Showing papers on "Reactivity (chemistry) published in 2010"

Thorough study of reactivity of various compound classes toward the Folin-Ciocalteu reagent.

Abstract: A thorough study was done to test the reactivity of the Folin−Ciocalteu (F-C) reagent toward various compound classes. Over 80 compounds were tested. Compound classes included phenols, thiols, vitamins, amino acids, proteins, nucleotide bases, unsaturated fatty acids, carbohydrates, organic acids, inorganic ions, metal complexes, aldehydes, and ketones. All phenols, proteins, and thiols tested were reactive toward the reagent. Many vitamin derivatives were also reactive, as were the inorganic ions Fe+2, Mn2+, I−, and SO32−. Other compounds showing reactivity included the nucleotide base guanine and the trioses glyceraldehyde and dihydroxyacetone. Copper complexation enhanced the reactivity of salicylate derivatives toward the reagent, whereas zinc complexation did not. Several amino acids and sugars that were reported to be reactive toward the F-C reagent in earlier studies were found not to be reactive in this study, at least in the concentrations used. Reaction kinetics of each compound with the F-C rea...

Ligand-Enabled Reactivity and Selectivity in a Synthetically Versatile Aryl C–H Olefination

Abstract: The Mizoroki-Heck reaction, which couples aryl halides with olefins, has been widely used to stitch together the carbogenic cores of numerous complex organic molecules. Given that the position-selective introduction of a halide onto an arene is not always straightforward, direct olefination of aryl carbon-hydrogen (C-H) bonds would obviate the inefficiencies associated with generating halide precursors or their equivalents. However, methods for carrying out such a reaction have suffered from narrow substrate scope and low positional selectivity. We report an operationally simple, atom-economical, carboxylate-directed Pd(II)-catalyzed C-H olefination reaction with phenylacetic acid and 3-phenylpropionic acid substrates, using oxygen at atmospheric pressure as the oxidant. The positional selectivity can be tuned by introducing amino acid derivatives as ligands. We demonstrate the versatility of the method through direct elaboration of commercial drug scaffolds and efficient syntheses of 2-tetralone and naphthoic acid natural product cores.

Rhodium(III)-Catalyzed Arene and Alkene C−H Bond Functionalization Leading to Indoles and Pyrroles

Abstract: Recently, the rhodium(III)-complex [Cp*RhCl(2)](2) 1 has provided exciting opportunities for the efficient synthesis of aromatic heterocycles based on a rhodium-catalyzed C-H bond functionalization event. In the present report, the use of complexes 1 and its dicationic analogue [Cp*Rh(MeCN)(3)][SbF(6)](2) 2 have been employed in the formation of indoles via the oxidative annulation of acetanilides with internal alkynes. The optimized reaction conditions allow for molecular oxygen to be used as the terminal oxidant in this process, and the reaction may be carried out under mild temperatures (60 °C). These conditions have resulted in an expanded compatibility of the reaction to include a range of new internal alkynes bearing synthetically useful functional groups in moderate to excellent yields. The applicability of the method is exemplified in an efficient synthesis of paullone 3, a tetracyclic indole derivative with established biological activity. A mechanistic investigation of the reaction, employing deuterium labeling experiments and kinetic analysis, has provided insight into issues of reactivity for both coupling partners as well as aided in the development of conditions for improved regioselectivity with respect to meta-substituted acetanilides. This reaction class has also been extended to include the synthesis of pyrroles. Catalyst 2 efficiently couples substituted enamides with internal alkynes at room temperature to form trisubstituted pyrroles in good to excellent yields. The high functional group compatibility of this reaction enables the elaboration of the pyrrole products into a variety of differentially substituted pyrroles.

