| Silylium Ion-Catalyzed Challenging Diels-Alder Reactions: The Danger of Hidden Proton Catalysis with Strong Lewis Acids. | |
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MedLine Citation:
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PMID: 22309027 Owner: NLM Status: Publisher |
Abstract/OtherAbstract:
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The pronounced Lewis acidity of tricoordinate silicon cations brings about unusual reactivity in Lewis acid catalysis. The downside of catalysis with strong Lewis acids is though that these do have the potential to mediate the formation of protons by various mechanisms, and the thus released Brønsted acid might even outcompete the Lewis acid as the true catalyst. That is an often ignored point. One way of eliminating a hidden proton-catalyzed pathway is to add a proton scavenger. The low-temperature Diels-Alder reactions catalyzed by our ferrocene-stabilized silicon cation are such a case where the possibility of proton catalysis must be meticulously examined. Addition of the common hindered base 2,6-di-tert-butylpyridine resulted, however, in slow decomposition along with formation of the corresponding pyridinium ion. Quantitative deprotonation of the silicon cation was observed with more basic (Mes)3P to yield the phosphonium ion. A deuterium-labeling experiment verified that the proton is abstracted from the ferrocene backbone. A reasonable mechanism of the proton formation is proposed based on quantum-chemical calculations. This is, admittedly, a particular case but suggests that the use of proton scavengers must be carefully scrutinized as proton formation might be provoked rather than prevented. Proton-catalyzed Diels-Alder reactions are not well-documented in the literature, and a representative survey employing TfOH is included here. The outcome of these catalyses is compared with our silylium ion-catalyzed Diels-Alder reactions, thereby clearly corroborating that hidden Brønsted acid catalysis is not operating with our Lewis acid. Several simple-looking but challenging Diels-Alder reactions with exceptionally rare dienophile/enophile combinations are reported. Another indication is obtained from the chemoselectivity of the catalyses. The silylium ion-catalyzed Diels-Alder reaction is general with regard to the oxidation level of the alpha,beta-unsaturated dienophile (carbonyl and carboxyl) whereas proton catalysis is limited to carbonyl compounds. |
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Authors:
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Ruth K Schmidt; Kristine Müther; Christian Mück-Lichtenfeld; Stefan Grimme; Martin Oestreich |
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Publication Detail:
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Type: JOURNAL ARTICLE Date: 2012-2-6 |
Journal Detail:
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Title: Journal of the American Chemical Society Volume: - ISSN: 1520-5126 ISO Abbreviation: - Publication Date: 2012 Feb |
Date Detail:
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Created Date: 2012-2-7 Completed Date: - Revised Date: - |
Medline Journal Info:
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Nlm Unique ID: 7503056 Medline TA: J Am Chem Soc Country: - |
Other Details:
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Languages: ENG Pagination: - Citation Subset: - |
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From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine
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