Alkali metal promoted transfer of dimethylsilylene from dodecamethylcyclohexasilane to silanes and dienes

Addition of sodium/potassium alloy to THF solutions containing dodecamethylcyclohexasilane and a diene or a silane, affords silacyclopentenes and disilanes, respectively.     

Photolysis of dodecamethylcyclohexasilane generation of dimethylsilylene and some of its insertion reactions

The UV photolysis of dodecamethylcyclohexasilane (I) in cyclohexane at ca. 45° proceeds readily will loss of dimethylsilylene species to give two of the lower homologs, viz, decamethylcyclopentasilane and octamethylcyclotetrasilane. Prolonged photolysis leads to the formation of a significant amount of 1,4- dihydrohexamethyltetrasilane, which can be ascribed to homolytic scission of the silicon-silicon bond of the cyclotetrasilane. The photochemically generated dimethylsilylene species readily insert into siliconhydrogen or siliconmethoxy bonds of appropriate monosilane derivatives. However, photolysis of (I) in dimethyldichlorosilane affords several products whose formation may be best explained in terms of a direct reaction of photochemically excited polysilanes with chlorosilanes. Other evidence for this comes from the photolysis of tetradecamethylhexasilane in dimethyldichlorosilane.     

Reactions of dodecamethylcyclohexasilane and polydimethylsilane with metal chlorides

The reactions of dodecamethylcyclohexasilane and high-molecular-weight polydimethylsilane with chlorides of I, II, IV-VI and VIII Group metals at high temperature in the absence of a solvent were studied. The interaction of (Me₂Si)₆ with metal chlorides proceeds with the cleavage of SiSi and SiC bonds with the formation of chloro derivatives of linear and cyclic permethyloligosilanes. The reactions of polydimethylsilane with metal chlorides afford mixtures of α,ω-dichlorooligosilanes, Cl(Me₂Si)_nCl (n=2-9). The influence of the reaction conditions (temperature, reaction time and the reagent ratio) on the composition and yields of the reaction products was examined.     

Light-emitting charge transfer species promoted by metal ion coordination

The coordinative interaction between zinc(II) and naphthalene-labeled diiminopyridine ligands induces the formation of light-emitting intracomplex charge transfer species.     

Tetrathiafulvalene-flavin dyads: electron transfer promoted by metal cations

Tetrathiafulvalene-flavin dyads 1 and 2 are reported. Both absorption and ESR spectral studies show that the intramolecular electron transfer occurs from TTF to flavin units in dyads 1 and 2 in the presence of Pb²⁺/Sc³⁺. But, the electron transfer is more efficient for dyad 1 in the presence of Pb²⁺/Sc³⁺. Electrochemical studies manifest that coordination of dyads 1 and 2 with Pb²⁺/Sc³⁺ play an important role in facilitating the electron transfer within dyads 1 and 2.     

Transition metal halide promoted hydride transfer in N,N-diisoalkyl-N-propargylamines

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Alkali metal additions to unsaturated systems

Treatment of tetrakis(dimethoxyboryl)methane, C[B(OMe)₂]₄, with lithium methoxide or butyllithium followed by triphenyltin chloride yields (triphenylstannyl)tris(dimethoxyboryl)methane, Ph₃SnC[B(OMe)₂]₃, which on further treatment with butyllithium disproportionates to form bis(triphenylstannyl)bis(dimethoxyboryl)methane, (Ph₃Sn)₂C[B(OMe)₂]₂. The analogous monolead compound proved too base-sensitive to isolate, and only bis(triphenylplumbyl)bis(dimethoxyboryl)methane, (Ph₃Pb)₂C[B(OMe)₂]₂, was obtained from C[B(OMe)₂]₄, BuLi, and Ph₃PbCl. A brief study of protodeboronation and other reactions of these compounds has been carried out.     

Transition metal free transfer hydrogenation of ketones promoted by 1,3-diarylimidazolium salts and KOH

An efficient transition metal free and greener catalytic system was developed for the selective transfer hydrogenation of saturated ketones to alcohols. This was achieved by the use of 1,3-diarylimidazolium salts in the presence of KOH as a promoter for the reaction. When the range of substrates was expanded to include unsaturated ketones, selective reduction of the double bond occurred. The catalyst efficiency was comparable to some established transition metal catalyzed systems. The current system utilizes mild aerobic reaction conditions compared to the inert atmosphere conditions required for the corresponding metal based systems.     

