RESUMO
The preparation of extremely thin samples, which are required for high-resolution electron microscopy, poses extreme risk of damaging biological macromolecules due to interactions with the air-water interface. Although the rapid increase in the number of published structures initially gave little indication that this was a problem, the search for methods that substantially mitigate this hazard is now intensifying. The two main approaches under investigation are (a) immobilizing particles onto structure-friendly support films and (b) reducing the length of time during which such interactions may occur. While there is little possibility of outrunning diffusion to the interface, intentional passivation of the interface may slow the process of adsorption and denaturation. In addition, growing attention is being given to gaining more effective control of the thickness of the sample prior to vitrification.
Assuntos
Microscopia Crioeletrônica/instrumentação , Microscopia Crioeletrônica/métodos , Complexos Multiproteicos/química , Ar , Carbono/química , Difusão , Grafite/química , Lipídeos/química , Complexos Multiproteicos/isolamento & purificação , Desnaturação Proteica , Manejo de Espécimes/métodos , Estreptavidina/química , ÁguaRESUMO
Emissions reduction and greenhouse gas removal from the atmosphere are both necessary to achieve net-zero emissions and limit climate change1. There is thus a need for improved sorbents for the capture of carbon dioxide from the atmosphere, a process known as direct air capture. In particular, low-cost materials that can be regenerated at low temperatures would overcome the limitations of current technologies. In this work, we introduce a new class of designer sorbent materials known as 'charged-sorbents'. These materials are prepared through a battery-like charging process that accumulates ions in the pores of low-cost activated carbons, with the inserted ions then serving as sites for carbon dioxide adsorption. We use our charging process to accumulate reactive hydroxide ions in the pores of a carbon electrode, and find that the resulting sorbent material can rapidly capture carbon dioxide from ambient air by means of (bi)carbonate formation. Unlike traditional bulk carbonates, charged-sorbent regeneration can be achieved at low temperatures (90-100 °C) and the sorbent's conductive nature permits direct Joule heating regeneration2,3 using renewable electricity. Given their highly tailorable pore environments and low cost, we anticipate that charged-sorbents will find numerous potential applications in chemical separations, catalysis and beyond.
Assuntos
Dióxido de Carbono , Dióxido de Carbono/análise , Dióxido de Carbono/química , Dióxido de Carbono/isolamento & purificação , Adsorção , Eletrodos , Hidróxidos/química , Atmosfera/química , Carbonatos/química , Ar , Temperatura , Carvão Vegetal/química , Porosidade , Carbono/químicaRESUMO
Dissolved organic matter (DOM) is one of the most complex, dynamic and abundant sources of organic carbon, but its chemical reactivity remains uncertain1-3. Greater insights into DOM structural features could facilitate understanding its synthesis, turnover and processing in the global carbon cycle4,5. Here we use complementary multiplicity-edited 13C nuclear magnetic resonance (NMR) spectra to quantify key substructures assembling the carbon skeletons of DOM from four main Amazon rivers and two mid-size Swedish boreal lakes. We find that one type of reaction mechanism, oxidative dearomatization (ODA), widely used in organic synthetic chemistry to create natural product scaffolds6-10, is probably a key driver for generating structural diversity during processing of DOM that are rich in suitable polyphenolic precursor molecules. Our data suggest a high abundance of tetrahedral quaternary carbons bound to one oxygen and three carbon atoms (OCqC3 units). These units are rare in common biomolecules but could be readily produced by ODA of lignin-derived and tannin-derived polyphenols. Tautomerization of (poly)phenols by ODA creates non-planar cyclohexadienones, which are subject to immediate and parallel cycloadditions. This combination leads to a proliferation of structural diversity of DOM compounds from early stages of DOM processing, with an increase in oxygenated aliphatic structures. Overall, we propose that ODA is a key reaction mechanism for complexity acceleration in the processing of DOM molecules, creation of new oxygenated aliphatic molecules and that it could be prevalent in nature.
