- Weakly nucleophilic conjugate bases of superacids as powerful nucleophiles in vinylic bimolecular nucleophilic substitutions of simple β- alkylvinyl(aryl)-λ3-bromanes
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We report herein, for the first time, the stereoselective synthesis of simple (E)-β-alkylvinyl(aryl)-λ3-bromanes via a boron-λ3-bromane exchange reaction and their unique bimolecular nucleophilic substitutions at the vinylic ipso carbon atom under mild conditions. Interestingly, even weakly nucleophilic anions such as conjugate bases of superacids (HBF4, TfOH, Tf2CH 2, Tf3CH, Tf2NH, etc.) function as nucleophiles toward the vinyl-λ3-bromanes. For instance, the vinylic SN2 reaction of (E)-vinyl-λ3-bromanes with potassium bis(triflyl)methanide stereoselectively produced (Z)-vinyloxy oxosulfonium ylides with exclusive inversion of configuration via oxygen attack, while that with potassium bis(triflyl)imide afforded predominantly (Z)-vinyloxysulfoximines. In marked contrast, (E)-β-alkylvinyl- λ3-iodanes do not undergo the vinylic SN2 reaction with these conjugate bases of superacids. The differences between the nucleofugalities of aryl-λ3-iodanyl and aryl- λ3-bromanyl groups (the latter being greater) probably play a pivotal role in these unique reactions.
- Ochiai, Masahito,Okubo, Takuji,Miyamoto, Kazunori
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p. 3342 - 3344
(2011/05/04)
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- Claisen rearrangements based on vinyl fluorides
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2-Fluoroalk-1-en-3-ols (4), available from terminal alkenes (1) by bromofluorination, subsequent dehydrobromination of the 1-bromo-2-fluoroalkanes (2) to form 2-fluoroalkenes (3) and selenium dioxide mediated allylic oxidation with tert-butylhydroperoxide, undergo Johnson-Claisen rearrangement on treatment with trimethyl orthoacetate to give methyl 4-fluoroalk-4-enoates (7) in high yields. In contrast Ireland-Claisen rearrangement of 3-acetoxy-2-fluorodec-1-ene (9b) with triethylamine and TMSOTf in ether failed. Instead of the expected formation of a carboxylic acid, selective C-silylation of the α-position to the carboxyl group to form 14 occurred. However, Ireland-Claisen rearrangement was successful with corresponding chloroacetates 10 and propionates 11 of four 2-fluoroalk-1-en-3-ols (4) to give 2-chloro-4-fluoroalk- 4-enecarboxylic acids (15) or its 2-methyl derivatives 16, respectively, in moderate yields. These [3,3]-sigmatropic rearrangements are diastereoselective giving trans-configured double bonds, exclusively. Corresponding esters derived from (Z)-2-fluorocyclododec-2-enol (22), did rearrange to yield mixtures of diastereomers much less selectively. Also 2-fluorodec-2-enol (6), which was prepared by rearrangement of 2-fluoro-2-octyloxirane (5) with TMSOTf and triethylamine, was successfully applied as a starting material for [3,3]-sigmatropic rearrangements. The corresponding 3-(1-fluoroethenyl)alkanoic acid derivatives 17 and 18 were formed in moderate yield. 2-Fluoroalk-1-en-3-ol esters prepared in two steps from 2-fluoroalk-1-enes undergo Claisen rearrangements to form 2-substistuted 4-fluoroalk-4-enecarboxylic acids.
- Tranel, Frank,Haufe, Günter
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p. 1593 - 1608
(2007/10/03)
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