The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds

Abstract: A series of manganese oxides differing in the structure, composition, average manganese oxidation state and specific surface area have been used in the total oxidation of volatile organic compounds (VOC). Ethanol, ethyl acetate and toluene were chosen as models of VOC. Among the manganese oxides tested, cryptomelane (KMn8O16) was found to be very active in the oxidation of VOC. The performance of cryptomelane was significantly affected by the presence of other phases, namely, Mn2O3 and Mn3O4. Temperature-programmed experiments combined with X-ray photoelectron spectroscopy (XPS) show that the mobility and reactivity of the oxygen species were significantly affected, explaining the catalytic performances of those samples. Mn3O4 improves the catalytic performance due to the increase of the reactivity and mobility of lattice oxygen, while Mn2O3 has the opposite effect. These results show that there is a correlation between the redox properties and the catalytic performance of the manganese oxides. Temperature-programmed surface reactions (TPSR) after adsorption of toluene or ethanol, in addition to reactions performed without oxygen in the feed, show that lattice oxygen is involved in the VOC oxidation mechanism. The conversion level was found to be influenced by the type of VOC, the reactivity into CO2 increasing in the following order: Toluene

Exploration of new C-O electrophiles in cross-coupling reactions.

Abstract: Since their development in the 1970s, cross-coupling reactions catalyzed by transition metals have become one of the most important tools for constructing both carbon−carbon and carbon−heteroatom bonds. Traditionally, organohalides were widely studied and broadly used as the electrophile, both in the laboratory and in industry. Unfortunately, the high cost, environmental toxicity, and sluggish preparation often associated with aryl halides can make them undesirable for the large-scale syntheses of industrial applications. However, with the further development of catalytic systems, and particularly of the ligands contained therein, a variety of electrophiles have now been successfully applied to cross-coupling reactions. Oxygen-based electrophiles have attracted much attention due to their ready availability from phenol and carbonyl compounds. Initially, aryl and alkenyl triflates were used in cross-coupling reactions due to their high reactivity; however, low moisture stability and high cost hampered thei...

Anomalously large reactivity of single graphene layers and edges toward electron transfer chemistries.

Abstract: The reactivity of graphene and its various multilayers toward electron transfer chemistries with 4-nitrobenzene diazonium tetrafluoroborate is probed by Raman spectroscopy after reaction on-chip Single graphene sheets are found to be almost 10 times more reactive than bi- or multilayers of graphene according to the relative disorder (D) peak in the Raman spectrum examined before and after chemical reaction in water A model whereby electron puddles that shift the Dirac point locally to values below the Fermi level is consistent with the reactivity difference Because the chemistry at the graphene edge is important for controlling its electronic properties, particularly in ribbon form, we have developed a spectroscopic test to examine the relative reactivity of graphene edges versus the bulk We show, for the first time, that the reactivity of edges is at least two times higher than the reactivity of the bulk single graphene sheet, as supported by electron transfer theory These differences in electron tr

Reactivity Theory of Transition-Metal Surfaces: A Brønsted−Evans−Polanyi Linear Activation Energy−Free-Energy Analysis

Abstract: The exponential increase in computational processor speed, the development of novel computational architectures, together with the tremendous advances in ab initio theoretical methods that have emerged over the past two decades have led to unprecedented advances in our ability to probe the fundamental chemistry that occurs on complex catalytic surfaces. In particular, advances in density functional theory (DFT) have made it possible to elucidate the elementary steps and mechanisms in surface-catalyzed processes that would be difficult to explore experimentally. The advanced state of plane wave DFT has made it possible to rapidly examine systematic changes to the metal or the reactant in order to establish structure-property relationships. As a result, extensive data based on the energetics for various different surface-catalyzed reactions has been generated. This invites a detailed theoretical analysis of the factors that control reaction paths and corresponding potentialenergy surfaces of surface reactions. Such a theoretical analysis will not only provide interesting new insights into the intricate relationship between the chemical bonding features, structure, and energies of transition states but also serve as a basis for the development of analytical expressions that relate transitionstate properties to more easily accessible thermodynamic properties. The Brønsted-Evans-Polanyi (BEP) relationship is one such example which has been widely applied in the analysis of surface elementary reaction steps.1-8 δEact )RδEr (1)

Carbon dioxide capture by superbase-derived protic ionic liquids.

Abstract: Protic ionic liquids (PILs) from a superbase and fluorinated alcohol, imidazole, pyrrolinone, or phenol were designed to capture CO{sub 2} based on the reactivity of their anions to CO{sub 2}. These PILs are capable of rapid and reversible capture of about one equivalent of CO{sub 2}, which is superior to those sorption systems based on traditional aprotic ILs.