Assuntos
Carbono , Água Doce , Carbono/análise , Carbono/química , Espectroscopia de Ressonância Magnética Nuclear de Carbono-13 , Água Doce/química , Lagos/química , Lignina/química , Oxirredução , Oxigênio/química , Polifenóis/química , Rios/química , Suécia , Taninos/química , Ciclo do CarbonoRESUMO
Heteroarenes are ubiquitous motifs in bioactive molecules, conferring favourable physical properties when compared to their arene counterparts1-3. In particular, semisaturated heteroarenes possess attractive solubility properties and a higher fraction of sp3 carbons, which can improve binding affinity and specificity. However, these desirable structures remain rare owing to limitations in current synthetic methods4-6. Indeed, semisaturated heterocycles are laboriously prepared by means of non-modular fit-for-purpose syntheses, which decrease throughput, limit chemical diversity and preclude their inclusion in many hit-to-lead campaigns7-10. Herein, we describe a more intuitive and modular couple-close approach to build semisaturated ring systems from dual radical precursors. This platform merges metallaphotoredox C(sp2)-C(sp3) cross-coupling with intramolecular Minisci-type radical cyclization to fuse abundant heteroaryl halides with simple bifunctional feedstocks, which serve as the diradical synthons, to rapidly assemble a variety of spirocyclic, bridged and substituted saturated ring types that would be extremely difficult to make by conventional methods. The broad availability of the requisite feedstock materials allows sampling of regions of underexplored chemical space. Reagent-controlled radical generation leads to a highly regioselective and stereospecific annulation that can be used for the late-stage functionalization of pharmaceutical scaffolds, replacing lengthy de novo syntheses.
Assuntos
Carbono , Técnicas de Química Sintética , Compostos Heterocíclicos com 1 Anel , Preparações Farmacêuticas , Carbono/química , Ciclização , Compostos Heterocíclicos com 1 Anel/síntese química , Compostos Heterocíclicos com 1 Anel/química , Solubilidade , Oxirredução , Fotoquímica , Preparações Farmacêuticas/síntese química , Preparações Farmacêuticas/química , Técnicas de Química Sintética/métodosRESUMO
Cytochrome P450 enzymes are known to catalyse bimodal oxidation of aliphatic acids via radical intermediates, which partition between pathways of hydroxylation and desaturation1,2. Developing analogous catalytic systems for remote C-H functionalization remains a significant challenge3-5. Here, we report the development of Cu(I)-catalysed bimodal dehydrogenation/lactonization reactions of synthetically common N-methoxyamides through radical abstractions of the γ-aliphatic C-H bonds. The feasibility of switching from dehydrogenation to lactonization is also demonstrated by altering reaction conditions. The use of a readily available amide as both radical precursor and internal oxidant allows for the development of redox-neutral C-H functionalization reactions with methanol as the sole side product. These C-H functionalization reactions using a Cu(I) catalyst with loading as low as 0.5 mol.% is applied to the diversification of a wide range of aliphatic acids including drug molecules and natural products. The exceptional compatibility of this catalytic system with a wide range of oxidatively sensitive functionality demonstrates the unique advantage of using a simple amide substrate as a mild internal oxidant.
Assuntos
Carbono , Cobre , Hidrogênio , Lactonas , Amidas/química , Amidas/metabolismo , Carbono/química , Catálise , Cobre/química , Sistema Enzimático do Citocromo P-450/metabolismo , Sistema Enzimático do Citocromo P-450/química , Hidrogênio/química , Hidrogenação , Lactonas/química , Metanol/química , Oxidantes/química , Oxidantes/metabolismo , OxirreduçãoRESUMO
The ability to tame high-energy intermediates is important for synthetic chemistry, enabling the construction of complex molecules and propelling advances in the field of synthesis. Along these lines, carbenes and carbenoid intermediates are particularly attractive, but often unknown, high-energy intermediates1,2. Classical methods to access metal carbene intermediates exploit two-electron chemistry to form the carbon-metal bond. However, these methods are usually prohibitive because of reagent safety concerns, limiting their broad implementation in synthesis3-6. Mechanistically, an alternative approach to carbene intermediates that could circumvent these pitfalls would involve two single-electron steps: radical addition to metal to forge the initial carbon-metal bond followed by redox-promoted α-elimination to yield the desired metal carbene intermediate. Here we realize this strategy through a metallaphotoredox platform that exploits iron carbene reactivity using readily available chemical feedstocks as radical sources and α-elimination from six classes of previously underexploited leaving groups. These discoveries permit cyclopropanation and σ-bond insertion into N-H, S-H and P-H bonds from abundant and bench-stable carboxylic acids, amino acids and alcohols, thereby providing a general solution to the challenge of carbene-mediated chemical diversification.
