Mechanism of di-tert-butylsilylene transfer from a silacyclopropane to an alkene

Kinetic and thermodynamic studies of the reactions of cyclohexene silacyclopropane 1 and monosubstituted alkenes suggested a possible mechanism for di-tert-butylsilylene transfer. The kinetic order in cyclohexene silacyclopropane 1 and cyclohexene were determined to be 1 and -1, respectively. Saturation kinetic behavior in monosubstituted alkene concentration was observed. Competition experiments between substituted styrenes and a deficient amount of di-tert-butylsilylene from 1 correlated well with the Hammett equation and provided a rho value of -0.666 +/- 0.008, using sigma(p) constants. These data supported a two-step mechanism involving reversible di-tert-butylsilylene extrusion from 1, followed by irreversible concerted electrophilic attack of the silylene on the monosubstituted alkene. Eyring activation parameters were found to be DeltaH++ = 22.1 +/- 0.9 kcal.mol(-1) and DeltaS++ = -15 +/- 2 eu. Competition experiments between cycloalkenes and allylbenzene determined cycloalkenes to be more efficient silylene traps (k(rel) =1.3, DeltaDeltaG++ = 0.200 kcal.mol(-1)). A summary of the data resulted in a postulated reaction coordinate diagram. The mechanistic studies enabled rational modification of reaction conditions that improved the synthetic utility of silylene transfer. Removal of the volatile cyclohexene from the reaction mixture into an evacuated headspace led to the formation of previously inaccessible cyclohexene-derived silacyclopropanes.     

Metal-catalyzed di-tert-butylsilylene transfer: synthesis and reactivity of silacyclopropanes

Metal-catalyzed di-tert-butylsilylene transfer was developed as a mild, operationally simple, functional-group-tolerant method for silacyclopropane formation. Di-tert-butylsilylene was transferred from cyclohexene silacyclopropane 1 to an alkene through the use of a metal salt. Silacyclopropanation occurred at temperatures as low as -27 degrees C when AgOTf or AgOC(O)CF(3) were used as catalysts. Complex silacyclopropanes were formed stereospecifically and diastereoselectively from functionalized alkenes. Silacyclopropanes reacted with various carbonyl compounds, including aldehydes, ketones, formate esters, and formamides, in an overall process that efficiently converts alkenes into oxasilacyclopentanes with defined stereochemistry.     

Mechanism of proton transfer from 2-nitrohexafluoropropane to amines

According to the analysis of NMR kinetic data, the proton transfer from the CH-acid 2-nitrohexafluoropropane to trioctylamine occurs in a hydrogen-bonded complex. The value of the k_H/k_D ratio (6) and the linearity of the ln k vs. T^–1 dependence indicate a classical mechanism of the process.

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2- . . , . k_H/k_D 6 .

     

Mechanism of ruthenium-catalyzed hydrogen transfer reactions. Concerted transfer of OH and CH hydrogens from an alcohol to a (Cyclopentadienone)ruthenium complex

Kinetic studies of the ruthenium-catalyzed dehydrogenation of 1-(4-fluorophenyl)ethanol (4) by tetrafluorobenzoquinone (7) using the Shvo catalyst 1 at 70 degrees C show that the dehydrogenation by catalytic intermediate 2 is rate-determining with the rate = k[4][1](1/2) and with deltaH++ = 17.7 kcal mol(-1) and deltaS++ = -13.0 eu. The use of specifically deuterated derivative 4-CHOD and 4-CDOH gave individual isotope effects of k(CHOH)/k(CHOD) = 1.87 +/- 0.17 and k(CHOH)/k(CDOH) = 2.57 +/- 0.26, respectively. Dideuterated derivative 4-CDOD gave a combined isotope effect of k(CHOH)/k(CDOD) = 4.61 +/- 0.37. These isotope effects are consistent with a concerted transfer of both hydrogens of the alcohol to ruthenium species 2.     

Synthesis of a trihydroxylated azepane from d-arabinose by way of an intramolecular alkene nitrone cycloaddition

The synthesis of 1,6-imino-1,5,6-trideoxy-l-xylo-hexitol, a trihydroxylated azepane, from d-arabinose was achieved by way of an intramolecular alkene nitrone cycloaddition. The final product as well as its bicyclic precursor, (3R,4S,5S)-3,4-dihydroxy-8-oxa-1-azabicyclo[3.2.1]octane, were evaluated as glycosidase inhibitors.     