Asymmetric Cooperative Catalysis of Strong Brønsted Acid–Promoted Reactions Using Chiral Ureas

Abstract: Cationic organic intermediates participate in a wide variety of useful synthetic transformations, but their high reactivity can render selectivity in competing pathways difficult to control. Here, we describe a strategy for inducing enantioselectivity in reactions of protio-iminium ions, wherein a chiral catalyst interacts with the highly reactive intermediate through a network of noncovalent interactions. This interaction leads to an attenuation of the reactivity of the iminium ion and allows high enantioselectivity in cycloadditions with electron-rich alkenes (the Povarov reaction). A detailed experimental and computational analysis of this catalyst system has revealed the precise nature of the catalyst-substrate interactions and the likely basis for enantioinduction.

Controlled selectivity for palladium catalysts using self-assembled monolayers

Abstract: The selective reaction of one part of a bifunctional molecule is a fundamental challenge in heterogeneous catalysis and for many processes including the conversion of biomass-derived intermediates. Selective hydrogenation of unsaturated epoxides to saturated epoxides is particularly difficult given the reactivity of the strained epoxide ring, and traditional platinum group catalysts show low selectivities. We describe the preparation of highly selective Pd catalysts involving the deposition of n-alkanethiol self-assembled monolayer (SAM) coatings. These coatings improve the selectivity of 1-epoxybutane formation from 1-epoxy-3-butene on palladium catalysts from 11 to 94% at equivalent reaction conditions and conversions. Although sulphur species are generally considered to be indiscriminate catalyst poisons, the reaction rate to the desired product on a catalyst coated with a thiol was 40% of the rate on an uncoated catalyst. Interestingly the activity decreased for less-ordered SAMs with shorter chains. The behaviour of SAM-coated catalysts was compared with catalysts where surface sites were modified by carbon monoxide, hydrocarbons or sulphur atoms. The results suggest that the SAMs restrict sulphur coverage to enhance selectivity without significantly poisoning the activity of the desired reaction.

Nucleophile-Initiated Thiol-Michael Reactions: Effect of Organocatalyst, Thiol, and Ene

Abstract: A detailed evaluation of the kinetics of the thiol-Michael reaction between hexanethiol and hexyl acrylate is described. It is shown that primary amines are more effective catalysts than either secondary or tertiary amines with, for example, quantitative conversion being achieved within 500 s in the case of hexylamine with an apparent rate constant of 53.4 mol L−1 s−1 at a catalyst loading of 0.057 mol %. Certain tertiary phosphines, and especially tri-n-propylphosphine and dimethylphenylphosphine, are shown to be even more effective species even at concentrations 2 orders of magnitude lower than employed for hexylamine and performed in solution with quantitative conversions reached within ca. 100 s for both species and apparent rate constants of 1810 and 431 mol L−1 s−1, respectively. The nature of the thiol is also demonstrated to be an important consideration with mercaptoglycolate and mercaptopropionate esters being significantly more reactive than hexanethiol with reactivity mirroring the pKa of the ...

Non-Oxidative Vanadium-Catalyzed CO Bond Cleavage: Application to Degradation of Lignin Model Compounds

