Showing papers on "Double bond published in 2003"
923 citations
TL;DR: The structures of naphthalene dioxygenase are determined that show a molecular oxygen species bound to the mononuclear iron in a side-on fashion and provide the basis for a reaction mechanism and for the high stereospecificity of the reaction catalyzed by naphthaene diodesase.
Abstract: Binding of oxygen to iron is exploited in several biological and chemical processes. Although computational and spectroscopic results have suggested side-on binding, only end-on binding of oxygen to iron has been observed in crystal structures. We have determined structures of naphthalene dioxygenase that show a molecular oxygen species bound to the mononuclear iron in a side-on fashion. In a complex with substrate and dioxygen, the dioxygen molecule is lined up for an attack on the double bond of the aromatic substrate. The structures reported here provide the basis for a reaction mechanism and for the high stereospecificity of the reaction catalyzed by naphthalene dioxygenase.
473 citations
TL;DR: A general and efficient copper-catalyzed method for the amidation of vinyl bromides and iodides has been developed and the double bond geometry of the vinyl halides was retained under the reaction conditions.
360 citations
TL;DR: It was shown, that hydroamination can be catalyzed by transition metals (dand f-block) and alkali metals.[1] Depending on the catalytic system either an activation of the C C multiple bond or the N H bond takes place.
Abstract: The catalytic addition of an organic amine R2N H bond to alkenes or alkynes (hydroamination) to give nitrogen containing molecules is of great interest for academic and industrial research. Since today most amines are made in multistep syntheses, hydroamination would offer the most attractive alternative synthetic route. It was shown, that hydroamination can be catalyzed by transition metals (dand f-block) and alkali metals.[1] Depending on the catalytic system either an activation of the C C multiple bond or the N H bond takes place. Alkene or alkyne activation is usually accomplished with late-transition metals through coordination of the C C multiple bond to the metal center. In contrast, the amino function can be activated by alkali or early transition metals, which generate an amido species, or by N H oxidative addition to electron-rich transition metals. Depending on the nature of the catalysts and the substrates either Markovnikov or anti-Markovnikov products are obtained [Eq. (1)]. The scope of catalytic hydroamination has been reviewed recently.[1–6] Today, the vast number of enantioselective syntheses of molecules bearing an amine functionality use classical stoichiometric reactions with chiral auxiliaries or utilize enantiomerically pure starting materials.[7] As a result of the increasing interest in the hydroamination reaction some research groups have started to investigate the enantioselective R2N H addition to C C double bonds. In early transition-metal chemistry Marks et al. implemented C1-symmetric organolanthanide ansa-metallocene catalysts of the type [Me2Si(hC5Me4)(h-C5H3R*)LnE(SiMe3)2] (1)[8–10] (R*= ( )-menthyl, (þ)-neomenthyl, ( )-phenylmenthyl; Ln=La, Nd, Sm, Y, Lu; E=CH, N) and [Me2Si(hOHF)(h-C5H3R*)LnN(SiMe3)2] (2)[11] (OHF= octahydrofluorenyl; R*= ( )menthyl; Ln= Sm, Y, Lu) in the enan-
315 citations
TL;DR: Examples will be presented for the interaction of HNE with various classes of biomolecules such as proteins and peptides, lipids and nucleic acids and the biochemical consequences will be discussed.
308 citations
TL;DR: It is proposed that the function of integral membrane proteins such as rhodopsin is sensitive to such a redistribution, and the loss of a single double bond from DHA to DPA results in a more even distribution of chain densities along the bilayer normal.
