Showing papers in "Journal of Organic Chemistry in 2010"
TL;DR: The practical method provides for safer handling and drying of solvents than methods calling for the use of reactive metals, metal hydrides, or solvent distillation and can be used to rapidly and reliably generatesolvents with low residual water content by means of commonly available materials found in most synthesis laboratories.
Abstract: Various commonly used organic solvents were dried with several different drying agents. A glovebox-bound coulometric Karl Fischer apparatus with a two-compartment measuring cell was used to determine the efficiency of the drying process. Recommendations are made relating to optimum drying agents/conditions that can be used to rapidly and reliably generate solvents with low residual water content by means of commonly available materials found in most synthesis laboratories. The practical method provides for safer handling and drying of solvents than methods calling for the use of reactive metals, metal hydrides, or solvent distillation.
648 citations
TL;DR: This perspective presents a simple and informative definition of "ideality" and demonstrates its use during the self-evaluation of several syntheses from the laboratory.
Abstract: The field of total synthesis has a rich history and a vibrant future. Landmark advances and revolutionary strides in the logic of synthesis have put the practicing chemist in the enviable position of being able to create nearly any molecule with enough time and effort. The stage is now set for organic chemists to aim for "ideality" in the way molecules are synthesized. This perspective presents a simple and informative definition of "ideality" and demonstrates its use during the self-evaluation of several syntheses from our laboratory.
483 citations
TL;DR: This Perspective summarizes a series of iron-catalyzed C-C bond formation reactions developed by us, which include (asymmetric) carbometalation of olefins, cross-coupling of alkyl halides, and activation of sp(2) and sp(3) C-H bonds.
Abstract: The use of iron as a catalyst for organic synthesis has been increasingly attracting the interest of chemists from economical and ecological points of view. While Fe(III) and Fe(II) catalysts have long been used as Lewis acids for synthesis, we have been interested in exploration of catalysis based on rather unexplored organoiron chemistry since the late 1990s. This Perspective summarizes a series of iron-catalyzed C−C bond formation reactions developed by us, which include (asymmetric) carbometalation of olefins, cross-coupling of alkyl halides, and activation of sp2 and sp3 C−H bonds.
417 citations
TL;DR: A methodology for the high yield and facile synthesis of isoquinolones from benzamide and alkynes via the oxidative ortho C-H activation of benzamides has been developed and Ag(2)CO(3) proved to be an optimal oxidant when MeCN was used as a solvent, and [RhCp*Cl(2)]( 2) was utilized as an efficient catalyst.
Abstract: A methodology for the high yield and facile synthesis of isoquinolones from benzamides and alkynes via the oxidative ortho C−H activation of benzamides has been developed. Ag2CO3 proved to be an optimal oxidant when MeCN was used as a solvent, and [RhCp*Cl2]2 was utilized as an efficient catalyst. Both N-alkyl and N-aryl secondary benzamides can be applied as effective substrates. Furthermore, primary benzamides react with two alkyne units, leading to tricyclic products via double C−H activation and oxidative coupling. The reactivity of the structurally related 1-hydroxyisoquinoline was also demonstrated, where both N- and O-containing rhodacyclic intermediates can be generated, leading to the construction of different O- or N-containing heterocycles.
282 citations
TL;DR: The impact of the C-Cl functional group on direct arylation reactivity has been evaluated mechanistically, and the observed reactivity profiles correlate very well with that predicted by a concerted metalation-deprotonation pathway.
Abstract: Through the introduction of an aryl chloride substituent, the selectivity of palladium-catalyzed direct arylation may be diverted to provide alternative regioisomeric products in high yields. In cases where low reactivity is typically observed, the presence of the carbon−chlorine bond can serve to enhance reactivity and provide superior outcomes. From a strategic perspective, the C−Cl bond is easily introduced and can be employed in a variety of subsequent transformations to provide a wealth of highly functionalized heterocycles with minimal substrate preactivation. The impact of the C−Cl functional group on direct arylation reactivity has also been evaluated mechanistically, and the observed reactivity profiles correlate very well with that predicted by a concerted metalation−deprotonation pathway.