Abstract: Lignocellulosic biomass has recently received great interest as a renewable source of fuel and chemicals.[1] Among the three major components of non-edible lignocellulose (cellulose, hemicellulose, and lignin), extensive efforts have been made to convert cellulose to ethanol and other biofuels. In contrast, research on the conversion of lignin has been limited to its removal from biomass either to enhance the accessibility of chemicals and enzymes to other components of biomass or to prevent photo-yellowing of paper and pulp. Despite the fact that lignin corresponds up to 30% of the weight and 40% of the energy content of lignocellulosic biomass, few novel processes aimed at producing high value compounds have been reported. Recently, several reports inspired by the pulp bleaching process have been published regarding the mechanism and product distribution of enzymatic and chemical oxidation reactions.[2] Using dimeric lignin model compounds (e.g. 1) containing a β-O-4 linkage that represents the most common substructure in lignin,[3] aromatic aldehydes were obtained as the main products in low yield. Although these methods show promises for selective conversion of lignin, fundamentally new catalytic processes need to be developed to fully realize lignin's potential as a chemical feedstock. In addition, thorough understanding of the mechanism of these processes is necessary to successfully achieve high selectivity. Aiming to develop a novel method to selectively convert lignin to highly functionalized aromatic compounds, we explored various homogeneous vanadium complexes for the conversion of 1 (Table 1).[4] Most of the vanadium catalysts tested yielded benzylic alcohol oxidation product 4 as the major product[5] in addition to small amounts of C–O bond cleavage products 2 and 3 (entries 2–7). In spite of the low yield, the formation of 2 distinguishes this reaction from previous reports: not only is 2 a novel product, but it is also a redox-neutral transformation. Excited by this new reactivity, we explored other vanadium catalysts and found that tridendate Schiff base ligands favor C–O bond cleavage over benzylic oxidation (entries 8–11). Higher selectivity for C–O bond cleavage was observed when ligands with larger bite angles were employed (entries 8 vs. 9 and 10 vs. 11).[6] The increased reactivity of catalyst 11 compared to 9 (entry 11 vs. 9) may be attributed to its tBu substituents, which enable intermediates from 11 to remain as catalytically active monomeric species instead of forming insoluble aggregates.[7] Thus, through subtle changes in the ligand structure, the reactivity of the vanadium(V)-oxo catalyst was tuned away from simple alcohol oxidation toward the cleavage of the β-O-4 carbon-oxygen bond. Table 1 Ligand effects on degradation of lignin model compound 1. To understand the role of oxygen in the formally non-oxidative process, the reaction was performed under anaerobic conditions with catalyst 11. After 24 h, the same products were obtained as those under aerobic conditions, albeit with lower conversion, along with pale purple precipitate (Scheme 1). The collected precipitate exhibited the same analytical properties as independently prepared V(IV) complex 12.[8] The EPR spectrum of the dark purple reaction mixture under aerobic conditions with 11 after 30 minutes also indicated the presence of V(IV) species. The V(IV) complex 12 is insoluble in various organic solvents and stable under air in the solid state. However, a heterogeneous reaction mixture with 12 turns into a dark purple solution under aerobic reaction conditions and furnishes a similar result to that with 11: >95% conversion, 80% 2, 58% 3, and 1% 4 (Scheme 2). These results indicate that the V(V) catalyst is reduced to V(IV) species during the reaction, but oxygen is not essential for the catalyst turnover, although it does increase the reaction rate. The facile interconversion between 11 and 12 allows one to employ the high reactivity of a homogeneous catalyst during the reaction and then easily recover the vanadium catalyst as an insoluble complex after the reaction by simply controlling the reaction atmosphere. Scheme 1 Degradation of 1 under anaerobic conditions. Scheme 2 Recovery of the reactivity of V(IV) complex 12 under aerobic conditions. To gain further insight on the mechanism of the non-oxidative C–O bond cleavage, we performed the reaction with various derivatives of 1 (Scheme 3). To test the possibility of oxidation of 1 to ketone 4 followed by reductive cleavage, 4 was subjected to the reaction conditions with or without benzylic alcohol 13. In both cases, 4 was recovered in high yield without any evidence of degradation to 2 or 3. The corresponding ketone 14 was obtained when 13 was added to reproduce the catalytic species formed after oxidation of 1 to 4. The lack of reactivity of 4 under the reaction conditions eliminates the possibility of a sequential oxidation and cleavage process of 1. Additionally, this experiment suggests that a V(III)–V(V) cycle,[9] which has been proposed for some vanadium-catalyzed aerobic alcohol oxidation,[4a,4c,4d] is likely not operational for this transformation. When the benzylic hydroxy group was replaced by a methoxy group (1a), the reaction proceeded with only 12% conversion, to afford conjugated aldehyde 15 as the product, indicating the importance of ligand exchange on 11 with the benzylic hydroxy group. In contrast, methyl ether 1b was converted to 2 and 3 with only slightly diminished yield and selectivity compared to 1. When the aryloxy group was absent (1c), only the corresponding ketone 16 was observed. Scheme 3 Reactivity study of analogues of 1. Based on these data, we propose a one-electron process as the most plausible mechanism (Figure 1). After ligand exchange on the vanadium complex with the benzylic hydroxy group, the benzylic hydrogen is abstracted to generate the ketyl radical, which eliminates the aryloxy radical.[10] The elimination of the hydroxy group from the resulting enolate produces enone 2 and a vanadium(IV) complex (17a or 17b), which is re-oxidized to vanadium(V) by the aryloxy radical.[11] Degradation of lignin via a ketyl radical had been suggested by several groups as a mechanism responsible for photo-yellowing of paper.[12] However, this hypothesis was later disputed because the ketyl radical generated by various methods reacted readily with oxygen to produce the corresponding ketone, and only a small amount of fragmentation products was formed by secondary photolysis of this ketone.[13] The high yield and good selectivity observed for β-O-4 cleavage of vanadium-catalyzed reaction indicates a dramatic reactivity change of the ketyl radical by coordination to vanadium. Figure 1 Plausible mechanism for vanadium-catalyzed non-oxidative cleavage of 1. Formation of the insoluble V(IV) complex 12 and only a slightly lower turnover number under anaerobic conditions suggests that regeneration of a V(V) complex by the aryloxy radical competes with formation of an insoluble V(IV) aggregate. Molecular oxygen probably accelerates the reaction by increasing the effective concentration of catalytically active species by converting insoluble 12 to soluble V(V) species. Although the vanadium-catalyzed non-oxidative degradation of 1 proceeds with high efficiency and selectivity in CH3CN, it is desirable to utilize solvents without nitrogen atoms for application to biofuel synthesis to prevent formation of NOx. When the reaction was performed in EtOAc on 1 mmol scale, the desired products were obtained with higher yields and selectivity than reactions run in CH3CN (Scheme 4).[14] Moreover, a more complex trimeric model compound 18[2f, 15] underwent clean C–O cleavage to provide three monomeric compounds, demonstrating the possibility of application of this reaction to more complex systems (Scheme 5).[16] Scheme 4 Conversion of 1 on 1 mmol scale in a solvent without nitrogen atoms. Scheme 5 Conversion of trimeric lignin model compound 18. In conclusion, we have demonstrated that changes in the ancilliary ligands can divert the reactivity of vanadium-oxo complexes from the typical alcohol oxidation to an unprecedented C-O bond cleavage reaction. This novel reactivity has been applied to a vanadium-catalyzed non-oxidative C–O bond cleavage reactions of dimeric lignin model compounds that produces aryl enones as unprecedented degradation products. This transformation is proposed to proceed via a ketyl radical generated by hydrogen atom transfer to a vanadium(V) complex. Oxygen is not essential for the reaction, although it increases the reaction rate. The novel reactivity of this transformation combined with good selectivity for a highly functionalized aryl enone demonstrates a great potential of lignin as a chemical feedstock. Further mechanistic studies and applications to biomass degradation are underway.

