- Allylmagnesium Halides Do Not React Chemoselectively because Reaction Rates Approach the Diffusion Limit
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Competition experiments demonstrate that additions of allylmagnesium halides to carbonyl compounds, unlike additions of other organomagnesium reagents, occur at rates approaching the diffusion rate limit. Whereas alkylmagnesium and alkyllithium reagents could differentiate between electronically or sterically different carbonyl compounds, allylmagnesium reagents reacted with most carbonyl compounds at similar rates. Even additions to esters occurred at rates competitive with additions to aldehydes. Only in the case of particularly sterically hindered substrates, such as those bearing tertiary alkyl groups, were additions slower.
- Read, Jacquelyne A.,Woerpel
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p. 2300 - 2305
(2017/02/26)
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- The retro Grignard addition reaction revisited: The reversible addition of benzyl reagents to ketones
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The Grignard addition reaction is known to be a reversible process with allylic reagents, but so far the reversibility has not been demonstrated with other alkylmagnesium halides. By using crossover experiments it has been established that the benzyl addition reaction is also a reversible transformation. The retro benzyl reaction was shown by the addition of benzylmagnesium chloride to di-tert-butyl ketone followed by exchange of both the benzyl and the ketone moiety with another substrate. Similar experiments were performed with phenylmagnesium bromide and tert-butylmagnesium chloride, but in these two cases the Grignard addition reaction did not show any sign of a reverse transformation.
- Christensen, Stig Holden,Holm, Torkil,Madsen, Robert
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p. 1478 - 1483
(2014/02/14)
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- Pivaloylmetals (tBu-COM: M=Li, MgX, K) as equilibrium components
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Short-lived pivaloylmetals, (H3C)3C-COM, were established as the reactive intermediates arising through thermal heterolytic expulsion of O=CtBu2 from the overcrowded metal alkoxides tBuC(=O)-C(-OM)tBu2 (M=MgX, Li, K). In all three cases, this fission step is counteracted by a faster return process, as shown through the trapping of tBu-COM by O=C(tBu)-C(CD3)3 with formation of the deuterated starting alkoxides. If generated in the absence of trapping agents, all three tBu-COM species "dimerize" to give the enediolates MO-C(tBu)=C(tBu)-OM along with O=CtBu2 (2 equiv). A common-component rate depression by surplus O=CtBu2 proves the existence of some free tBu-COM (separated from O=CtBu2); but companion intermediates with the traits of an undissociated complex such as tBu-COM & O=CtBu2 had to be postulated. The slow fission step generating tBu-COMgX in THF levels the overall rates of dimerization, ketone addition, and deuterium incorporation. Formed by much faster fission steps, both tBu-COLi and tBu-COK add very rapidly to ketones and dimerize somewhat slower (but still fairly fast, as shown through trapping of the emerging O=CtBu2 by H3CLi or PhCH2K, respectively). At first sight surprisingly, the rapid fission, return, and dimerization steps combine to very slow overall decay rates of the precursor Li and K alkoxides in the absence of trapping agents: A detailed study revealed that the fast fission step, generating tBu-COLi in THF, is followed by a kinetic partitioning that is heavily biased toward return and against the product-forming dimerization. Both tBu-COLi and tBu-COK form tBu-CH=O with HN(SiMe3)3, but only tBu-COK is basic enough for being protonated by the precursor acyloin tBuC(=O)-C(-OH)tBu2. Copyright
- Knorr,Boehrer,Schubert,Boehrer
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p. 7506 - 7515
(2012/07/27)
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- Synthesis of Some Carboxylic Acid Analogs Cleaved between the C-2 and C-3 Bond of Tetramethylcyclopropanecarboxylic Acid, and Insecticidal Activities of Their Esters
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Carboxylic acids with a cleaved type of cyclopropane ring between the C-2 and C-3 bond in tetramethylcyclopropanecarboxylic acid, the acid part of fenpropathrin, were prepared.Several of their esters with pyrethroidal alcohols exhibited good insecticidal activity, but their activity was a little weaker than that of fenpropathrin.
- Furuhata, Akimichi,Hirano, Masachika,Fujimoto, Izumi,Matsui, Masano
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p. 1633 - 1640
(2007/10/02)
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- SYNTHESE EN SERIE ENCOMBREE. PREPARATION DE L'ACIDE DITERTIOBUTYL-METHYLACETIQUE tBu2MeCCOOH ET DE QUELQUES COMPOSES CETONIQUES DERIVES DE CETTE STRUCTURE
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The synthesis of tBu2MeCCOOH, which is among the most sterically hindered known acids is described.Only Newman's sequence via tBu2C=O, tBu2MeCOH, tBu2C=CH2, tBu2CHCH2OH, tBu2CHCOOH, tBu2CHCOCl and tBu2C=C=O, which has been optimized in this work by a direct access to tBu2CHCOOH, permits the preparation of tBu2MeCCOOH.The condensation of the corresponding chloride with a Grignard reagent yields new highly-hindered ketones tBu2MeCCOR wich by alkylation give more substituted structures.The limitations of each method have been studied in this work.
- Dubois, Jacques-Emile,Zhang, Ben Li,Lion, Claude
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p. 4189 - 4194
(2007/10/02)
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