Ligand-Accelerated Activation of Strong CH Bonds of Alkanes by a (Salen)ruthenium(VI)-Nitrido Complex

Kinetic and mechanistic studies on the intermolecular activation of strong C?H bonds of alkanes by a (salen)ruthenium(VI) nitride were performed. The initial, rate-limiting step, the hydrogen atom transfer (HAT) from the alkane to Ru(VI) ?N, generates Ru(V) ?NH and RC(.) HCH(2) R. The following steps involve N-rebound and desaturation.     

Enantioselective rhodium-catalyzed nucleophilic allylation of cyclic imines with allylboron reagents

Chiral allylrhodium nucleophiles: The highly diastereo- and enantioselective title reaction of a range of cyclic imines with various potassium allyltrifluoroborates most likely proceeds via allylrhodium(I) intermediates, and represents the first rhodium-catalyzed enantioselective nucleophilic allylation of π?electrophiles with allylboron compounds.     

Experimental and theoretical study of the reaction of the ethynyl radical with nitrous oxide, C2H + N2O

We investigated the rate constants and reaction mechanism of the gas phase reaction between the ethynyl radical and nitrous oxide (C(2)H + N(2)O) using both experimental methods and electronic structure calculations. A pulsed-laser photolysis/chemiluminescence technique was used to determine the absolute rate coefficient over the temperature range 570 K to 836 K. In this experimental temperature range, the measured temperature dependence of the overall rate constants can be expressed as: k(T) (C(2)H + N(2)O) = 2.93 × 10(-11) exp((-4000 ± 1100) K/T) cm(3) s(-1) (95% statistical confidence). Portions of the C(2)H + N(2)O potential energy surface (PES), containing low-energy pathways, were constructed using the composite G3B3 method. A multi-step reaction route leading to the products HCCO + N(2) is clearly preferred. The high selectivity between product channels favouring N(2) formation occurs very early. The pathway corresponds to the addition of the terminal C atom of C(2)H to the terminal N atom of N(2)O. Refined calculations using the coupled-cluster theory whose electronic energies were extrapolated to the complete basis set limit CCSD(T)/CBS led to an energy barrier of 6.0 kcal mol(-1) for the entrance channel. The overall rate constant was also determined by application of transition-state theory and Rice-Ramsperger-Kassel-Marcus (RRKM) statistical analyses to the PES. The computed rate constants have similar temperature dependence to the experimental values, though were somewhat lower.     

Antioxidant benzylidene 2-aminoimidazolones from the Mediterranean sponge Phorbas topsenti

Three new benzylidene 2-aminoimidazolones, named phorbatopsins A-C, have been isolated from the Mediterranean marine sponge Phorbas topsenti, in addition to the known astaxanthin, adonirubin, taurine, and taurobetain. Their structures were elucidated through mass spectrometric and detailed 1D and 2D NMR spectroscopic data. The relative configuration of the molecules was determined on the basis of ¹H and ¹³C chemical shifts and proton-proton coupling constants. The absolute configuration of the chiral carbon C-6 of phorbatopsin B was determined by modified Mosher's method. The antioxidant activity, evaluated through the Oxygen Radical Absorbance Capacity (ORAC) assay, was also reported for the seven isolated compounds and structure-activity relationships are discussed. Phorbatopsin A appeared as the most active with an ORAC value of 0.88. Further investigations to support the promising antioxidant activity of phorbatopsin A will be followed.     

One new and several minor diarylheptanoids from Amomum muricarpum

A new natural diarylheptanoid, designated muricarpin, together with four diarylheptanoids were isolated from the rhizomes of Amomum muricarpum Elmer (Zingiberaceae) growing in Vietnam. Three known compounds, 1,7-bis(3,4-dihydroxyphenyl)heptan-3-yl acetate, 1-(4'-hydroxyphenyl)-7- (3″,4″-dihydroxyphenyl)heptan-3-yl acetate and 1-(3',4'-dihydroxyphenyl)-7-(4″-hydroxyphenyl)-heptan-3-one were isolated for the first time from the genus Amomum, meanwhile (5R)-5-hydroxy-1,7-bis(4-hydroxyphenyl)-heptan-3-one was found for the first time in plants. Their structures were determined using spectroscopic analyses.     