Catalytic Water Oxidation by Single-Site Ruthenium Catalysts

Abstract: A series of monomeric ruthenium polypyridyl complexes have been synthesized and characterized, and their performance as water oxidation catalysts has been evaluated. The diversity of ligand environments and how they influence rates and reaction thermodynamics create a platform for catalyst design with controllable reactivity based on ligand variations.

Modulating Reactivity and Diverting Selectivity in Palladium-Catalyzed Heteroaromatic Direct Arylation Through the Use of a Chloride Activating/Blocking Group

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.

Carboxylation of N ? H/C ? H Bonds Using N‐Heterocyclic Carbene Copper(I) Complexes

Abstract: Transition-metal-mediated carboxylation of N H and C H bonds represents a nascent area in organic chemistry, because these reactions enable the efficient construction of valuable synthons. Palladium-catalyzed N-carbonylation–oxidation sequences are well-documented, but they often require high catalyst loadings and the use of either gaseous carbon monoxide or Group VI metal–carbonyl complexes. An analogous transformation sequence is also promoted by molybdenum and tungsten carbonyl amine species under forcing temperatures. Important advances in C-carboxylation reactions have been made using ruthenium and nickel complexes; however, examples under mild conditions are elusive. The carboxylation of allylstannanes, organozincs, and organoboronic esters have been described as a new method to improve functional group tolerance, but the stoichiometric consumption of an organometallic reagent remains a disadvantage. The reactivity of allylstannanes and organozinc compounds necessitates handling under an inert atmosphere, while organoboronic esters are expensive. A protocol has recently been developed for the C-carboxylation of simple aromatic groups under very mild reaction conditions. In this case the strongly basic [Au(IPr)(OH)] (IPr=1,3-bis(diisopropyl)phenylimidazol-2-ylidene) complex (pKaDMSO= 30.3(2)) was used, [11] which contains an N-heterocyclic carbene (NHC) ligand [Eq. (1)].