Abstract: Insufficient supply to the developing brain of docosahexaenoic acid (22:6n3, DHA), or its omega-3 fatty acid precursors, results in replacement of DHA with docosapentaenoic acid (22:5n6, DPA), an omega-6 fatty acid that is lacking a double bond near the chain's methyl end. We investigated membranes of 1-stearoyl(d(35))-2-docosahexaenoyl-sn-glycero-3-phosphocholine and 1-stearoyl(d(35))-2-docosapentaenoyl-sn-glycero-3-phosphocholine by solid-state NMR, X-ray diffraction, and molecular dynamics simulations to determine if the loss of this double bond alters membrane physical properties. The low order parameters of polyunsaturated chains and the NMR relaxation data indicate that both DHA and DPA undergo rapid conformational transitions with correlation times of the order of nanoseconds at carbon atom C(2) and of picoseconds near the terminal methyl group. However, there are important differences between DHA- and DPA-containing lipids: the DHA chain with one additional double bond is more flexible at the methyl end and isomerizes with shorter correlation times. Furthermore, the stearic acid paired with the DHA in mixed-chain lipids has lower order, in particular in the middle of the chain near carbons C(10)(-)(12), indicating differences in the packing of hydrocarbon chains. Such differences are also reflected in the electron density profiles of the bilayers and in the simulation results. The DHA chain has a higher density near the lipid-water interface, whereas the density of the stearic acid chain is higher in the bilayer center. The loss of a single double bond from DHA to DPA results in a more even distribution of chain densities along the bilayer normal. We propose that the function of integral membrane proteins such as rhodopsin is sensitive to such a redistribution.
229 citations
TL;DR: The metal-silicon double bond in silylene complexes is highly polarized in a Mδ−−Siδ+ manner as discussed by the authors, and silyl(silylene) complexes exhibit high reactivities toward nucleophiles, such as water, alcohols, ketones, isocyanates, and phosphorus ylides.
Abstract: The metal–silicon bond in silylene complexes is highly polarized in a Mδ−–Siδ+ manner. Accordingly, silylene complexes show high reactivities toward nucleophiles, such as water, alcohols, ketones, isocyanates, and phosphorus ylides. The metal–silicon double bond can also activate aromatic carbon–hydrogen bonds. Among the various silylene complexes, silyl(silylene) complexes occupy a unique position; these complexes undergo intramolecular 1,3-migration, which is postulated as a key step in the metal-mediated redistribution of substituents on organosilicon compounds. Alkyl(silylene) complexes are not stable and undergo 1,2-alkyl migration to yield alkylsilyl complexes.
195 citations
Patent•
25 Dec 2003TL;DR: In this article, the general formula (I), salts thereof, and hydrates of both: (I) wherein T is a single bond, optionally substituted C1-4 alkylene, or the like; the symbols ˙˙ ǫ˫˒˔˒ ˫ are each a single or a double bond; A is a divalent group derived from a 5- to 14-membered heterocycle which may be substituted, or a like; Y is a one-to-four independent hydrogen atoms or the
Abstract: Compounds represented by the general formula (I), salts thereof, and hydrates of both: (I) wherein T is a single bond, optionally substituted C1-4 alkylene, or the like; the symbols ˙˙˙˙˙˙ are each a single bond or a double bond; A is a single bond, a divalent group derived from a 5- to 14-membered heterocycle which may be substituted, or the like; Y is a single bond or the like; Z is methylene or the like; G is phenylene optionally fused with a 5- or 6-membered ring which may have a heteroatom, or the like; Ra and Rb are each independently hydrogen or the like; W is a single bond or the like; R’ represents one to four independent hydrogen atoms or the like; and R” represents one to four independent hydrogen atoms or the like.
158 citations
TL;DR: The relationship of their structure to the inhibition of low-density lipoprotein (LDL) oxidation in vitro was investigated, and the contribution of different functional groups among the flavonoid subclasses to in vitro inhibition of LDL oxidation is assigned.
151 citations
TL;DR: The highly strained ingenane skeleton was constructed through an intramolecular cyclization reaction of an acetylene dicobalt complex followed by a rearrangement Reaction of an epoxy alcohol.
Abstract: Total synthesis of ingenol, a diterpene isolated from the genus Euphorbia, was accomplished on the basis of the novel key reactions. The highly strained ingenane skeleton was constructed through an intramolecular cyclization reaction of an acetylene dicobalt complex followed by a rearrangement reaction of an epoxy alcohol. The C(3),C(4),C(5)-triol moiety was introduced by a stereoselective double dihydroxylation reaction of a diene having C(2)−C(3) and C(4)−C(5) double bonds.
142 citations
TL;DR: In this article, the catastrophe theory has been used to investigate the reorganization of the localization basins, within the electron localization function formalism, along the intrinsic reaction coordinate associated with the reaction pathway of the Diels−Alder reaction.
