- Direct cyclopropanation of 1-alkynylphosphonates by Cp2ZrCl 2/2EtMgBr/2AlCl3 to afford cyclopropylmethylphosphonates
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The reagent system Cp2ZrCl2/2EtMgBr/2AlCl3 converts 1-alkynylphosphonates into cyclopropylmethylphosphonates 3 in good isolated yields. Ethers, chlorides, and other cyclopropyl groups are compatible with the reaction condi
- Quntar, Abed Al Aziz,Srebnik, Morris
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- Selective cyclometalation of disubstituted acetylenes and ethylene with diethylmagnesium and ethylmagnesium halides in the presence of zirconium complexes
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Catalytic cyclometalation of disubstituted acetylenes and ethylene with ethylmagnesium halides EtMgHlg (Hlg = Cl, Br) and diethylmagnesium Et 2Mg in the presence of Cp2ZrCl2 gave tetrasubstituted magnesacyclopenta-2,4-dienes and disubstituted magnesacyclopent-2-enes. A probable scheme of formation of cyclic unsaturated organomagnesium compounds was proposed, according to which the reactive intermediates in the cyclometalation process are zirconacyclopentadienes and zirconacyclopentenes generated from Cp2ZrCl2, EtMgHlg, Et2Mg, acetylenes, and ethylene.
- Sultanov,Vasil'Ev,Dzhemilev
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- Ruthenium(II)-catalyzed olefination: Via carbonyl reductive cross-coupling
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Natural availability of carbonyl groups offers reductive carbonyl coupling tremendous synthetic potential for efficient olefin synthesis, yet the catalytic carbonyl cross-coupling remains largely elusive. We report herein such a reaction, mediated by hydrazine under ruthenium(ii) catalysis. This method enables facile and selective cross-couplings of two unsymmetrical carbonyl compounds in either an intermolecular or intramolecular fashion. Moreover, this chemistry accommodates a variety of substrates, proceeds under mild reaction conditions with good functional group tolerance, and generates stoichiometric benign byproducts. Importantly, the coexistence of KOtBu and bidentate phosphine dmpe is vital to this transformation.
- Wei, Wei,Dai, Xi-Jie,Wang, Haining,Li, Chenchen,Yang, Xiaobo,Li, Chao-Jun
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p. 8193 - 8197
(2017/11/27)
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- Stereoselective synthesis of vicinal dilithioalkenes by addition of lithium metal, to carbon-carbon triple bonds
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The reaction of various cyclic and acyclic alkynes with lithium dust (2% sodium) to form vicinal dilithioalkenes has been investigated. Aliphatic alkynes, e.g. 3-hexyne (27a), exclusively afford the corresponding (E)-dilithioalkenes, insoluble solids which are stable at room temperature and allow access to a variety of tetrasubstituted olefins in acceptable yields.
- Maercker,Girreser
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p. 8019 - 8034
(2007/10/02)
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- Pair-Selective Coupling of Alkynes with Alkenes on Zirconocene Complex
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When ethylene and alkynes such as 4-octyne and diphenylacetylene were treated with Cp2ZrBu2, highly pair selective coupling products were formed in high yields.Similarly, styrene or trimethylvinylsilane also afforded cross coupling products with alkynes on zirconocene complex.
- Takahashi, Tamotsu,Xi, Zhenfeng,Rousset, Christophe J.,Suzuki, Noriyuki
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p. 1001 - 1004
(2007/10/02)
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- Outer-sphere and inner-sphere processes in reductive elimination. Direct and indirect electrochemical reduction of vicinal dibromoalkanes
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The reduction of vicinal dibromoalkanes is investigated as an example of the dichotomy between outer-sphere and inner-sphere processes in reductive elimination. As a result from the analysis of the kinetic data, outer- sphere reagents such as carbon electrodes and aromatic anion radicals react with vicinal dibromoalkanes according to an "ET" mechanism in which the rate-determining step is a concerted electron- transfer bond-breaking reaction leading to the β-bromoalkyl radical. The latter is then reduced very rapidly, in a second step, most probably along another concerted electro- transfer bond-breaking pathway leading directly to the olefin in the heterogeneous case and through halogen atom expulsion in the homogeneous case. In the absence of steric constraints, the reduction goes entirely through the antiperiplanar conformer because the resulting β-bromoalkyl radical is then stabilized by delocalization of the unpaired electron over the C-C-X framework due to a favorable interaction between the pz orbital of the radical carbon and the σ* orbital of the C-Br bond. This interaction is enhanced by alkyl substitution at the reacting carbons, resulting in an approximately linear correlation between the reduction potential and the C-X bond energy of OlX2 on one hand and the vertical ionization potential of the olefin on the other. The stabilization energy is of the order of 0.2-0.3 eV for the anti conformers. It can also be taken as a measure of the rotation barrier around the C-C bond responsible for the loss of stereospecificity in the reduction. This competes with the reduction of the two stable conformers of the OlX* radicals and for the expulsion of the halogen atom. There is a remarkably good agreement between the ensuing prediction of the E:Z olefin ratio that should be found upon reduction of threo and erythro OlX2 isomers by outer-sphere reagents such as aromatic anion radicals and the experimental data. Although members of perfectly reversible redox couples, iron(I), iron("0"), and cobalt(I) porphyrins offer typical examples of inner-sphere reagents in their reaction with vicinal dibromoalkanes. They indeed react much more rapidly than outer-sphere electron donors (aromatic anion radicals) of the same standard potential. On the basis of steric hindrance experiments, it was shown that they do not react according to an SN2 rate-determining step involving the transient formation of an organometallic species. Complete stereospecificity is obtained, showing that they react along a halonium transfer E2 elimination mechanism rather than by an E1 elimination or a halogen atom transfer mechanism. As shown on a quantitative basis, this is related to the large driving force offered to halonium abstraction by the strong affinity of the iron(III) and cobalt (III) complexes toward halide ions. In regards to catalysis, the investigated systems provide typical examples showing the superiority of inner-sphere (chemical) catalysis over outer-sphere (redox) catalysis of electrochemical reactions. Not only is the catalytic efficiency much better since the rate constants of the key steps are larger, given the standard potential of the catalyst, but also selectivity is dramatically improved.
- Lexa, Doris,Savéant, Jean-Michel,Sch?fer, Hans J.,Su, Khac-Binh,Vering, Birgit,Wang, Dan Li
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p. 6162 - 6177
(2007/10/02)
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- Direct and Regioselective Transformation of α-Chloro Carbonyl Compounds into Alkenes and Deuterioalkenes
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The successive treatment ethyl chloroacetate or chloroacetyl chloride with Grignard reagents and lithium powder leads to symmetrical terminal olefins in a regioselective manner.The best results are obtained with acid chlorides.The influence of the temperature and the reaction time on overall yield of the process are studied; in general, yields are increased by working at low temperature (-60 deg C).Internally substituted olefins are obtained from α-chloro acid chlorides through a similar process.The treatment of α-chloro aldehydes, ketones and carboxylic acid derivatives (esters or acid chlorides) with lithium aluminium hydride or lithium aluminium hydride/aluminium chloride and lithium powder at low temperature (-60 deg C) leads in a regioselective manner to olefins with the same carbon skeleton as the starting carbonyl compound.Reactions with lithium aluminium deuteride lead to incorporation of deuterium at predetermined positions in the alkene.
- Barluenga, Jose,Yus, Miguel,Concellon, Jose M.,Bernad, Pablo
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p. 2721 - 2726
(2007/10/02)
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