Role of the secondary coordination sphere in metal-mediated dioxygen activation.

Abstract: Alfred Werner proposed nearly 100 years ago that the secondary coordination sphere has a role in determining the physical properties of transition-metal complexes. We now know that the secondary coordination sphere impacts nearly all aspects of transition-metal chemistry, including the reactivity and selectivity in metal-mediated processes. These features are highlighted in the binding and activation of dioxygen by transition-metal complexes. There are clear connections between control of the secondary coordination sphere and the ability of metal complexes to (1) reversibly bind dioxygen or (2) bind and activate dioxygen to form highly reactive metal−oxo complexes. In this Forum Article, several biological and synthetic examples are presented and discussed in terms of structure−function relationships. Particular emphasis is given to systems with defined noncovalent interactions, such as intramolecular H-bonds involving dioxygen-derived ligands. To further illustrate these effects, the homolytic cleavage o...

Discovery of 4-tert-Butyl-2,6-dimethylphenylsulfur Trifluoride as a Deoxofluorinating Agent with High Thermal Stability as Well as Unusual Resistance to Aqueous Hydrolysis, and Its Diverse Fluorination Capabilities Including Deoxofluoro-Arylsulfinylation with High Stereoselectivity

Abstract: Versatile, safe, shelf-stable, and easy-to-handle fluorinating agents are strongly desired in both academic and industrial arenas, since fluorinated compounds have attracted considerable interest in many areas, such as drug discovery, due to the unique effects of fluorine atoms when incorporated into molecules. This article describes the synthesis, properties, and reactivity of many substituted and thermally stable phenylsulfur trifluorides, in particular, 4-tert-butyl-2,6-dimethylphenylsulfur trifluoride (Fluolead, 1k), as a crystalline solid having surprisingly high stability on contact with water and superior utility as a deoxofluorinating agent compared to current reagents, such as DAST and its analogues. The roles of substiuents on 1k in thermal and hydrolytic stability, fluorination reactivity, and the high-yield fluorination mechanism it undergoes have been clarified. In addition to fluorinations of alcohols, aldehydes, and enolizable ketones, 1k smoothly converts non-enolizable carbonyls to CF2 gr...

Pd-catalyzed ortho-arylation of phenylacetamides, benzamides, and anilides with simple arenes using sodium persulfate

Abstract: We report a mild and efficient Pd-catalyzed ortho-arylation of phenylacetamides, benzamides, and anilides with a range of simple arenes using sodium persulfate (Na2S2O8). This green strategy generates biaryl C–C bonds from two unactivated sp2 hybridized C–H bonds. Electron-rich and electron-neutral arenes underwent oxidative arylation under our optimized reaction conditions. In substrates bearing two reactive ortho C–H bonds, selective diarylation via quadruple C–H bond functionalization was possible. The same reaction conditions were extended to an intramolecular cross-coupling for preparing lactams. The synthesis of relevant trifluoroacetate-bridged bimetallic Pd complexes derived from anilides and their stoichiometric reactivity were investigated.

Metal-organic frameworks as efficient heterogeneous catalysts for the regioselective ring opening of epoxides.

Abstract: An iron-based metal―organic framework, [Fe(BTC)] (BTC: 1,3,5-benzenetricarboxylate) is an efficient catalyst in the ring opening of styrene oxide with alcohols and aniline under mild reaction conditions. Out of the various alcohols tested for ring opening of styrene oxide, methanol was found to be the most reactive in terms of percentage conversion and reactivity. The rate of the ring-opening reaction of styrene oxide decreases as the size of the alcohol is increased, suggesting the location of active sites in micropores. [Fe(BTC)] was a truly heterogeneous catalyst and could be reused without loss of activity. The analogous compound [Cu 3 (BTC) 2 ] was also found to be effective, although with somewhat lower activity than [Fe(BTC)]. The present heterogeneous protocol is compared with a homogeneous catalyst to give an insight into the reaction mechanism.