Abstract: The catastrophe theory has been used to investigate the reorganization of the localization basins, within the electron localization function formalism, along the intrinsic reaction coordinate associated with the reaction pathway of the Diels−Alder reaction between ethylene and 1,3-butadiene. There are distinguished seven phases (I−VII) characterized by a decay and formation of the double bonds, an accumulation of the nonbonding electron density on the C atoms involved in the formation of two sigma bonds and a ring closure processes. During the reaction 10 catastrophes occur belonging to two elementary types: fold and cusp. The transition structure is located in phase III, being determined by a “reduction” of the double CC bond of ethylene to the single bond, and it is not associated with any special event on the intrinsic reaction coordinate path. For the first time, it is shown that formation of two new sigma C−C bonds between ethylene and 1,3-butadiene begins in phase VI at 2.044A.
TL;DR: A comparison of the two structures reveals subtle but important conformational changes that aid in the stabilization of the Trp191 cationic radical in Compound I, and higher-resolution data enable a more accurate determination of changes in heme parameters.
Abstract: Cytochrome c peroxidase (CCP) is a 32.5 kDa mitochondrial intermembrane space heme peroxidase from Saccharomyces cerevisiae that reduces H(2)O(2) to 2H(2)O by oxidizing two molecules of cytochrome c (cyt c). Here we compare the 1.2 A native structure (CCP) with the 1.3 A structure of its stable oxidized reaction intermediate, Compound I (CCP1). In addition, crystals were analyzed by UV-vis absorption and electron paramagnetic resonance spectroscopies before and after data collection to determine the state of the Fe(IV) center and the cationic Trp191 radical formed in Compound I. The results show that X-ray exposure does not lead to reduction of Fe(IV) and only partial reduction of the Trp radical. A comparison of the two structures reveals subtle but important conformational changes that aid in the stabilization of the Trp191 cationic radical in Compound I. The higher-resolution data also enable a more accurate determination of changes in heme parameters. Most importantly, when one goes from resting state Fe(III) to Compound I, the His-Fe bond distance increases, the iron moves into the porphyrin plane leading to shorter pyrrole N-Fe bonds, and the Fe(IV)-O bond distance is 1.87 A, suggesting a single Fe(IV)-O bond and not the generally accepted double bond.
TL;DR: Analyses of IR, NMR, crystallographic, and DFT data are consistent with the presence of an Fe=B double bond.
Abstract: The synthesis and the spectroscopic and structural characterization of the cationic terminal borylene complex [Cp*Fe(CO)2(BMes)]+ are reported. Halide abstraction from the corresponding bromoboryl species using Na[BAr f4] generates the borylene as the [BAr f4]- salt in ca. 50% yield. Analyses of IR, NMR, crystallographic, and DFT data are consistent with the presence of an Fe=B double bond.
TL;DR: Computer modeling study indicates that trehalose forms a stable complex with an olefin double bond through OH...pi and CH...O types of hydrogen bonding.
Abstract: Trehalose is a disaccharide that attracts much attention as a stress protectant. In this study, we investigated the mechanism of the antioxidant function of trehalose. The spin-lattice relaxation times (T(1)) of (1)H and (13)C NMR spectra were measured to investigate the interaction between trehalose and unsaturated fatty acid (UFA). We selected several kinds of UFA that differ in the number of double bonds and in their configurations (cis or trans). Several other disaccharides (sucrose, maltose, neotrehalose, maltitol, and sorbitol) were also analyzed by NMR. The T(1) values for the (1)H and (13)C signals assigned to the olefin double bonds in UFA decrease with increasing concentration of trehalose and the changes reaches plateaus at integer ratios of trehalose to UFA. The characteristic T(1) change is observed only for the combination of trehalose and UFA with cis double bond(s). On the other hand, from the (13)C-T(1) measurements for trehalose, the T(1) values of the C-3 (C-3') and C-6' (C-6) are found to change remarkably by addition of UFA. (1)H[bond](1)H NOESY measurements provide direct evidence for complexation of trehalose with linoleic acid. These results indicate that one trehalose molecule stoichiometrically interacts with one cis-olefin double bond of UFA. Computer modeling study indicates that trehalose forms a stable complex with an olefin double bond through OH...pi and CH...O types of hydrogen bonding. Furthermore, a significant increase in the activation energy is found for hydrogen abstraction reaction from the methylene group located between the double bonds that are both interacting with the trehalose molecules. Therefore, trehalose has a significant depression effect on the oxidation of UFA through the weak interaction with the double bond(s). This is the first study to elucidate the antioxidant function of trehalose.