Assuntos
Álcoois , Aminoácidos , Ácidos Carboxílicos , Técnicas de Química Sintética , Ferro , Metano , Fotoquímica , Álcoois/química , Aminoácidos/química , Carbono/química , Ácidos Carboxílicos/química , Catálise , Ciclopropanos/química , Ciclopropanos/síntese química , Ferro/química , Metano/análogos & derivados , Metano/química , Oxirredução , Fotoquímica/métodos , Técnicas de Química Sintética/métodos , ElétronsRESUMO
The carbon skeleton of any organic molecule serves as the foundation for its three-dimensional structure, playing a pivotal role in determining its physical and biological properties1. As such, taxane diterpenes are one of the most well-known natural product families, primarily owing to the success of their most prominent compound, paclitaxel, an effective anticancer therapeutic for more than 25 years2-6. In contrast to classical taxanes, the bioactivity of cyclotaxanes (also referred to as complex taxanes) remains significantly underexplored. The carbon skeletons of these two groups of taxanes differ significantly, and so would typically their own distinct synthetic approaches. Here we report a versatile synthetic strategy based on the interconversion of complex molecular frameworks, providing general access to the wider taxane diterpene family. A range of classical and cyclotaxane frameworks was prepared including, among others, the total syntheses of taxinine K (2), canataxapropellane (5) and dipropellane C from a single advanced intermediate. The synthetic approach deliberately eschews biomimicry, emphasizing instead the power of stereoelectronic control in orchestrating the interconversion of polycyclic frameworks.
Assuntos
Hidrocarbonetos Aromáticos com Pontes , Técnicas de Química Sintética , Diterpenos , Taxoides , Produtos Biológicos/síntese química , Produtos Biológicos/química , Hidrocarbonetos Aromáticos com Pontes/síntese química , Hidrocarbonetos Aromáticos com Pontes/química , Diterpenos/síntese química , Diterpenos/química , Estereoisomerismo , Taxoides/química , Taxoides/síntese química , Carbono/químicaRESUMO
Carbon-oxygen bonds are commonplace in organic molecules, including chiral bioactive compounds; therefore, the development of methods for their construction with simultaneous control of stereoselectivity is an important objective in synthesis. The Williamson ether synthesis, first reported in 18501, is the most widely used approach to the alkylation of an oxygen nucleophile, but it has significant limitations (scope and stereochemistry) owing to its reaction mechanism (SN2 pathway). Transition-metal catalysis of the coupling of an oxygen nucleophile with an alkyl electrophile has the potential to address these limitations, but progress so far has been limited2-7, especially with regard to controlling enantioselectivity. Here we establish that a readily available copper catalyst can achieve an array of enantioconvergent substitution reactions of α-haloamides, a useful family of electrophiles, by oxygen nucleophiles; the reaction proceeds under mild conditions in the presence of a wide variety of functional groups. The catalyst is uniquely effective in being able to achieve enantioconvergent alkylations of not only oxygen nucleophiles but also nitrogen nucleophiles, giving support for the potential of transition-metal catalysts to provide a solution to the pivotal challenge of achieving enantioselective alkylations of heteroatom nucleophiles.
Assuntos
Alquilação , Catálise , Cobre , Oxigênio , Carbono/química , Cobre/química , Oxigênio/químicaRESUMO
When searching for the ideal molecule to fill a particular functional role (for example, a medicine), the difference between success and failure can often come down to a single atom1. Replacing an aromatic carbon atom with a nitrogen atom would be enabling in the discovery of potential medicines2, but only indirect means exist to make such C-to-N transmutations, typically by parallel synthesis3. Here, we report a transformation that enables the direct conversion of a heteroaromatic carbon atom into a nitrogen atom, turning quinolines into quinazolines. Oxidative restructuring of the parent azaarene gives a ring-opened intermediate bearing electrophilic sites primed for ring reclosure and expulsion of a carbon-based leaving group. Such a 'sticky end' approach subverts existing atom insertion-deletion approaches and as a result avoids skeleton-rotation and substituent-perturbation pitfalls common in stepwise skeletal editing. We show a broad scope of quinolines and related azaarenes, all of which can be converted into the corresponding quinazolines by replacement of the C3 carbon with a nitrogen atom. Mechanistic experiments support the critical role of the activated intermediate and indicate a more general strategy for the development of C-to-N transmutation reactions.
