- Novel organoborane Lewis acids via selective boron-tin exchange processes - Steric constraints to electrophilic initiation by the boron halide
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With the purpose of preparing novel mono- and bidentate organoboron Lewis acids, the scope and limitations of synthesizing the requisite organoboranes by the boron-tin exchange between a boron halide and the appropriate organostannane have been examined in detail. The following organotin derivatives have been obtained either from the corresponding RMgBr or RLi reagent and MenSnCl4-n or from a Barbier procedure using the organic halide, Me3SnCl and magnesium metal: 1,2-bis(trimethylstannyl)ethyne, o-, m-, and p-bis(trimethylstannyl)benzenes, α,o-bis(trimethylstannyl)toluene, α,α-bis(trimethylstannyl)-o-xylene, and 2,2-dimethyl-2-stannaindane. The individual interaction of the 1,2-bis(trimethylstannyl)ethyne and the isomeric bis(trimethylstannyl)benzenes with Et2BBr produced the corresponding bis(diethylboryl)-derivatives. By contrast, with Et2BCl the α,o-bis(trimethylstannyl)toluene gave only o-diethylboryl-α-trimethylstannyltoluene and with BCl3 the α,α′-bis(trimethylstannyl)-o-xylene formed only α,α′-bis-(chlorodimethylstannyl)-o-xylene. Furthermore, in the attempted double boron-tin exchange between o-bis(trimethylstannyl)benzene and BCl3, an unprecedented rearrangement of the 1-(dichloroboryl)-2-(trimethylstannyl)benzene intermediate into its 1-[chloro(methyl)boryl]-2-(chlorodimethylstannyl) isomer was observed. Likewise, o-bis(trimethylstannyl)benzene with PhBCl2 produced by a similar rearrangement 1-[methyl(phenyl)boryl]-2-(chloro-dimethylstannyl)benzene. The thermolysis of such boranes led variously to definite dimers or ill-defined oligomers. Preliminary studies of the properties of these organoboranes have identified the heightened Lewis acidity of 1,2-bis(diethylboryl)ethyne and the π-electron delocalization involving the 2pΖ-boron orbitals in the 9,10-dihydro-9,10-diboraanthracene system. Finally, an electronic mechanism for the boron-tin exchange has been developed to account for the selectivity of the boron halide's attack at unsaturated carbon-tin bonds.
- Eisch, John J.,Kotowicz, Boguslaw W.
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p. 761 - 769
(2007/10/03)
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- Di(lithiomethyl)benzenes from di(chloromethyl)benzenes by a DTBB-catalysed lithiation under Barbier-type conditions
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The reaction of 1,2-, 1,3- and 1,4-di(chloromethyl)benzenes (1a-c) and different electrophiles [Pr(i)CHO, Bu(t)CHO, PhCHO, (CH2)5CO, PhCOMe, Me3SiCl] with an excess of lithium powder and a catalytic amount of DTBB (4 mol%)
- Gomez, Cecilia,Huerta, Fernando F.,Yus, Miguel
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p. 13897 - 13904
(2007/10/03)
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- Ni(0)-Catalyzed Cross Coupling of Aryl O-Carbamates and Aryl Triflates with Grignard Reagents. Directed Ortho Metalation-Aligned Synthetic Methods for Polysubstituted Aromatics via a 1,2-Dipole Equivalent
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The first Ni(0)-catalyzed cross-coupling reactions of aryl O-carbamates and aryl triflates with Grignard reagents (Scheme I) to give diversely polysubstituted aromatics 2d and 2e (Table I) which feature regiospecificity based on directed ortho metalation (carbamate), minimal β-hydride elimination (triflate), and dependence on steric and electronic effects are described.
- Sengupta, Saumitra,Leite, Magda,Raslan, Delio Soares,Quesnelle, Claude,Snieckus, Victor
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p. 4066 - 4068
(2007/10/02)
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- The Metallation-Elimination Reaction, II. Monocyclic Anions and Polyanions
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Equimolar mixtures of n-butyllithium and potassium tert-amyloxide effect metallation and metal hydride elimination of cyclic olefins directly to give conjugated anions and polyanions in a single operation. 1-Methyl-1-cycloalkenes or methylenecycloalkanes are particularly well suited and give highly unsaturated products.Odd-membered rings eliminate better than their even-membered counterparts, and eight-membered rings eliminate particularly slowly.Reactions with methyl iodide and trimethylsilyl chloride proceed smoothly in high yield.
- Wilhelm, Dieter,Clark, Timothy,Friedl, Thomas,Schleyer, Paul von Rague
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p. 751 - 760
(2007/10/02)
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- Crystalline Alkyl-lithiums from α-Trimethylsilyl-substituted o-Xylenes; X-Ray Crystal Structure of 2> (tmeda = Me2NCH2CH2NMe2)
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Reaction of (R, R', R'', R''' = H or SiMe3) with LiBun in n-hexane in the presence of Me2NCH2CH2NMe2 (tmeda) or MeN(CH2CH2NMe2)2 (pmdeta) results in selective lithiation of the α-carbon in a manner dependent on the na
- Lappert, Michael F.,Raston, Colin L.,Skelton, Brian W.,White, Allan H.
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- Delocalized Dicarbanions and Higher Delocalized Carbanions
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Simple routes to four new dianions with high resonance energy per atom (REPA) and improved preparations for several other di- and trianions are given.REPA is calculated for delocalized dicarbanions and higher delocalized carbanions which have been prepare
- Bates, Robert B.,Hess, B. Andes,Ogle, Craig A.,Schaad, L.J.
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p. 5052 - 5058
(2007/10/02)
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