相转移条件下过渡金属促进有机反应的一些新进展

本文综述了近年来，在相转移条件下，过渡金属催化氧化、还原、羰基化以及偶合反应及其在有机合成中应用新进展。     

Oxy-function promoted hydroboration of conjugated dienes with controlled allylic borane rearrangement

Hydroboration of conjugated dienes is promoted by the hydroxy and methoxy groups, which also control the rearrangement of the initially produced allylic boranes.     

Vibrationally promoted electron emission from low work-function metal surfaces

We observe electron emission when vibrationally excited NO molecules with vibrational state v, in the range of 9 < or = v < or =18, are scattered from a Cs-dosed Au surface. The quantum efficiency increases strongly with v, increasing up to 10(-2) electrons per NO (v) collision, a value several orders of magnitude larger than that observed in experiments with similar molecules in the ground vibrational state. The electron emission signal, as a function of v, has a threshold where the vibrational excitation energy slightly exceeds the surface work function. This threshold behavior strongly suggests that we are observing the direct conversion of NO vibrational energy into electron kinetic energy. Several potential mechanisms for the observed electron emission are explored, including (1) vibrational autodetachment, (2) an Auger-type two-electron process, and (3) vibrationally promoted dissociation. The results of this work provide direct evidence for nonadiabatic energy-transfer events associated with large amplitude vibrational motion at metal surfaces.     

Alkali metal reductions of organic molecules: why mediated electron transfer from lithium is faster than direct reduction

Lithium metal reductions are widely employed in organic synthesis, where it is common to employ a "mediator" to speed up the electron transfer kinetics. We present experimental data for the electrode kinetics of the reduction of the most common mediator, 4,4'-di-tert-butyl-1,1'-biphenyl (DBB) in tetrahydrofuran (THF) over a range of temperatures. Using corresponding data for the oxidation of lithium we present quantitative estimates of the kinetic advantage for the use of DBB as a mediator in lithium reductions, over, in particular, direct reduction using lithium metal.     

Cycloreversion of acylquadricyclane to acylnorbornadiene promoted by metal oxides

The very mild, stable, immobilized, and inexpensive metal oxides effectively catalyzed the cycloreversion of methyl 2,4-dimethyl 6,7-diphenyl-5-propionyltetracyclo[3.2.0.0^2,−7.04,6] heptan-1-carboxylate (1).     

Titanium promoted reduction of imines with Grignards,silanes, and zinc: identification of a new mechanism with silanes

Aldimines react with reducing agents, such as Grignards, phenylsilane or zinc in the presence of titanium(IV) isopropoxide to form amines and reductively coupled imines (diamines). Using deuterium labeled reagents, the mechanism of reduction to form amines is described. Reducing agents, such as the Grignard and zinc result in the formation of low valent titanium (LVT), which in turn reduces the imine. On the other hand, phenylsilane reacts by a distinctly different mechanism and where a hydrogen atom from silicon is directly transferred to the titanium coordinated imine.     

Highly stereo- and regioselective ni-catalyzed homoallylation of aldimines with conjugated dienes promoted by diethylzinc

Under the nickel catalysis, in one pot, diethylzinc serves as a reducing agent to connect the three components of 1,3-dienes, alkyl or aryl aldehydes, and anilines to furnish 3-substituted 3,5-syn-5-(arylamino)pent-1-enes 1, the homoallylation products of aldimines with dienes, in excellent yields and with high regio- and stereoselectivity.     

A second transition state for chain transfer to monomer in olefin polymerization promoted by group 4 metal catalysts

beta-Hydrogen transfer (BHT) to monomer is the dominant chain termination pathway for olefin polymerization promoted by group 4 metal catalysts. The transition state (TSA) for BHT studied in earlier work is characterized by a strong metal-hydrogen interaction. Our theoretical study of a series of homogeneous single-site polymerization catalysts reveals the existence of a second transition state (TSC), competitive with TSA, which has no direct metal-hydrogen interaction and strongly resembles that for the main-group metal aluminum. The balance between the two reaction paths is sensitive to choice of metal and ligand structure.