274 citations
TL;DR: These reagents are typically more selective than DAST and Deoxo-Fluor and exhibit superior performance by providing significantly less elimination side products.
Abstract: Diethylaminodifluorosulfinium tetrafluoroborate (XtalFluor-E) and morpholinodifluorosulfinium tetrafluoroborate (XtalFluor-M) are crystalline fluorinating agents that are more easily handled and significantly more stable than Deoxo-Fluor, DAST, and their analogues. These reagents can be prepared in a safer and more cost-efficient manner by avoiding the laborious and hazardous distillation of dialkylaminosulfur trifluorides. Unlike DAST, Deoxo-Fluor, and Fluolead, XtalFluor reagents do not generate highly corrosive free-HF and therefore can be used in standard borosilicate vessels. When used in conjunction with promoters such as Et3N·3HF, Et3N·2HF, or DBU, XtalFluor reagents effectively convert alcohols to alkyl fluorides and carbonyls to gem-difluorides. These reagents are typically more selective than DAST and Deoxo-Fluor and exhibit superior performance by providing significantly less elimination side products.
254 citations
TL;DR: This strategy demonstrated four-component coupling reactions of 1,3-dicarbonyl compounds, amines, aromatic aldehydes, and nitroalkanes without an inert atmosphere to provide an alternative approach for easy access of highly substituted pyrroles in moderate to very good yields.
Abstract: A simple, convenient, and multicomponent coupling strategy for the synthesis of highly functionalized pyrroles catalyzed by iron(III) salts has been developed. This strategy demonstrated four-component coupling reactions of 1,3-dicarbonyl compounds, amines, aromatic aldehydes, and nitroalkanes without an inert atmosphere. This methodology provides an alternative approach for easy access of highly substituted pyrroles in moderate to very good yields using four simple and readily available building blocks via one-pot tandem reaction. Notably, this method is very cheap, straightforward, and environmentally friendly compared to the existing methods.
232 citations
TL;DR: A new class of chiral phosphoric acids with spirobiindane as scaffold were conveniently synthesized from (S)-1,1'- SpINOL and employed to catalyze the asymmetric Friedel-Crafts reaction of indoles with imines to afford 3-indolyl methanamines.
Abstract: A new class of chiral phosphoric acids with spirobiindane as scaffold were conveniently synthesized from (S)-1,1′-spirobiindane-7,7′-diol ((S)-SPINOL) and employed to catalyze the asymmetric Friedel−Crafts reaction of indoles with imines to afford 3-indolyl methanamines. High yields (68−97%) and excellent enantioselectivities (up to 99% ee) were obtained.
219 citations
TL;DR: A CuI-catalyzed direct access to sulfides from disulfides via C-H bond cleavage of di- or trimethoxybenzene is described, which represents an atom-economical procedure for the synthesis of sulfides and selenides.
Abstract: A CuI-catalyzed direct access to sulfides from disulfides via C−H bond cleavage of di- or trimethoxybenzene is described. The procedure utilizes O2 as a clean and cheap oxidant. Direct selenation of the C−H bond also took place under this procedure. Furthermore, the system enables the use of two RS in (RS)2. Thus, it represents an atom-economical procedure for the synthesis of sulfides and selenides.
213 citations
TL;DR: Two new organic dyes adopting coplanar diphenyl-substituted dithienosilole as the central linkage have been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs).
Abstract: Two new organic dyes adopting coplanar diphenyl-substituted dithienosilole as the central linkage have been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs). The best DSSC exhibited a high power conversion efficiency up to 7.6% (TP6CADTS) under AM 1.5G irradiation, reaching ∼96% of the ruthenium dye N719-based reference cell under the same conditions.
196 citations
TL;DR: Biophysical evaluation of the cMOE- and cEt-containing oligonucleotides revealed that they possess hybridization and mismatch discrimination attributes similar to those of LNA but greatly improved resistance to exonuclease digestion.