TL;DR: In less than three decades, the chemistry of low-coordinated compounds of the heavier elements of group 14 has made unforeseen and unprecedented advances as mentioned in this paper, including homonuclear double bonds between all elements of this group and the first examples of compounds analogous to the alkynes have been isolated.
TL;DR: In this paper, a self-organization of the Fischer-Tropsch regime is established from time dependence of activity, selectivity and catalyst structure, and a process of thermodynamically controlled restructuring/segregation of the cobalt surface is addressed and understood as activating the catalyst and specifically, disproportionating on-plane sites into sites of lower coordination (on-top sites) and higher coordination (inhole sites).
Abstract: Minor reactions, accompanying the major reactions for building straight-chains of aliphatic hydrocarbons from the reactants CO and H2 on the surface of cobalt catalysts, can contribute substantially to the understanding of the regime of Fischer–Tropsch synthesis. This goal affords precise mass balances, precise determination of product composition and consistent kinetic schemes for obtaining the right kinetic coefficients. The concept of self-organization of the Fischer–Tropsch regime is established from time dependence of activity, selectivity and catalyst structure. A process of thermodynamically controlled restructuring/segregation of the cobalt surface is addressed and understood as activating the catalyst and specifically, disproportionating on-plane sites into sites of lower coordination (on-top sites) and higher coordination (in-hole sites). These different sites appear to collaborate in the Fischer–Tropsch regime, with steps of coordination chemistry (comparable to those of transition metal complexes) on on-top sites and dissociation (specifically of CO) on in-hole sites and further in principle suppressed reactions on on-plane sites. This concept is developed and illustrated here with the results of several investigations such as tracing of activity and selectivity during the initial episodes of synthesis, experiments with added (14C-labeled) olefins and variation of synthesis parameters to see their specific influences. As minor reactions of coordination chemistry on on-top sites, reversible CH2 cleavage from alkyl chains, CO insertion and ethene insertion are visualized. On on-plane sites CO methanation, olefin hydrogenation and olefin double bond shift are noticed, but much inhibited. As compared to Fischer–Tropsch on iron catalysts, the common Fischer–Tropsch principle appears to be the inhibition of chain desorption to allow for growth reactions of the adsorbed chains. Minor reactions and detailed kinetics on iron and cobalt catalysts differ basically.
TL;DR: The results clearly demonstrate that the molecular engineering criteria for metal-containing nonlinear optical chromophores can differ dramatically from those for purely organic compounds.
Abstract: We have used several techniques, including hyper-Rayleigh scattering and Stark spectroscopy, to investigate the effects of polyene chain length on the optical properties of complexes containing ruthenium(II) electron donor groups and pyridinium electron acceptors. In marked contrast with all other known donor-acceptor polyenes, conjugation extension beyond a single double bond in the dipolar complexes studied leads to blue-shifting of the intramolecular charge-transfer absorptions. Furthermore, the static first hyperpolarizabilities beta0 become maximized with trans-1,3-butadienyl linkages and then decrease in complexes with three CH=CH bonds. Our results clearly demonstrate that the molecular engineering criteria for metal-containing nonlinear optical chromophores can differ dramatically from those for purely organic compounds.
TL;DR: In this article, the apparent rate constants and turnover frequencies of the catalytic reactions were determined using an iterative kinetics-fitting program, which is similar to those reported for many homogeneous nitrile hydration catalysts described in the literature.
TL;DR: In this article, the direct conversion of allyl ethers to cyclic enol ethers using an olefin metathesis/double bond migration sequence was described, where Ruthenium carbene complexes were activated to catalyze the double bond migration step by addition of hydride sources, such as NaH or NaBH4.
TL;DR: The spectroscopy measurement showed an increase in the double bond amount with increasing enzymatic digestion time, and this double bond forms a complex with the radicals during UV irradiation, and reduced the toxicity of the radicals before they are absorbed in the human skin.
TL;DR: Both the amplitude and frequency of motion increase toward the terminal methyl group of DHA, and the low barriers to torsional rotation about C-C bonds that link the cis-locked double bonds with the methylene carbons between them are responsible for this unusual flexibility.
