- Geometric E→Z Isomerisation of Alkenyl Silanes by Selective Energy Transfer Catalysis: Stereodivergent Synthesis of Triarylethylenes via a Formal anti-Metallometallation
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An efficient geometrical E→Z isomerisation of alkenyl silanes is disclosed via selective energy transfer using an inexpensive organic sensitiser. Characterised by operational simplicity, short reaction times (2 h), and broad substrate tolerance, the reaction displays high selectivity for trisubstituted systems (Z/E up to 95:5). In contrast to thermal activation, directionality results from deconjugation of the π-system in the Z-isomer due to A1,3-strain thereby inhibiting re-activation. The structural importance of the β-substituent logically prompted an investigation of mixed bis-nucleophiles (Si, Sn, B). These versatile linchpins also undergo facile isomerisation, thereby enabling a formal anti-metallometallation. Mechanistic interrogation, supported by a theoretical investigation, is disclosed together with application of the products to the stereospecific synthesis of biologically relevant target structures.
- Fa?bender, Svenja I.,Molloy, John J.,Mück-Lichtenfeld, Christian,Gilmour, Ryan
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supporting information
p. 18619 - 18626
(2019/11/16)
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- Yttrium-mediated conversion of vinyl grignard reagent to a 1,2-dimetalated ethane and its synthetic application
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A new 1,2-dimetalated ethane was generated from a vinyl Grignard reagent in the presence of yttrium(III) chloride, sodium cyclopentadienide, and diisobutylaluminum hydride (Dibal). This reagent enables the regio- and stereoselective (metaloethyl)metalation of acetylenes to produce ethylated alkenes after hydrolysis. In addition to simple hydrolysis, further carbon-carbon bond formation or functionalization at the metaloethyl group is possible, demonstrating that the yttrium-based 1,2-dimetalated ethane works as a conjunctive reagent. In addition, the reactive dicarbanionic character of this reagent effects one-pot synthesis of cyclopronanols from esters via consecutive double carbonyl addition. Copyright
- Tanaka, Ryoichi,Sanjiki, Hiroaki,Urabe, Hirokazu
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p. 2904 - 2905
(2008/09/19)
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- A Nickel-catalyzed carbozincation of aryl-substituted alkynes
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The addition of dialkylzincs or diphenylzinc to substituted phenylacetylenes in the presence of catalytic amounts of Ni(acac)2 in THF:NMP mixtures produces syn-carbozincation products with good to excellent regio- and stereoselectivity. After quenching with an electrophile (iodine, acyl chloride, allyl bromide) tetrasubstituted olefines are obtained in good to satisfactory yields. An intramolecular version of the reaction is possible using a terminal triple bond bearing an iodine at a remote position. More substituted iodo-alkynes furnish only reductive elimination products. An application to a stereoselective synthesis of (Z)-tamoxifen (Z:E > 99:1) has been developed.
- Stuedemann, Thomas,Ibrahim-Ouali, Malika,Knochel, Paul
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p. 1299 - 1316
(2007/10/03)
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- The Effect of the Ring Size of the Dithiolato Leaving Group on the Orientation of β-Elimination in the Nickel-catalysed Alkenation of Cyclic Dithioacetals with Me3SiCH2MgCl
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The product distribution (allyl- vs. vinyl-silanes) of the reactions of dithioacetals derived from alkyl aryl ketones with Me3SiCH2MgCl surprisingly depends on ring size of the cyclic dithioacetals, the five-membered ring substrates affording allylsilane as the major product while the six-membered analogues yield vinylsilanes predominantly.These results indicate that the leaving group, dithiolato moiety, may remain coordinated to the metal centre during the course of the catalytic process.
- Wong, Ken-Tsung,Ni, Zhi-Jie,Luh, Tien-Yau
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p. 3113 - 3116
(2007/10/02)
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- Regiospecific and Stereospecific Synthesis of E- and Z-Trisubstituted Alkenes via 2,2-Disubstituted Vinylsilanes
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Treatment of terminal alkynes 1 with the organocopper reagents derived from Grignard reagents, cuprous iodide, and lithium bromide (molar ratio 2:1:2) at low temperature followed by the addition of chlorotrimethylsilane gave the 2,2-disubstituted vinylsilanes 2 with complete regio- and stereospecificity (syn addition).Electrophilic substitution of 2 with ICl, Br2, and acetyl chloride gave the corresponding vinyl iodides, bromides, and α,β-unsaturated ketones 5 with retention of configuration.Epoxidation of 2 with MCPBA gave the epoxy silanes 6, which upon treatment with concentrated HX and BF3*Et2O gave the vinyl halides 7 with net inversion of configuration.If the epoxy silanes 6 were first converted to the β-hydroxy silanes 8 by Gilman's reagents, either E- or Z-trisubstituted alkenes 9 and 10 could be obtained by treatment with acid or base.Vinyl halides 7 could also be stereoselectively converted to other functionalities via the vinyllithium intermediates.
- Chou, Shang-Shing P.,Kuo, Hwei-Lon,Wang, Chung-Jen,Tsai, Chung-Ying,Sun, Chung-Ming
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p. 868 - 872
(2007/10/02)
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- Formation and Reactions of Olefins with Vicinal Silyl and Stannyl Substituents
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The silicon-tin bond in Me3SiSnR3 (R=Me, n-Bu) adds regio- and stereospecifically to 1-alkynes and also to a limited number of nonterminal alkynes when Pd(PPh3)4 is added as a catalyst.The use of the (Z)-silylstannylalkenes thus formed in synthesis either
- Mitchell, T. N.,Wickenkamp, R.,Amamria, A.,Dicke, R.,Schneider, U.
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p. 4868 - 4874
(2007/10/02)
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- Dihexylmagnesium Promoted Carboalumination of Silylacetylenes
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The bimetallic species, n-Bu2Mg.2Et3Al has been found to be effective for the carboalumination of silylacetylenes.
- Hayami, Hiroshi,Oshima, Koichiro,Nozaki, Hitosi
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p. 4433 - 4436
(2007/10/02)
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