Assuntos
Carbono , Técnicas de Química Sintética , Nitrogênio , Quinazolinas , Quinolinas , Carbono/química , Nitrogênio/química , Quinazolinas/síntese química , Quinazolinas/química , Quinolinas/química , Oxirredução , Desenho de Fármacos , Preparações Farmacêuticas/síntese química , Preparações Farmacêuticas/químicaRESUMO
Cyclic organic molecules are common among natural products and pharmaceuticals1,2. In fact, the overwhelming majority of small-molecule pharmaceuticals contain at least one ring system, as they provide control over molecular shape, often increasing oral bioavailability while providing enhanced control over the activity, specificity and physical properties of drug candidates3-5. Consequently, new methods for the direct site and diastereoselective synthesis of functionalized carbocycles are highly desirable. In principle, molecular editing by C-H activation offers an ideal route to these compounds. However, the site-selective C-H functionalization of cycloalkanes remains challenging because of the strain encountered in transannular C-H palladation. Here we report that two classes of ligands-quinuclidine-pyridones (L1, L2) and sulfonamide-pyridones (L3)-enable transannular γ-methylene C-H arylation of small- to medium-sized cycloalkane carboxylic acids, with ring sizes ranging from cyclobutane to cyclooctane. Excellent γ-regioselectivity was observed in the presence of multiple ß-C-H bonds. This advance marks a major step towards achieving molecular editing of saturated carbocycles: a class of scaffolds that are important in synthetic and medicinal chemistry3-5. The utility of this protocol is demonstrated by two-step formal syntheses of a series of patented biologically active small molecules, prior syntheses of which required up to 11 steps6.
Assuntos
Produtos Biológicos , Carbono , Ácidos Carboxílicos , Cicloparafinas , Hidrogênio , Produtos Biológicos/química , Ácidos Carboxílicos/química , Cicloparafinas/química , Preparações Farmacêuticas/química , Piridonas/química , Carbono/química , Hidrogênio/química , Sulfonamidas/química , Ligantes , Química Farmacêutica , Quinuclidinas/química , Ciclobutanos/químicaRESUMO
Calcium carbonate formation is the primary pathway by which carbon is returned from the ocean-atmosphere system to the solid Earth1,2. The removal of dissolved inorganic carbon from seawater by precipitation of carbonate minerals-the marine carbonate factory-plays a critical role in shaping marine biogeochemical cycling1,2. A paucity of empirical constraints has led to widely divergent views on how the marine carbonate factory has changed over time3-5. Here we use geochemical insights from stable strontium isotopes to provide a new perspective on the evolution of the marine carbonate factory and carbonate mineral saturation states. Although the production of carbonates in the surface ocean and in shallow seafloor settings have been widely considered the predominant carbonate sinks for most of the history of the Earth6, we propose that alternative processes-such as porewater production of authigenic carbonates-may have represented a major carbonate sink throughout the Precambrian. Our results also suggest that the rise of the skeletal carbonate factory decreased seawater carbonate saturation states.
Assuntos
Carbonatos , Sedimentos Geológicos , Água do Mar , Animais , Organismos Aquáticos/química , Organismos Aquáticos/metabolismo , Carbonato de Cálcio/análise , Carbonato de Cálcio/química , Carbonato de Cálcio/metabolismo , Carbono/análise , Carbono/química , Carbono/metabolismo , Sequestro de Carbono , Carbonatos/análise , Carbonatos/química , Carbonatos/metabolismo , Sedimentos Geológicos/análise , Sedimentos Geológicos/química , Água do Mar/análise , Água do Mar/química , Isótopos de Estrôncio , História AntigaRESUMO
Biaryl compounds, with two connected aromatic rings, are found across medicine, materials science and asymmetric catalysis1,2. The necessity of joining arene building blocks to access these valuable compounds has inspired several approaches for biaryl bond formation and challenged chemists to develop increasingly concise and robust methods for this task3. Oxidative coupling of two C-H bonds offers an efficient strategy for the formation of a biaryl C-C bond; however, fundamental challenges remain in controlling the reactivity and selectivity for uniting a given pair of substrates4,5. Biocatalytic oxidative cross-coupling reactions have the potential to overcome limitations inherent to numerous small-molecule-mediated methods by providing a paradigm with catalyst-controlled selectivity6. Here we disclose a strategy for biocatalytic cross-coupling through oxidative C-C bond formation using cytochrome P450 enzymes. We demonstrate the ability to catalyse cross-coupling reactions on a panel of phenolic substrates using natural P450 catalysts. Moreover, we engineer a P450 to possess the desired reactivity, site selectivity and atroposelectivity by transforming a low-yielding, unselective reaction into a highly efficient and selective process. This streamlined method for constructing sterically hindered biaryl bonds provides a programmable platform for assembling molecules with catalyst-controlled reactivity and selectivity.