Abstract: Early experiments and molecular simulations of PUFA favored a rigid arrangement of double bonds in U-shaped or extended conformations such as angle-iron or helical. Although results of recent solid-state NMR measurements and molecular simulations have confirmed the existence of these structural motifs, they portray an image of DHA (22∶6n−3) as a highly flexible molecule with rapid transitions between large numbers of conformers on the time scale from picoseconds to hundreds of nanoseconds. The low barriers to torsional rotation about C-C bonds that link the cis-locked double bonds with the methylene carbons between them are responsible for this unusual flexibility. Both the amplitude and frequency of motion increase toward the terminal methyl group of DHA.
TL;DR: In this paper, a density functional theory (DFT) study of propene, 1-hexene, and 3hexene protonation over representative H-ZSM-5 clusters to give covalent alkoxide intermediates is presented.
Abstract: We present a density functional theory (DFT) study of propene, 1-hexene, and 3-hexene protonation over representative H-ZSM-5 clusters to give covalent alkoxide intermediates. The influence of cluster size, olefin carbon number, olefin conformation, proton siting, aluminum siting, and bonding configuration (primary vs secondary) of the alkoxide intermediate was analyzed. We found the formation of a physisorbed π-complex involving the olefin double bond and the acidic proton to be relatively independent of olefin structure and site geometry. However, we show that the proton-transfer process for formation of the covalent alkoxide intermediate involves a carbenium-ion-like transition state, with an activation energy that is (1) dependent on the protonation site of the olefin and (2) relatively independent of the carbon number and double bond location of the olefin. Accessibility of the alkoxide oxygen site in the cavity was observed to play a significant role in the stability of the alkoxy species. We find t...
TL;DR: The reaction of the three-coordinate complex LFeCl (L = bulky β-diketiminate) with KBEt3H gives a dark red iron(II) hydride complex as discussed by the authors.
Abstract: Reaction of the three-coordinate complex LFeCl (L = bulky β-diketiminate) with KBEt3H gives a dark red iron(II) hydride complex. The complex is a dimer in the solid state, but spectroscopy and kinetics suggest that an orange three-coordinate monomer is in equilibrium with the dimer in solution. The double bond of azobenzene is completely cleaved by heating with the hydride complex, and a hydrazido intermediate can be isolated.
TL;DR: The structurally characterized molybdoenzyme carbon monoxide dehydrogenase (CODH) catalyzes the oxidation of CO to CO2 in the aerobic bacterium Oligotropha carboxidovorans and demonstrates that Se is coordinated by two C atoms at 1.94-1.95 A distance, interpreted as a replacement of the S in methionine residues.
Abstract: The structurally characterized molybdoenzyme carbon monoxide dehydrogenase (CODH) catalyzes the oxidation of CO to CO2 in the aerobic bacterium Oligotropha carboxidovorans. The active site of the enzyme was studied by Mo- and Cu-K-edge X-ray absorption spectroscopy. This revealed a bimetallic [Cu(I)SMo(VI)(double bond O)2] cluster in oxidized CODH which was converted into a [Cu(I)SMo(IV)(double bond O)OH2] cluster upon reduction. The Cu...Mo distance is 3.70 A in the oxidized form and is increased to 4.23 A upon reduction. The bacteria contain CODH species with the complete and functional bimetallic cluster along with enzyme species deficient in Cu and/or bridging S. The latter are precursors in the posttranslational biosynthesis of the metal cluster. Cu-deficient CODH is the most prominent precursor and contains a [HSMo(double bond O)OH2] cluster. Se-K-edge X-ray absorption spectroscopy demonstrates that Se is coordinated by two C atoms at 1.94-1.95 A distance. This is interpreted as a replacement of the S in methionine residues. In contrast to a previous report [Dobbek, H., Gremer, L., Meyer, O., and Huber, R. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 8884-8889] Se was not identified in the active site of CODH.