Uniquely Clean Elements in Rings

It is well known that every uniquely clean ring is strongly clean. In this article, we investigate the question of when this result holds element-wise. We first construct an example showing that uniquely clean elements need not be strongly clean. However, in case every corner ring is clean the uniquely clean elements are strongly clean. Further, we classify the set of uniquely clean elements for various classes of rings, including semiperfect rings, unit-regular rings, and endomorphism rings of continuous modules.     

An exact algorithm and a metaheuristic for the multi-vehicle covering tour problem with a constraint on the number of vertices

The multi-vehicle covering tour problem (m-CTP) involves finding a minimum-length set of vehicle routes passing through a subset of vertices, subject to constraints on the length of each route and the number of vertices that it contains, such that each vertex not included in any route lies within a given distance of a route. This paper tackles a particular case of m-CTP where only the restriction on the number of vertices is considered, i.e., the constraint on the length is relaxed. The problem is solved by a branch-and-cut algorithm and a metaheuristic. To develop the branch-and-cut algorithm, we use a new integer programming formulation based on a two-commodity flow model. The metaheuristic is based on the evolutionary local search (ELS) method proposed in [23]. Computational results are reported for a set of test problems derived from the TSPLIB.     

Surprising acid/base and ion-sequestration chemistry of Sn9(4-): HSn9(3-), Ni@HSn9(3-), and the Sn9(3-) ion revisited

K(4)Sn(9) dissolves in ethylenediamine (en) to give equilibrium mixtures of the diamagnetic HSn(9)(3-) ion along with K(x)Sn(9)((4-x)-) ion pairs, where x = 0, 1, 2, 3. The HSn(9)(3-) cluster is formed from the deprotonation of the en solvent and is the conjugate acid of Sn(9)(4-). DFT studies show that the structure is quite similar to the known isoelectronic RSn(9)(3-) ions (e.g., R = i-Pr). The hydrogen atom of HSn(9)(3-) (δ = 6.18 ppm) rapidly migrates among all nine Sn atoms in an intramolecular fashion; the Sn(9) core is also highly dynamic on the NMR time scale. The HSn(9)(3-) cluster reacts with Ni(cod)(2) to give the Ni@HSn(9)(3-) ion containing a hydridic hydrogen (δ = -28.3 ppm) that also scrambles across the Sn(9) cluster. The Sn(9)(4-) ion competes effectively with 2,2,2-crypt for binding K(+) in en solutions, and the pK(a) of HSn(9)(3-) is similar to that of en (i.e., Sn(9)(4-) is a very strong Br?nsted base with a pK(b) comparable to that of the NH(2)CH(2)CH(2)NH(-) anion). Competition studies show that the HSn(9)(3-) ? Sn(9)(4-) + H(+) equilibrium is fully reversible. The HSn(9)(3-) anion is present in significant concentrations in en solutions containing 2,2,2-crypt, yet it has gone undetected for over 30 years.     

Stereoselectivities of histidine-catalyzed asymmetric aldol additions and contrasts with proline catalysis: a quantum mechanical analysis

Quantum mechanical calculations reveal the origin of diastereo- and enantioselectivities of aldol reactions between aldehydes catalyzed by histidine, and differences between related reactions catalyzed by proline. A stereochemical model that explains both the sense and the high levels of the experimentally observed stereoselectivity is proposed. The computations suggest that both the imidazolium and the carboxylic acid functionalities of histidine are viable hydrogen-bond donors that can stabilize the cyclic aldolization transition state. The stereoselectivity is proposed to arise from minimization of gauche interactions around the forming C-C bond.