Efficient photocatalytic oxygenation of aromatic alkene to 1,2-dioxetane with oxygen via electron transfer

[reaction: see text] Photocatalytic oxygenation of tetraphenylethylene (TPE) with oxygen occurs efficiently via electron-transfer reactions of TPE and oxygen with a photogenerated electron transfer state of 9-mesityl-10-methylacridniium ion, followed by the radical-coupling reaction between TPE radical cation and O2*- to produce 1,2-dioxetane selectively. The further photocatalytic cleavage of the O-O bond of dioxetane affords benzophenone as the final oxygenated product.     

Catalytic enantioselective alkene aminohalogenation/cyclization involving atom transfer

Problem solved: the title reaction was used for the synthesis of chiral 2-bromo, chloro, and iodomethyl indolines and 2-iodomethyl pyrrolidines. Stereocenter formation is believed to occur by enantioselective cis?aminocupration and C-X bond formation is believed to occur by atom transfer. The ultility of the products as versatile synthetic intermediates was demonstrated, as was a radical cascade cyclization sequence.     

Reversible alkene extrusion from platinaoxetanes

Reversible alkene extrusion from platinaoxetanes has been observed. Catalytic amounts of electrophilic Pt complexes, HOTf, Ag+, or BF3 (best) result in dramatic rate enhancements.     

Carbon-sulfur bond formation via alkene addition to an oxidized ruthenium thiolate

Bulk oxidation of [Ru(DPPBT)3], 1b, (DPPBT = 2-diphenylphosphinobenzenethiolate) in the presence of ethylene yields [(ethane-1,2-diylbis(thio-2,1-phenylene)diphenylphosphine) ruthenium(II)] hexafluorophosphate, [2a]PF6, from the addition of the alkene across cis sulfur sites. During oxidation, the absorption bands of 1b at 540, 797, and 1041 nm decrease in intensity. The resulting complex [2a]+ displays a single redox couple at +804 mV. The +ESI-MS of [2a]+ shows a parent ion peak at m/z = 1009.1013, and the 31P NMR spectrum displays chemical shift values of delta(1) = 61.0, delta(2) = 40.3, and delta(3) = 37.5 with coupling constants of J(12) approximately J(13) approximately 30 Hz and J(23) = 304 Hz. Oxidation of [2a]+ by one electron at a holding potential of +1000 mV yields [(ethane-1,2-diylbis(thio-2,1-phenylene)diphenyl phosphine)ruthenium(III)] hexafluorophosphate, [2b][PF6]2. The EPR of [2b][PF6]2 displays a rhombic signal with g(1) = 2.09, g(2) = 2.04, and g(3) = 2.03. Oxidation of 1b in the presence of alkenes including 1-hexene, styrene, cyclohexene, and norbornene yields products similar to [2a]+. Each of these products was further oxidized to an analogue of [2b]2+. Complex [2a]+ was also prepared, as the bromide salt, from [PPN][Ru(DPPBT)3] (PPN [1a]; PPN = bis(triphenylphosphoranylidene)ammonium) and 1,2-dibromoethane. The complex [2a]Br crystallizes as thin yellow plates in the monoclinic space group P21/c with unit cell dimensions of a = 10.2565(9) A, b = 13.2338(12) A, c = 38.325(3) A, and beta = 93.3960(10) degrees.     

Mechanism of hydrogen transfer to imines from a hydroxycyclopentadienyl ruthenium hydride. Support for a stepwise mechanism

The negligible double kinetic deuterium isotope effect (k(HH)/k(DD)= 1.05) in the reaction where [2,3,4,5-Ph4(eta5-C4COH)Ru(CO)2H (2) transfers a hydride and a proton to N-phenyl-[1-(4-methoxyphenyl)ethylidene]amine (4) indicates that no bond to hydrogen is broken or formed in the rate-determining step.     

Fragmentation of carbonyl oxides by N-oxides: an improved approach to alkene ozonolysis

[Structure: see text] Ozonolysis of alkenes in the presence of amine N-oxides results in the direct formation of aldehydes. This reaction, which appears to involve an unprecedented trapping and fragmentation of the short-lived carbonyl oxide intermediates, avoids the hazards associated with generation and isolation of ozonides or other peroxide products.     