TL;DR: The boron trifluoride-catalyzed Rothemund condensation of triisopropylsilyl (TIPS) propynal 1 with 3,4-diethylpyrrole in dichloromethane, followed by oxidation with 2,3-dichloro-5,6-dicyano-1, 4-benzoquinone (DDQ) generates a mixture of products, including triphyrin H
Abstract: The boron trifluoride-catalyzed Rothemund condensation of triisopropylsilyl (TIPS) propynal 1 with 3,4-diethylpyrrole in dichloromethane, followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) generates a mixture of products, including [15]triphyrin(1.1.3) H3, corrole H(3)4, porphyrin H(2)2, [24]pentaphyrin(1.1.1.1.1) H(4)5, [28]hexaphyrin(1.1.1.1.1.1) H(4)6, and two linear tripyrromethenes H(2)7 and H(2)8. We report the spectroscopic characteristics of these unusual chromophores, together with the crystal structures of triphyrin H3 (and its zinc complex ZnCl3), porphyrin H(2)2 (and its metal complexes Zn2, Ni2 and Pt2), hexaphyrin H(4)6, and tripyrromethene nickel(II) complex Ni7. When the condensation is catalyzed with trifluoroacetic acid, rather than boron trifluoride, the triphyrin H3 become the main product (26% yield). This novel macrocycle is linked with a TIPS-substituted exocyclic double bond. This C=C bond makes an eta(2)-interaction with the zinc center in ZnCl3 with C-Zn distances of 2.863 and 3.025 A. The porphyrin H(2)2 is severely ruffled, and its absorption spectrum is red-shifted and broadened compared with the analogous compound without ethyl substituents. The hexaphyrin H(4)6 adopts a figure-of-eight conformation with virtual C(2) symmetry in the solid state and C(2) symmetry in solution on the NMR time scale. Oxidation with DDQ appears to convert this nonaromatic [28]hexaphyrin into an aromatic [26]hexaphyrin with a strongly red-shifted absorption spectrum, but the oxidized macrocyle is too unstable to isolate.
TL;DR: The pi complexes first formed as essential intermediates from alkenes, alkynes, and allenes with bromine have been investigated in different solvents by UV-spectroscopy in combination with stopped-flow techniques allowing the determination of the equilibrium constants, K(f), which reveals that complexes are stabilized in solution owing to reduction of the entropy loss by restricted translations and rotation.
Abstract: The pi complexes first formed as essential intermediates from alkenes, alkynes, and allenes with bromine have been investigated in different solvents by UV-spectroscopy in combination with stopped-flow techniques allowing the determination of the equilibrium constants, K(f). Using alkenes with sterically protected double bonds, such as di-tert-butylstilbene and tetraneopentylethylene, the reaction stops at the stage of the 1:1 and 1:2 pi complex of the alkene with bromine as persistent species in 1,2-dichlorethane as solvent. Calculations by state-of-art ab initio and DFT methods reproduces the experimentally determined thermodynamic values quite well, and reveal the preferred structures and nature of both complexes for ethene, ethyne, and allene. Consideration of the entropy term reveals that complexes are stabilized in solution owing to reduction of the entropy loss by restricted translations and rotation. According to calculations these species are Mulliken-outer-type complexes with no or little charge transfer from bromine to the double or triple bond, respectively. The 1:2 complex has a close structural relationship to the bromonium- or bromirenium ion, which is the subsequent intermediate on the reaction coordinate. Steric influences show a strong effect on the K(f) value, which can be explained by the polarizibility of the parent system. Addition-elimination often occurs. In bromination of adamantylidenadamantane and its derivatives the reaction stops at the stage of the bromonium ion. The effect of various polar groups situated in equatorial homoallyl positions on the stability of corresponding pi complex and bromonium ion has been studied in this series.
TL;DR: The reaction of hydrated electrons with 8-bromo-2'-deoxyadenosine has been investigated by radiolytic methods coupled with product studies and addressed computationally by means of DFT-B3LYP calculations, and the reaction mechanism was defined in some detail.