Selectivity enhancement in functionalization of C–H bonds: A review

Abstract: The selectivity is an extremely important characteristic of a chemical reaction. This review deals mainly with supramolecular and nano-chemical approaches to the problem of selectivity enhancement in various functionalizations of C–H compounds. Enzyme mimics is a very fruitful method to achieve the predominant formation of desirable products and isomers. By obstructing the approach of certain C–H bonds of a substrate to the active catalytic centre we simultaneously increase the relative reactivity of other fragments. This can be done by creating steric hindrance around the active centre. Spatial restrictions can be made if we place the catalyst into a nano-cavity. We can achieve discrimination in reactivity of different C–H bonds if we allow certain fragments to approach closely the active centre. In order to do this chemists use coordination of the catalyst to some groups of the substrate with the participation of relatively strong binding (chelate control) or relatively weak forces (molecular recognition).

Highly Active, Bifunctional Co(III)-Salen Catalyst for Alternating Copolymerization of CO2 with Cyclohexene Oxide and Terpolymerization with Aliphatic Epoxides

Abstract: The cobalt(III) complex of a salen ligand bearing one quaternary ammonium salt on the three-position of one aromatic ring is a highly active catalyst for the alternating copolymerization of cyclohexene oxide (CHO) and CO2 to afford the corresponding poly(cyclohexene carbonate) (PCHC) at various temperatures. The cobalt-based catalyst exhibited excellent activity and selectivity for polymer formation at high temperatures up to 120 °C and even under low CO2 pressures of 0.1 MPa. Also, the cobalt(III)-salen complex could operate very efficiently for the terpolymerization of CHO and aliphatic epoxides with CO2 to provide selectively polycarbonates with a narrow polydispersity at high temperatures. The polycarbonates resulting from the terpolymerization of equimolar CHO and propylene oxide (PO) with CO2 have a close content for both cyclohexene carbonate and propylene carbonate units. This is ascribed to the presence of CHO significantly inhibiting the reactivity of PO and thereby causing a matched reactivity ...

EPR Studies of the Generation, Structure, and Reactivity of N-Heterocyclic Carbene Borane Radicals

Abstract: N-Heterocyclic carbene boranes (NHC−boranes) are a new “clean” class of reagents suitable for reductive radical chain transformations. Their structures are well suited for their reactivity to be tuned by inclusion of different NHC ring units and by appropriate placement of diverse substituents. EPR spectra were obtained for the boron-centered radicals generated on removal of one of the BH3 hydrogen atoms. This spectroscopic data, coupled with DFT computations, demonstrated that the NHC−BH2• radicals are planar π-delocalized species. tert-Butoxyl radicals abstracted hydrogen atoms from NHC−boranes more than 3 orders of magnitude faster than did C-centered radicals, although the rate decreased markedly for sterically shielded NHC−BH3 centers. Combinations of two NHC−boryl radicals afforded 1,2-bis-NHC−diboranes at rates which also depended strongly on steric shielding. The termination rate increased to the diffusion-controlled limit for sterically unhindered NHC−boryls. Bromine atoms were rapidly transferre...

Synthesis and reactivity of rhodium(II) N-triflyl azavinyl carbenes.

Abstract: Highly reactive rhodium(II) N-trifluoromethylsulfonyl azavinyl carbenes are formed in situ from NH-1,2,3-triazoles, triflic anhydride, and rhodium carboxylates. They rapidly and selectively react with olefins, providing cyclopropane carboxaldehydes and 2,3-dihydropyrroles in generally excellent yields and high enantio- and diastereoselectivity.

Activation of Si-H, B-H, and P-H bonds at a single nonmetal center.