Assuntos
Biocatálise , Técnicas de Química Sintética , Sistema Enzimático do Citocromo P-450/metabolismo , Oxidantes/química , Carbono/química , Cumarínicos/química , Sistema Enzimático do Citocromo P-450/química , Sistema Enzimático do Citocromo P-450/genética , Hidrogênio/química , Oxirredução , Especificidade por SubstratoRESUMO
Archaea synthesize isoprenoid-based ether-linked membrane lipids, which enable them to withstand extreme environmental conditions, such as high temperatures, high salinity, and low or high pH values1-5. In some archaea, such as Methanocaldococcus jannaschii, these lipids are further modified by forming carbon-carbon bonds between the termini of two lipid tails within one glycerophospholipid to generate the macrocyclic archaeol or forming two carbon-carbon bonds between the termini of two lipid tails from two glycerophospholipids to generate the macrocycle glycerol dibiphytanyl glycerol tetraether (GDGT)1,2. GDGT contains two 40-carbon lipid chains (biphytanyl chains) that span both leaflets of the membrane, providing enhanced stability to extreme conditions. How these specialized lipids are formed has puzzled scientists for decades. The reaction necessitates the coupling of two completely inert sp3-hybridized carbon centres, which, to our knowledge, has not been observed in nature. Here we show that the gene product of mj0619 from M. jannaschii, which encodes a radical S-adenosylmethionine enzyme, is responsible for biphytanyl chain formation during synthesis of both the macrocyclic archaeol and GDGT membrane lipids6. Structures of the enzyme show the presence of four metallocofactors: three [Fe4S4] clusters and one mononuclear rubredoxin-like iron ion. In vitro mechanistic studies show that Csp3-Csp3 bond formation takes place on fully saturated archaeal lipid substrates and involves an intermediate bond between the substrate carbon and a sulfur of one of the [Fe4S4] clusters. Our results not only establish the biosynthetic route for tetraether formation but also improve the use of GDGT in GDGT-based paleoclimatology indices7-10.
Assuntos
Proteínas Arqueais , Éteres de Glicerila , Lipídeos de Membrana , Methanocaldococcus , Proteínas Arqueais/química , Proteínas Arqueais/isolamento & purificação , Proteínas Arqueais/metabolismo , Carbono/química , Carbono/metabolismo , Glicerol/química , Glicerol/metabolismo , Éteres de Glicerila/química , Éteres de Glicerila/metabolismo , Lipídeos de Membrana/biossíntese , Lipídeos de Membrana/química , Lipídeos de Membrana/metabolismo , Methanocaldococcus/química , Methanocaldococcus/enzimologia , Methanocaldococcus/metabolismo , S-Adenosilmetionina/metabolismo , Terpenos/química , Terpenos/metabolismoRESUMO
Fluoroalkyl groups profoundly affect the physical properties of pharmaceuticals and influence almost all metrics associated with their pharmacokinetic and pharmacodynamic profile1-4. Drug candidates increasingly contain trifluoromethyl (CF3) and difluoromethyl (CF2H) groups, and the same trend in agrochemical development shows that the effect of fluoroalkylation translates across human, insect and plant life5,6. New fluoroalkylation reactions have undoubtedly stimulated this shift; however, methods that directly convert C-H bonds into C-CF2X groups (where X is F or H) in complex drug-like molecules are rare7-13. Pyridines are the most common aromatic heterocycles in pharmaceuticals14, but only one approach-via fluoroalkyl radicals-is viable for achieving pyridyl C-H fluoroalkylation in the elaborate structures encountered during drug development15-17. Here we develop a set of bench-stable fluoroalkylphosphines that directly convert the C-H bonds in pyridine building blocks, drug-like fragments and pharmaceuticals into fluoroalkyl derivatives. No preinstalled functional groups or directing groups are required. The reaction tolerates a variety of sterically and electronically distinct pyridines, and is exclusively selective for the 4-position in most cases. The reaction proceeds through initial formation of phosphonium salts followed by sp2-sp3 coupling of phosphorus ligands-an underdeveloped manifold for forming C-C bonds.