Abstract: We have recently shown that combining the structural elements of 2'O-methoxyethyl (MOE) and locked nucleic acid (LNA) nucleosides yielded a series of nucleoside modifications (cMOE, 2',4'-constrained MOE; cEt, 2',4'-constrained ethyl) that display improved potency over MOE and an improved therapeutic index relative to that of LNA antisense oligonucleotides. In this report we present details regarding the synthesis of the cMOE and cEt nucleoside phosphoramidites and the biophysical evaluation of oligonucleotides containing these nucleoside modifications. The synthesis of the cMOE and cEt nucleoside phosphoramidites was efficiently accomplished starting from inexpensive commercially available diacetone allofuranose. The synthesis features the use of a seldom used 2-naphthylmethyl protecting group that provides crystalline intermediates during the synthesis and can be cleanly deprotected under mild conditions. The synthesis was greatly facilitated by the crystallinity of a key mono-TBDPS-protected diol intermediate. In the case of the cEt nucleosides, the introduction of the methyl group in either configuration was accomplished in a stereoselective manner. Ring closure of the 2'-hydroxyl group onto a secondary mesylate leaving group with clean inversion of stereochemistry was achieved under surprisingly mild conditions. For the S-cEt modification, the synthesis of all four (thymine, 5-methylcytosine, adenine, and guanine) nucleobase-modified phosphoramidites was accomplished on a multigram scale. Biophysical evaluation of the cMOE- and cEt-containing oligonucleotides revealed that they possess hybridization and mismatch discrimination attributes similar to those of LNA but greatly improved resistance to exonuclease digestion.
TL;DR: It was found that Pd(OAc)(2) plays a pivotal role in the reaction mechanism as more than merely a precatalyst, but also as a source of acetate base required for the C-H bond cleavage step.
Abstract: Detailed mechanistic studies on the palladium-catalyzed direct arylation of pyridine N-oxides are presented. The order of each reaction component is determined to provide a general mechanistic picture. The C−H bond cleaving step is examined in further detail through computational studies, and the calculated results are in support of an inner-sphere concerted metalation−deprotonation (CMD) pathway. Competition experiments were conducted with N-oxides of varying electronic characters, and results revealed an enhancement of rate when using a more electron-deficient species, which is in support of a CMD transition state. The effect of base on reaction rate was also examined and it was found that a carboxylate base was required for the reaction to proceed. This led to the conclusion that Pd(OAc)2 plays a pivotal role in the reaction mechanism as more than merely a precatalyst, but also as a source of acetate base required for the C−H bond cleavage step.
TL;DR: Novel and environmentally benign methods for palladium-catalyzed dihydroxylation and oxidative cleavage of olefins with oxygen as sole oxidant are presented.
Abstract: 1,2-Diols, aldehydes, and ketones are important intermediates in chemical synthesis, and alkenes are possible precursors for 1,2-diols, aldehydes, and ketones. Herein, novel and environmentally benign methods for palladium-catalyzed dihydroxylation and oxidative cleavage of olefins with oxygen as sole oxidant are presented. The cleavage reactions were performed with acid as additive in aqueous solution, whereas 1,2-diols were formed in the presence of base. A broad substrate scope has been demonstrated allowing monosubstituted aromatic and aliphatic terminal alkenes, 1,2-disubstituted, and 1,1-disubstituted olefins. The cleavage reactions of dioxo−PdII complexes implicate 1,2-diol might act as a key intermediate of olefin cleavage.
TL;DR: An in situ generated catalyst from readily available RuH(2)(PPh(3))(4), an N-heterocyclic carbene (NHC) precursor, NaH, and acetonitrile was developed and showed high activity for the amide synthesis directly from either alcohols or aldehydes with amines.
Abstract: An in situ generated catalyst from readily available RuH2(PPh3)4, an N-heterocyclic carbene (NHC) precursor, NaH, and acetonitrile was developed. The catalyst showed high activity for the amide synthesis directly from either alcohols or aldehydes with amines. When a mixture of an alcohol and an aldehyde was reacted with an amine, both of the corresponding amides were obtained with good yields. Homogeneous Ru(0) complexes such as (η4-1,5-cyclooctadiene)(η6-1,3,5-cyclooctatriene)ruthenium [Ru(cod)(cot)] and Ru3(CO)12 were also active in the amidation of an alcohol or an aldehyde with the help of an in situ generated NHC ligand.
