- Reactivity of Benzylic Carbanions. 10. Rearrangement of the Me3SI Group in 9,9-Bis(trimethylsilyl)- and 9-Alkyl-9-(trimethylsilyl)-10-lithio-9,10-dihydroanthracenes
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A Me3Si migration has been shown to occur in 9,9-bis(trimethylsilyl)-10-lithio-9,10-dihydroanthracene (C1); the resulting carbanion (C2), protonated by H2O, gives rise exclusively to the cis-9,10-bis(trimethylsilyl)-9,10-dihydroanthracene ((Me3Si)2DHA) in high yield.The same migration was also observed in a series of 9-(trimethylsilyl)-9-alkyl-10-lithio-9,10-dihydroanthracenes (alkyl = Me, Et, i-Pr), which were transformed into a mixture of cis and trans 9,10-disubstituted DHAs (VII, VIII, IX).Experimental evidence was obtained for an intramolecular shift, and the mechanism of the reaction was considered, particularly the possibility of a reversible Me3Si migration (C2 -> C1).The driving force of the Me3Si shift is believed to involve easy formation of pentacoordinated silicon derivatives combined with relief of strain in, and formation of, more stable ion pairs.In light of these results, the rearrangements observed in 9-(trimethylsilyl)-9-deuterio-10-lithio-9,10-dihydroanthracenes were reexamined.A comparison with other well-known R3Si migrations in carbanions, mainly in systems containing heteroatoms, was made, and the structural requirements to observe such reactions in genuine carbanions were delineated; the DHA substrate appears to be well suited for this rearrangement.
- Daney, Michele,Lapouyade, Rene,Bouas-Laurent, Henri
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- Nucleophilic displacement versus electron transfer in the reactions of alkyl chlorosilanes with electrogenerated aromatic anion radicals
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Anion radicals of a series of aromatic compounds (C6H5CN, C6H5COOEt, anthracene, 9,10-dimethyl-, 9,10-diphenyl-and 9-phenylanthracene, pyrene and naphthalene) react with trialkyl chlorosilanes R1R2R3SiCl (R1-3 = Me, Et; R1,2 = Me, R3 = t-Bu) in multiple ways, following classical bimolecular schemes. The ratio of one-electron transfer (ET) to a two-electron process (SN2-like nucleophilic attack of the reduced form of mediator on the chlorosilane, with k2 ? 102-108 M-1 s-1) is inversely related to the steric availability of Si for nucleophilic displacement reactions. The nucleophilic substitution pathway mainly results in mono-and disilylated aromatic products. Paralleling the electrochemical data with DFT calculations, the role of silicophilic solvent (DMF) in SN process was shown to be quite complex because of its involvement into coordination extension at silicon, dynamically modifying energetics of the process along the reaction coordinate. Although 2,2'-bipyridine also forms delocalized persistent anion radicals, they do not induce neither ET nor SN reactions in the same manner as aromatic mediators. Silicophilicity of 2,2'-bipyridine being superior to that of DMF, a R3SiCl·bipy complex of hypercoordinated silicon with electroactive ligand was formed instead, whose reduction requires about 1 V less negative potentials than bipyridine itself.
- Soualmi, Saida,Dieng, Mamadou,Ourari, Ali,Gningue-Sall, Diariatou,Jouikov, Viatcheslav
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p. 457 - 469
(2015/03/04)
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- Silylated cyclohexadienes as new radical chain reducing reagents: Preparative and mechanistic aspects
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Various silylated 1,4-cyclohexadienes are presented as superior tin hydride substitutes for the conduction of various radical chain reductions. Debrominations, deiodinations, and deselenations can be performed using these environmentally benign reagents. Furthermore, Barton - McCombie-type deoxygenations using silylated cyclohexadienes are described. Radical cyclizations, ring expansions, and Giesetype addition reactions with the new tin hydride substitutes are presented. The polymerization of styrene can be regulated using silylated cyclohexadienes. Rate constants for hydrogen atom abstraction from two 1-silyl-cyclohexadienes by primary C-radicals were determined. The effects of the cyclohexadiene substituents on the reaction outcomes are discussed. Finally, qualitative EPR experiments on silyl radical expulsion from silylated cyclohexadienyl radicals are presented.
- Studer, Armido,Amrein, Stephan,Schleth, Florian,Schulte, Tobias,Walton, John C.
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p. 5726 - 5733
(2007/10/03)
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- A Convenient Synthesis of 9,9-Dialkyl-9,10-dihydroanthracenes and 10,10-Dialkylanthrones: Silicon-Mediated Regioselective Dialkylation of 9,10-Dihydroanthracene
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Described is a short and convenient approach to the synthesis of 9,9-dialkyl-9,10-dihydroanthracenes, 9,9,10-trialkyl-9,10-dihydroanthracenes, and 10,10-dialkylanthrones, some of which are otherwise unknown or inaccessible by conventional methods.Deprotonation of 9-(trimethylsilyl)-9,10-dihydroanthracene (2; 9-(trimethylsilyl)-9,10-DHA) followed by reaction with alkyl halides (RX) produces 9-alkyl-9-(trimethylsilyl)-9,10-DHAs 3-7 in 80-90percent yields.Treatment of 3-7 with n-BuLi produces the 10-lithio derivatives that rearrange to 9-alkyl-9-lithio-10-(trimethylsilyl) intermediates; subsequent alkylation with RX generates 9,9-dialkyl-10-(trimethylsilyl)-9,10-DHAs 8-19.Formation of single stereoisomers 13-19 was suggested by NMR and confirmed in two cases, 15 and 16, by X-ray structure determination.The trimethylsilyl group is removed by tetrabutylammonium fluoride (TBAF) to provide 9,9-dialkyl-9,10-DHAs 20-29 with impressive yields.Oxidation of either the 9,9-dialkyl-9,10-DHAs or 9,9-dialkyl-10-(trimethylsilyl)-9,10-DHAs with Cr(VI) oxidant furnished 10,10-dialkylanthrones 36-41 in 80-90percent yields.
- Dhar, Raj K.,Clawson, D. K.,Fronczek, Frank R.,Rabideau, Peter W.
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p. 2917 - 2921
(2007/10/02)
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- Silicon Mediated Alkylations in the 9,10-Dihydroanthracene System: A Convenient Synthesis of 9,9-Dialkyl-9,10- Dihydroanthracenes
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A trimethylsilyl (TMS) substituent is used to control the regiochemistry of alkylation in 9,10-dihydroanthracene (9,10-DHA) furnishing 9,9-dialkyl-10-TMS-9,10-DHAs. The TMS group is subsequently removed resulting in the first convenient synthesis of a var
- Rabideau, Peter W.,Dhar,Clawson,Zhan, Zhengyun
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p. 3969 - 3972
(2007/10/02)
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- SILICON-MODIFIED BIRCH REDUCTION AND REDUCTIVE ALKYLATION OF POLYNUCLEAR AROMATICS
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A trimethylsilyl substituent is used to control regiochemistry, overreduction, and prevent bond cleavage during the metal/ammonia reduction of aromatic and polynuclear aromatic compounds.The trimethylsilyl group is then removed by tetrabutylammonium fluoride and replaced by either hydrogen or primary alkyl, the latter case representing overall reductive alkylation.Results are presented for naphtalene together with its 1-methyl, 2-methyl and 2-methoxy derivatives, phenanthrene and its 9-methyl and 9-ethyl derivatives, biphenyl and triptycene.
- Marcinow, Z.,Clawson, D. K.,Rabideau, P. W.
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p. 5441 - 5448
(2007/10/02)
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