Abstract: The reaction of hydrated electrons (e(aq)(-)) with 8-bromo-2'-deoxyadenosine has been investigated by radiolytic methods coupled with product studies and addressed computationally by means of DFT-B3LYP calculations. Pulse radiolysis revealed that this reaction was complete in approximately 0.3 mus, and, at this time, no significant absorption was detected. The spectrum of a transient developed in 20 mus has an absorbance in the range 300-500 nm (epsilon(max) congruent with 9600 M(-1) cm(-1) at 360 nm), and it was assigned to aromatic aminyl radical 3. Computed vertical transitions (TD-UB3LYP/6-311+G) are in good agreement with the experimental observations. Radical 3 is obtained by the following reaction sequence: one-electron reductive cleavage of the C-Br bond that gives the C8 radical, a fast radical translocation from the C8 to C5' position, and an intramolecular attack of the C5' radical at the C8,N7 double bond of the adenine moiety. The rate constant for the cyclization is 1.6 x 10(5) s(-1). On the basis of the theoretical findings, the cyclization step is highly stereospecific. The rate constants for the reactions of C5' and aminyl 3 radicals with different oxidants were determined by pulse radiolysis methods. The respective rate constants for the reaction of 2'-deoxyadenosin-5'-yl radical with dioxygen, Fe(CN)(6)(3)(-), and MV(2+) in water at ambient temperature are 1.9 x 10(9), 4.2 x 10(9), and 2.2 x 10(8) M(-1) s(-1). The value for the reaction of aminyl radical 3 with Fe(CN)(6)(3-) is 8.3 x 10(8) M(-1) s(-1), whereas the reaction with dioxygen is reversible. Tailored experiments allowed the reaction mechanism to be defined in some detail. A synthetically useful radical cascade process has also been developed that allows in a one-pot procedure the conversion of 8-bromo-2'-deoxyadenosine to 5',8-cyclo-2'-deoxyadenosine in a diastereoisomeric ratio (5'R):(5'S) = 6:1 and in high yield, by reaction with hydrated electrons in the presence of K(4)Fe(CN)(6).
TL;DR: A highly chemoselective oxidation of allylic and vinylic sulfides to the corresponding sulfoxides has been developed using flavin 1 as the oxidation catalyst and hydrogen peroxide as the terminal oxidant.
Abstract: A highly chemoselective oxidation of allylic and vinylic sulfides to the corresponding sulfoxides has been developed using flavin 1 as the oxidation catalyst and hydrogen peroxide as the terminal oxidant. The sulfoxides were formed in good to excellent yields in a highly selective manner even when an excess of the oxidant was used. Sulfone formation was completely suppressed to <0.5% (in one single case 1.5% sulfone was detected). No epoxidation of double bonds or interference with other functional groups was observed under the reaction conditions. The general applicability was demonstrated by the selective oxidation of various allylic and vinylic sulfides having different electronic properties. A number of functionalities including hydroxy, acetoxy, amino, silyloxy, and formyl groups are tolerated under these mild reaction conditions.
TL;DR: These neutral systems are shown to be highly active, even under conditions incompatible with their isostructural, but formally cationic, relatives.
Abstract: Formally zwitterionic bis(phosphanyl)- and bis(amino)borate rhodium(I) complexes (see picture) can catalytically mediate the hydrogenation, hydroacylation, hydroboration, and hydrosilation of double bonds. These neutral systems are shown to be highly active, even under conditions incompatible with their isostructural, but formally cationic, relatives.
TL;DR: QM/MM (ONIOM) calculations, supporting a mixed Fischer-Schrock-type biscarbene complex, show that protonation occurs preferentially at the carbene carbon C(1) adjacent to Ru, in the relative cis position with respect to the Ru-Cl bond.
Abstract: The reaction of a variety of alkynes RC⋮CH with a variety of carboxylic acids R1CO2H, in the presence of 5% of RuCl(COD)C5Me5, selectively leads to the dienylesters (1E,3E)-RCH1CH2−CH3C(R)(O2CR1). The reaction also applies to amino acid and dicarboxylic acid derivatives. It is shown that the first step of the reaction consists of the head-to-head alkyne coupling and of the formation of the metallacyclic biscarbene−ruthenium complex isolated for R = Ph and catalyzing the formation of dienylester. D-labeled reactions show that the alkyne protons remain at the alkyne terminal carbon atoms and carboxylic acid protonates the C1 carbon atom. QM/MM (ONIOM) calculations, supporting a mixed Fischer−Schrock-type biscarbene complex, show that protonation occurs preferentially at the carbene carbon C1 adjacent to Ru, in the relative cis position with respect to the Ru−Cl bond, to give a mixed C(1)alkyl−C(4)carbene complex in which the C4 carbene is conjugated with the noncoordinated C2C3 double bond. This 16-electron...