TL;DR: It is demonstrated that the reaction proceeds largely via a cation-radical mechanism, and the feasibility of the dicationic intermediate, formed by loss of two electrons, has been demonstrated by its generation from a tetraphenylene derivative using DDQ/H(+) as an oxidant.
Abstract: DDQ/H+ system readily oxidizes a variety of electron donors with oxidation potential as high as ∼1.7 V to the corresponding cation radicals. A re-examination of the controversial arenium-ion versus cation-radical mechanisms for Scholl reaction using DDQ/H+ together with commonly utilized FeCl3 as oxidants led us to demonstrate that the reaction proceeds largely via a cation-radical mechanism. The critical experimental evidence in support of a cation-radical pathway for the Scholl reaction includes the following: (i) There is no reaction in Scholl precursors in a mixture of dichloromethane and various acids (10% v/v). (ii) The necessity to use powerful oxidants such as ferric chloride (FeCl3) or DDQ/H+ for Scholl reactions is inconsistent with the arenium-ion mechanism in light of the fact that aromatization of the dihydro intermediates (formed via arenium-ion mechanism) can be easily accomplished with rather weak oxidants such as iodine or air. (iii) Various Scholl precursors with oxidation potentials ≤1....
TL;DR: Symmetrical diarylalkynes were obtained from propiolic acid and aryl halides in good yields and showed tolerance for functional groups such as ester, ketone, and aldehyde and exhibited chemoselectivity.
Abstract: Symmetrical diarylalkynes were obtained from propiolic acid (or 2-butynedioic acid) and aryl halides in good yields. The optimized reaction conditions were 2.0 equiv of aryl halide, 1.0 equiv of propiolic acid, 5.0 mol % Pd(PPh3)2Cl2, 10.0 mol % 1,4-bis(diphenylphosphino)butane (dppb), 2.0 equiv of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and dimethyl sulfoxide (DMSO) as the solvent. The coupling reaction of 2-butynedioic acid with aryl halides required 110 °C. The coupling reaction showed tolerance for functional groups such as ester, ketone, and aldehyde and exhibited chemoselectivity. In the coupling reaction of propiolic acid with aryl bromide, the diarylated product was the major one at 80 °C, even though 1 equiv of aryl halides was employed. However, among the monoarylated products that were formed predominantly at 25 and 50 °C in the coupling reaction with aryl iodide, more Sonogashira coupling product was obtained than the decarboxylative coupling product. Unsymmetrical diarylalkynes were also sy...
TL;DR: Cu-catalyzed O-arylation of phenols with aryl iodides and bromides can be performed under mild condition in DMSO/K(3)PO(4) with use of picolinic acid as the ligand for copper.
Abstract: Cu-catalyzed O-arylation of phenols with aryl iodides and bromides can be performed under mild condition in DMSO/K3PO4 with use of picolinic acid as the ligand for copper. This method tolerates a variety of functional groups and is effective in the synthesis of hindered diaryl ethers and heteroaryl ethers.
TL;DR: It is demonstrated that racemic cyclopropane 1,1-diesters can undergo dynamic kinetic asymmetric annulations catalyzed by (pybox)MgI(2) complexes and an unusual mechanism emerges in which the aldehyde traps a configurationally stable intimate ion pair to stereospecifically construct the C-O bond.
Abstract: This Perspective details a developing research program that emerged from simple plans for achieving the synthesis of tetrahydrofurans from cyclopropanes and C═O π bonds. Lewis acid catalyzed annulations of malonate-derived donor−acceptor cyclopropanes with aldehydes are unusually broad in scope and lead to the synthesis of structurally diverse tetrahydrofurans. The reactions are stereospecific, with inversion observed at the cyclopropane donor site. Substituent effects on the aldehyde suggest that it acts as a nucleophile in the reaction. An unusual mechanism emerges in which the aldehyde traps a configurationally stable intimate ion pair to stereospecifically construct the C−O bond. In addition to the stereospecific conversion of enantiomerically enriched cyclopropanes into nonracemic heterocycles, we have also demonstrated that racemic cyclopropane 1,1-diesters can undergo dynamic kinetic asymmetric annulations catalyzed by (pybox)MgI2 complexes. Asymmetric syntheses of (+)-polyanthellin A and (+)-virga...