High-efficiency energy transfer from carotenoids to a phthalocyanine in an artificial photosynthetic antenna

Two carotenoid pigments have been linked as axial ligands to the central silicon atom of a phthalocyanine derivative, forming molecular triad 1. Laser flash studies on the femtosecond and picosecond time scales show that both the carotenoid S1 and S2 excited states act as donor states in 1, resulting in highly efficient singlet energy transfer from the carotenoids to the phthalocyanine. Triplet energy transfer in the opposite direction was also observed. In polar solvents efficient electron transfer from a carotenoid to the phthalocyanine excited singlet state yields a charge-separated state that recombines to the ground state of 1.     

Electrospun polystyrene nanofibers as a novel adsorbent to transfer an organic phase from an aqueous phase

The aim of this work is to develop a simple phase‐transfer method for dispersive liquid–liquid microextraction. For this purpose, a polystyrene nanofiber was prepared by a facile electrospinning strategy and used for the first time as an adsorbent to transfer the organic phase in dispersive liquid–liquid microextraction procedure. The fiber was characterized and its chemical stability and excellent hydrophobicity enable it to selectively adsorb the organic solvent in an aqueous sample. High porosity and specific surface area provide a large adsorption capacity. Under the optimal conditions, the developed dispersive liquid–liquid microextraction with high‐performance liquid chromatography method was successfully applied to the analysis of aldehydes in environmental water samples. The merits of this approach are that it is easy‐to‐operate, low‐cost, time‐saving, and has satisfactory sensitivity. It provides an alternative way for fast and convenient phase transfer of the hydrophobic organic solvent from the aqueous phase.     

Evidence for a reactive (alkene)peroxoiridium(III) intermediate in the oxidation of an alkene complex with O2

An (alkene)peroxoiridium(III) complex, [Ir(L)(cod)(O(2))] [where LH = PhN=C(NMe(2))NHPh and cod = 1,5-cyclooctadiene], was identified as an intermediate in the reaction of the Ir(I) precursor [Ir(L)(cod)] with O(2) and characterized by spectroscopic methods. Decay of the intermediate and further reaction with 1,5-cyclooctadiene produced 4-cycloocten-1-one.     

Facilitated electron transfer from an electrode to horseradish peroxidase in a biomembrane-like surfactant film

Direct electron transfer was found to be greatly facilitated for horseradish peroxidase (HRP) in a didodecyldimethylammonium bromide (DDAB) biomembrane-like film at a pyrolytic graphite (PG) electrode involving the Fe^III Fe^II couple. The heterogeneous electron transfer rate constant k_s was fitted as 9.0 s⁻¹ using the non-linear regression analysis of the square wave voltammograms at a series of frequencies and pulse heights. The pH dependence of the formal potential for HRP in DDAB film at medium pH environments suggested one-proton transfer coupled with a one-electron transfer reaction. Scanning electron microscopy (SEM) showed different film morphology for HRP and HRP---DDAB films. UV–vis and reflectance absorption infrared (RAIR) spectra inferred that the heme state of HRP in DDAB film was similar to that in its native state. Circular dichroism (CD) results indicated slight perturbation of DDAB on the second structure of HRP. Thus, the embedded HRP in the biomembrane-like DDAB film showed nearly native structural properties and improved electrochemical characteristics. This has potential value for the basic and applied bioelectrochemistry of enzymes.     

Mechanism of alkene isomerization by bifunctional ruthenium catalyst: A theoretical study

The molecular mechanism of the isomerization of 1-pentene to form (E)-2-pentene catalyzed by the bifunctional ruthenium catalyst has been investigated using density functional theory calculations. The reaction is likely to proceed through the following steps: 1) the β-H elimination to generate the ruthenium hydride intermediate; 2) the reductive elimination of the hydride intermediate to generate the nitrogen-protonated allyl intermediate; 3) the transportation of the hydrogen by the dihedral rotation with Ru–P bond acting as axis; 4) the oxidative addition to afford another hydride complex; 5) the reductive elimination of the hydride intermediate to form the C₂-C₃ π-coordinated agostic intermediate; 6) the coordination of the nitrogen to the ruthenium center to give the final product. The rate-determining step is the oxidative addition step (the process of the hydrogen moves to ruthenium center from the nitrogen atom) with the free energy of 31.2 kcal/mol in the acetone solvent. And the N-heterocyclic ligand in the catalyst mainly functions in the two aspects: affords an important internal-basic center (nitrogen atom) and works as a transporter of hydrogen. Our results would be helpful for experimentalists to design more effective bifunctional catalysts for isomerization of a variety of heterofunctionalized alkene derivatives.