Assuntos
Carbono/química , Flúor/química , Hidrogênio/química , Fósforo/química , Piridinas/química , Alquilação , Animais , Humanos , Ligantes , Preparações Farmacêuticas/química , Farmacocinética , Fosfinas/químicaRESUMO
Synthetic chemistry aims to build up molecular complexity from simple feedstocks1. However, the ability to exert precise changes that manipulate the connectivity of the molecular skeleton itself remains limited, despite possessing substantial potential to expand the accessible chemical space2,3. Here we report a reaction that 'deletes' nitrogen from organic molecules. We show that N-pivaloyloxy-N-alkoxyamides, a subclass of anomeric amides, promote the intermolecular activation of secondary aliphatic amines to yield intramolecular carbon-carbon coupling products. Mechanistic experiments indicate that the reactions proceed via isodiazene intermediates that extrude the nitrogen atom as dinitrogen, producing short-lived diradicals that rapidly couple to form the new carbon-carbon bond. The reaction shows broad functional-group tolerance, which enables the translation of routine amine synthesis protocols into a strategy for carbon-carbon bond constructions and ring syntheses. This is highlighted by the use of this reaction in the syntheses and skeletal editing of bioactive compounds.
Assuntos
Aminas/química , Técnicas de Química Sintética , Nitrogênio/química , Amidas/química , Produtos Biológicos/síntese química , Produtos Biológicos/química , Carbono/química , Indicadores e Reagentes/químicaRESUMO
The substitution of an alkyl electrophile by a nucleophile is a foundational reaction in organic chemistry that enables the efficient and convergent synthesis of organic molecules. Although there has been substantial recent progress in exploiting transition-metal catalysis to expand the scope of nucleophilic substitution reactions to include carbon nucleophiles1-4, there has been limited progress in corresponding reactions with nitrogen nucleophiles5-8. For many substitution reactions, the bond construction itself is not the only challenge, as there is a need to control stereochemistry at the same time. Here we describe a method for the enantioconvergent substitution of unactivated racemic alkyl electrophiles by a ubiquitous nitrogen-containing functional group, an amide. Our method uses a photoinduced catalyst system based on copper, an Earth-abundant metal. This process for asymmetric N-alkylation relies on three distinct ligands-a bisphosphine, a phenoxide and a chiral diamine. The ligands assemble in situ to form two distinct catalysts that act cooperatively: a copper/bisphosphine/phenoxide complex that serves as a photocatalyst, and a chiral copper/diamine complex that catalyses enantioselective C-N bond formation. Our study thus expands enantioselective N-substitution by alkyl electrophiles beyond activated electrophiles (those bearing at least one sp- or sp2-hybridized substituent on the carbon undergoing substitution)8-13 to include unactivated electrophiles.
Assuntos
Amidas/química , Cobre/química , Fotoquímica , Brometos/química , Carbono/química , Catálise , Ciclização , Diaminas/química , Ligantes , Nitrogênio/química , Fosfinas/químicaRESUMO
Tritium labelling is a critical tool for investigating the pharmacokinetic and pharmacodynamic properties of drugs, autoradiography, receptor binding and receptor occupancy studies1. Tritium gas is the preferred source of tritium for the preparation of labelled molecules because it is available in high isotopic purity2. The introduction of tritium labels from tritium gas is commonly achieved by heterogeneous transition-metal-catalysed tritiation of aryl (pseudo)halides. However, heterogeneous catalysts such as palladium supported on carbon operate through a reaction mechanism that also results in the reduction of other functional groups that are prominently featured in pharmaceuticals3. Homogeneous palladium catalysts can react chemoselectively with aryl (pseudo)halides but have not been used for hydrogenolysis reactions because, after required oxidative addition, they cannot split dihydrogen4. Here we report a homogenous hydrogenolysis reaction with a well defined, molecular palladium catalyst. We show how the thianthrene leaving group-which can be introduced selectively into pharmaceuticals by late-stage C-H functionalization5-differs in its coordinating ability to relevant palladium(II) catalysts from conventional leaving groups to enable the previously unrealized catalysis with dihydrogen. This distinct reactivity combined with the chemoselectivity of a well defined molecular palladium catalyst enables the tritiation of small-molecule pharmaceuticals that contain functionality that may otherwise not be tolerated by heterogeneous catalysts. The tritiation reaction does not require an inert atmosphere or dry conditions and is therefore practical and robust to execute, and could have an immediate impact in the discovery and development of pharmaceuticals.