TL;DR: The palladium-catalyzed ortho-alkoxylation of N-methoxybenzamides has been demonstrated with the CONHOMe group as a directing group, and the aromatic C-H bond can be functionalized efficiently to generate Ortho-alksoxylated derivatives in moderate to good yields.
Abstract: The palladium-catalyzed ortho-alkoxylation of N-methoxybenzamides has been demonstrated. With the CONHOMe group as a directing group, the aromatic C−H bond can be functionalized efficiently to generate ortho-alkoxylated derivatives in moderate to good yields.
TL;DR: A straightforward method for the synthesis of polysubstituted pyrroles was achieved easily from oxidative cyclization of beta-enamino ketones or esters and alkynoates catalyzed by CuI in the presence of O(2).
Abstract: A straightforward method for the synthesis of polysubstituted pyrroles was achieved easily from oxidative cyclization of β-enamino ketones or esters and alkynoates catalyzed by CuI in the presence of O2.
TL;DR: This facile method would be useful for the synthesis of aza-π-conjugated materials having boron-nitrogen coordination.
Abstract: Pyridine−borane complexes were synthesized from 2-arylpyridines through an electrophilic aromatic borylation reaction with BBr3. The intermediate 2-(2-dibromoborylaryl)pyridines were stable enough to be handled in air and served as the synthetic platform for variously substituted pyridine−borane complexes. This facile method would be useful for the synthesis of aza-π-conjugated materials having boron−nitrogen coordination.
TL;DR: A one-pot approach to C3-position acetoxylated biindolyls is realized via palladium catalysis by the use of AgOAc under oxygen atmosphere as oxidants, which tolerates the bromide substituent on indoles.
Abstract: Mild conditions have been developed to achieve Pd-catalyzed homocoupling of indoles with excellent regioselectivity in the presence of Cu(OAc)2·H2O in DMSO at room temperature in high efficiency. This method provides a simple route to 2,3′-biindolyls from the electron-rich to moderately electron-poor indoles. The reaction tolerates the bromide substituent on indoles. In addition, a one-pot approach to C3-position acetoxylated biindolyls is realized via palladium catalysis by the use of AgOAc under oxygen atmosphere as oxidants.
TL;DR: A series of pillar[5]arene derivatives with alkyl groups of different length were synthesized and the conformational characteristics of the pillars were investigated by dynamic (1)H NMR measurements.
Abstract: A series of pillar[5]arene derivatives with alkyl groups of different length were synthesized. The new alkyl-substituted pillar[5]arene derivatives 1,4-bis(ethoxy)pillar[5]arene (C2), 1,4-bis(propoxy)pillar[5]arene (C3), 1,4-bis(butoxy)pillar[5]arene (C4), 1,4-bis(pentyloxy)pillar[5]arene (C5), 1,4-bis(hexyloxy)pillar[5]arene (C6), and 1,4-bis(dodecanoxy)pillar[5]arene (C12) were obtained by Lewis acid-catalyzed condensation of dialkoxybenzene monomers with paraformaldehyde. The conformational characteristics of the pillar[5]arene derivatives were investigated by dynamic (1)H NMR measurements. When the alkyl substituents were bulkier than methyl groups, the rotation of phenolic units in the pillar[5]arenes was suppressed and their conformation was immobilized. As their length increased, the alkyl substituents packed at the upper and lower rims and thus lowered the conformational freedom of the pillar[5]arenes.
TL;DR: An efficient one-pot synthesis of 4,5,6-triaryl-3,4-dihydropyrimidin-2(1H)-ones via a three-component Biginelli-type condensation of aldehyde, 2-phenylacetophenone, and urea/thiourea in the presence of a catalytic amount of t-BuOK is described.