Abstract: For many years, it was believed that only transition-metal centers could activate small molecules and enthalpically strong bonds. However, it has recently been shown that several nonmetallic systems are capable of some of these tasks. For example, stable singlet carbenes can activate CO, H2, [3b] and P4. [3c–e] Such reactions have long been known for transition metals. However, stable singlet carbenes can also activate NH3; [3b] a much more difficult task for transition metals. The oxidative addition of hydrosilanes, hydroboranes, and hydrophosphines at vacant coordination sites of transition metals are well-exemplified and are considered as key steps in the transition-metal-catalyzed hydrosilylation, hydroboration, and hydrophosphination of multiple bonds. Herein, we report the first examples of the activation of E H bonds (E=Si, B, P) at a single nonmetal center. On the basis of our successful results with H2, [3b] we began our study with the activation of Si H bonds. Indeed, silanes are similar to H2 in that they lack both nonbonding electron pairs and p electrons. They can bind to various metal centers to form stable Si H s complexes, which undergo subsequent oxidative addition. To test the possible activation of Si H bonds with carbenes, we treated the cyclic (alkyl)(amino)carbenes (CAACs) 1a and 1b with primary, secondary, and tertiary silanes. The addition of phenylsilane to 1a and 1b occurred readily at room temperature, and the corresponding adducts 2a,b were isolated in 91 and 83% yield, respectively (Scheme 1). As expected, in the case of the enantiomerically pure CAAC 1a, two diastereomers 2a,a’ were formed (in a 2:1 ratio), as shown by two singlets at d= 36.4 and 29.3 ppm in the Si NMR spectrum. The C NMR spectrum revealed the loss of the carbene signal and a new C H peak at d= 63.2 (2a) and 65.5 ppm (2b). The H NMR spectrum of the major isomer 2a revealed a pseudotriplet at d= 4.78 ppm (SiCH) and two doublets at d= 4.29 and 4.21 ppm corresponding to the diastereotopic hydrogen atoms of the SiH2 fragment. The structure of 2a was confirmed by X-ray crystallography (Figure 1, top), whereas the presence of a triplet at d= 4.53 ppm and a doublet at d= 4.08 ppm in the H NMR spectrum confirmed the identity of adduct 2b. CAACs 1a,b also reacted with (EtO)3SiH to afford 3a (d.r. 3:1) and 3b in 64 and 73% yield, respectively. However, when Ph2SiH2 was used, only the less bulky carbene 1b underwent insertion into the Si H bond (to give 4b in 65% yield), and a reaction time of 16 hours at 80 8C was necessary for the reaction to reach completion. Surprisingly, although it has been shown that, in contrast to CAACs, N-heterocyclic carbenes (NHCs) do not react with H2, [11] we found that imidazolidin-2-ylidene 5 also reacted at room temperature with phenylsilane to afford the Si H insertion product 6 in 88% yield (Figure 1, bottom). The formation of 6 raises the question of the mechanism of the activation of Si H bonds with carbenes. Why should NHCs react with silanes although they are inert towards hydrogen? The evident difference is the presence of low-lying vacant orbitals in silanes. In other words, the observed reactivity might be due to the Lewis acid character of silanes; indeed, several NHC–SiX4 adducts are known. [13]

The Role of One- and Two-Electron Transfer Reactions in Forming Thermodynamically Unstable Intermediates as Barriers in Multi-Electron Redox Reactions

Abstract: In the aquatic geochemical literature, a redox half-reaction is normally written for a multi-electron process (n > 2); e.g., sulfide oxidation to sulfate. When coupling two multi-electron half-reactions, thermodynamic calculations indicate possible reactivity, and the coupled half-reactions are considered favorable even when there is a known barrier to reactivity. Thermodynamic calculations should be done for one or two-electron transfer steps and then compared with known reactivity to determine the rate controlling step in a reaction pathway. Here, thermodynamic calculations are presented for selected reactions for compounds of C, O, N, S, Fe, Mn and Cu. Calculations predict reactivity barriers and agree with one previous analysis showing the first step in reducing O2 to O2− with Fe2+ and Mn2+ is rate limiting. Similar problems occur for the first electron transfer step in these metals reducing NO3−, but if reactive oxygen species form or if two-electron transfer steps with O atom transfer occur, reactivity becomes favorable. H2S and NH4+ oxidation in a one-electron transfer step by O2 is also not favorable unless activation of oxygen can occur. H2S oxidation by Cu2+, Fe(III) and Mn(III, IV) phases in two-electron transfer steps is favorable but not in one-electron steps indicating that (nano)particles with bands of orbitals are needed to accept two electrons from H2S. NH4+ oxidation by Fe(III) and Mn(III, IV) phases is generally not favorable for both one- and two-electron transfer steps, but their reaction with hydroxylamine and hydrazine to form N2O and N2, respectively, is favorable. The anammox reaction using hydroxylamine via nitrite reduction is the most favorable for NH4+ oxidation. Other chemical processes including photosynthesis and chemosynthesis are considered for these element–element transformations.