Assuntos
Compostos Heterocíclicos/química , Paládio/química , Sais/química , Trítio/química , Carbono/química , Catálise , Deutério/química , Hidrogênio/química , Marcação por Isótopo , Oxirredução , Preparações Farmacêuticas/química , Especificidade por SubstratoRESUMO
Metal-catalysed cross-couplings are a mainstay of organic synthesis and are widely used for the formation of C-C bonds, particularly in the production of unsaturated scaffolds1. However, alkyl cross-couplings using native sp3-hybridized functional groups such as alcohols remain relatively underdeveloped2. In particular, a robust and general method for the direct deoxygenative coupling of alcohols would have major implications for the field of organic synthesis. A general method for the direct deoxygenative cross-coupling of free alcohols must overcome several challenges, most notably the in situ cleavage of strong C-O bonds3, but would allow access to the vast collection of commercially available, structurally diverse alcohols as coupling partners4. We report herein a metallaphotoredox-based cross-coupling platform in which free alcohols are activated in situ by N-heterocyclic carbene salts for carbon-carbon bond formation with aryl halide coupling partners. This method is mild, robust, selective and most importantly, capable of accommodating a wide range of primary, secondary and tertiary alcohols as well as pharmaceutically relevant aryl and heteroaryl bromides and chlorides. The power of the transformation has been demonstrated in a number of complex settings, including the late-stage functionalization of Taxol and a modular synthesis of Januvia, an antidiabetic medication. This technology represents a general strategy for the merger of in situ alcohol activation with transition metal catalysis.
Assuntos
Álcoois/química , Brometos/química , Carbono/química , Cloretos/química , Metais/química , Oxigênio/química , Fotoquímica , Catálise , Metano/análogos & derivados , Metano/química , Nitrogênio/química , Oxirredução , Paclitaxel/química , Sinvastatina/síntese química , Sinvastatina/químicaRESUMO
Decomposition of dead organic matter is fundamental to carbon (C) and nutrient cycling in terrestrial ecosystems, influencing C fluxes from the biosphere to the atmosphere. Theory predicts and evidence strongly supports that the availability of nitrogen (N) limits litter decomposition. Positive relationships between substrate N concentrations and decomposition have been embedded into ecosystem models. This decomposition paradigm, however, relies on data mostly from short-term studies analyzing controls on early-stage decomposition. We present evidence from three independent long-term decomposition investigations demonstrating that the positive N-decomposition relationship is reversed and becomes negative during later stages of decomposition. First, in a 10-y decomposition experiment across 62 woody species in a temperate forest, leaf litter with higher N concentrations exhibited faster initial decomposition rates but ended up a larger recalcitrant fraction decomposing at a near-zero rate. Second, in a 5-y N-enrichment experiment of two tree species, leaves with experimentally enriched N concentrations had faster decomposition initial rates but ultimately accumulated large slowly decomposing fractions. Measures of amino sugars on harvested litter in two experiments indicated that greater accumulation of microbial residues in N-rich substrates likely contributed to larger slowly decomposing fractions. Finally, a database of 437 measurements from 120 species in 45 boreal and temperate forest sites confirmed that higher N concentrations were associated with a larger slowly decomposing fraction. These results challenge the current treatment of interactions between N and decomposition in many ecosystems and Earth system models and suggest that even the best-supported short-term controls of biogeochemical processes might not predict long-term controls.
Assuntos
Florestas , Nitrogênio , Folhas de Planta , Árvores , Nitrogênio/metabolismo , Nitrogênio/química , Folhas de Planta/química , Folhas de Planta/metabolismo , Árvores/metabolismo , Carbono/metabolismo , Carbono/química , Ecossistema , Taiga , Ciclo do CarbonoRESUMO
Mitigating climate change and sustainably feeding our growing population in the changing climate are 2 significant challenges facing the global community. Engineering photosynthesis, nature's carbon capture machinery, can help us surmount these threats.