Abstract: An efficient one-pot synthesis of 4,5,6-triaryl-3,4-dihydropyrimidin-2(1H)-ones via a three-component Biginelli-type condensation of aldehyde, 2-phenylacetophenone, and urea/thiourea in the presence of a catalytic amount of t-BuOK (20 mol %) is described. The reactions proceeded efficiently at 70 °C to afford the desired products in moderate to good yields. Detailed mechanistic study shows that the Biginelli-type reaction using urea and thiourea proceeds through two totally different pathways. Enone 5 and bis-urea 8 were highly suggested as respective reaction intermediates for reactions involving thiourea and urea as substrates.
TL;DR: The carbon azides capable of binding to the catalytic copper center at the alkylated azido nitrogen in a chelatable fashion were indeed shown to be superior substrates under the reported conditions and led to the hypothesis of an additional effect of chelating azides on the efficiencies of Cu(OAc)(2)-accelerated AAC reactions, which is to facilitate the rapid reduction ofCu(II) to highly catalytic Cu(I) species.
Abstract: We described in a previous communication a variant of the popular Cu(I)-catalyzed azide-alkyne cycloaddition (AAC) process where 5 mol % of Cu(OAc)(2) in the absence of any added reducing agent is sufficient to enable the reaction. 2-Picolylazide (1) and 2-azidomethylquinoline (2) were found to be by far the most reactive carbon azide substrates that convert to 1,2,3-triazoles in as short as a few minutes under the discovered conditions. We hypothesized that the abilities of 1 and 2 to chelate Cu(II) contribute significantly to the observed high reaction rates. The current work examines the effect of auxiliary ligands near the azido group other than pyridyl for Cu(II) on the efficiency of the Cu(OAc)(2)-accelerated AAC reaction. The carbon azides capable of binding to the catalytic copper center at the alkylated azido nitrogen in a chelatable fashion were indeed shown to be superior substrates under the reported conditions. The chelation between carbon azide 11 and Cu(II) was demonstrated in an X-ray single-crystal structure. In a limited set of examples, the ligand tris(benzyltriazolylmethyl)amine (TBTA), developed by Fokin et al. for assisting the original Cu(I)-catalyzed AAC reactions, also dramatically enhances the Cu(OAc)(2)-accelerated AAC reactions involving nonchelating azides. This observation leads to the hypothesis of an additional effect of chelating azides on the efficiencies of Cu(OAc)(2)-accelerated AAC reactions, which is to facilitate the rapid reduction of Cu(II) to highly catalytic Cu(I) species. Mechanistic studies on the AAC reactions with particular emphasis on the role of carbon azide/copper interactions will be conducted based on the observations reported in this work. Finally, the immediate utility of the product 1,2,3-triazole molecules derived from chelating azides as multidentate metal coordination ligands is demonstrated. The resulting triazolyl-containing ligands are expected to bind with transition metal ions via the N(2) nitrogen of the 1,2,3-triazolyl group to form nonplanar coordination rings. The Cu(II) complexes of bidentate T1 and tetradentate T6 and the Zn(II) complex of T6 were characterized by X-ray crystallography. The structure of [Cu(T1)(2)(H(2)O)(2)](ClO(4))(2) reveals the interesting synergistic effect of hydrogen bonding, π-π stacking interactions, and metal coordination in forming a one-dimensional supramolecular construct in the solid state. The tetradentate coordination mode of T6 may be incorporated into designs of new molecule sensors and organometallic catalysts.
TL;DR: The synthesis, structures, characterization, and applications to field-effect transistors of naphtho[1,2-b:5,6-b']dithiophene and -diselenophene derivatives indicated that NDT and NDS are new potential heteroarene core structures for organic semiconducting materials.
Abstract: In this paper we present the synthesis, structures, characterization, and applications to field-effect transistors (FETs) of naphtho[1,2-b:5,6-b′]dithiophene (NDT) and -diselenophene (NDS) derivatives. Treatment of 1,5-dichloro-2,6-diethynylnaphthalenes, easily derived from commercially available 2,6-dihydroxynaphthalene, with sodium chalcogenide afforded a straightforward access to NDTs and NDSs including the parent and dioctyl and diphenyl derivatives. Physicochemical evaluations of NDT and NDS derivatives showed that these heteroarenes have a similar electronic structure with isomeric [1]benzothieno[2,3-b][1]benzothiophene (BTBT) and [1]benzoselenopheneno[2,3-b][1]benzoselenophene (BSBS) derivatives, respectively. Although attempts to fabricate solution-processed field-effect transistors (FETs) with soluble dioctyl-NDT (C8-NDT) and -NDS (C8-NDS) failed, diphenyl derivatives (DPh-NDT and DPh-NDS) afforded vapor-processed FETs showing field-effect mobility as high as 0.7 cm2 V−1 s−1. These results indica...
