen, as well as the planarity of the environment around the nitrogen atom.