TL;DR: A novel calix[4]arene derivative locked in the 1,3-alternate conformation bearing two pyrene and rhodamine fluorophores was synthesized as a selective sensor for the Hg(2+) ion and it was found that the pyrene excimer emissions are more effective in obtaining strong FRET bands than those by intermolecular π-π interactions.
Abstract: A novel calix[4]arene derivative locked in the 1,3-alternate conformation (2) bearing two pyrene and rhodamine fluorophores was synthesized as a selective sensor for the Hg2+ ion. The sensoring is based on FRET from pyrene excimer emissions to ring-opened rhodamine absorption upon complexation of the Hg2+ ion. Addition of Hg2+ to a mixed solution of 2 gave significantly enhanced fluorescence at ∼576 nm via FRET with excitation at 343 nm. We also found that the pyrene excimer emissions formed by the intramolecular π−π interactions are more effective in obtaining strong FRET bands than those by intermolecular π−π interactions.
TL;DR: This work reports an alternative and more scalable approach where polymer-supported chiral enamine and iminium organocatalysts are prepared in a bottom-up fashion where methacrylic functional monomers are Prepared in an entirely nonchromatographic manner and subsequently copolymerized with suitable comonomers to give cross-linked polymer beads.
Abstract: Polymer-supported chiral organocatalysts, as well as most other forms of immobilized catalysts, are traditionally prepared by a postmodification approach where modified catalyst precursors are anchored onto prefabricated polymer beads. Herein, we report an alternative and more scalable approach where polymer-supported chiral enamine and iminium organocatalysts are prepared in a bottom-up fashion where methacrylic functional monomers are prepared in an entirely nonchromatographic manner and subsequently copolymerized with suitable comonomers to give cross-linked polymer beads. All syntheses have been conducted on multigram scale for all intermediates and finished polymer products, and the catalysts have proven successful in reactions taking place in solvents spanning a wide range of solvent polarity. While polymer-supported proline and prolineamides generally demonstrated excellent results and recycling robustness in asymmetric aldol reactions of ketones and benzaldehydes, the simplest type of Jorgensen/Hayashi diarylprolinol TMS-ether showed excellent selectivity, but rather sluggish reactivity in the Enders-type asymmetric cascade. The polymer-supported version of the first-generation MacMillan imidazolidinone had a pattern of reactivity very similar to that of the monomeric catalyst, but is too unstable to allow recycling.
TL;DR: Rate constants for reactions of cysteine/selenocystine are similar and Reaction rates of selenium as a nucleophile and as an electrophile are 2-3 and 4 orders of magnitude higher, respectively, than those of sulfur.
Abstract: Cysteamine reduces selenocystamine to form hemiselenocystamine and then cystamine. The rate constants are k1 = 1.3 × 105 M−1 s−1; k−1 = 2.6 × 107 M−1 s−1; k2 = 11 M−1 s−1; and k−2 = 1.4 × 103 M−1 s−1, respectively. Rate constants for reactions of cysteine/selenocystine are similar. Reaction rates of selenium as a nucleophile and as an electrophile are 2−3 and 4 orders of magnitude higher, respectively, than those of sulfur. Sulfides and selenides are comparable as leaving groups.
TL;DR: The microreactor provided a very convenient means for running sequential reactions rapidly with small amounts of reagents under well-controlled conditions, thereby allowing reaction kinetics to be followed and Arrhenius activation energies to be measured.
Abstract: A microreactor was applied to produce ortho-substituted [18F]fluoroarenes from the reactions of cyclotron-produced [18F]fluoride ion (t1/2 = 109.7 min) with diaryliodonium salts. The microreactor p...