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Concerted vs Stepwise Mechanisms in Dehydro-Diels-Alder Reactions.

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Title: Concerted vs Stepwise Mechanisms in Dehydro-Diels-Alder Reactions.
Authors: Ajaz, Aida1, Bradley, Alexander Z.1, Burrell, Richard C.1, Hong Li, William Hoi1, Daoust, Kimberly J.1, Bovee, Laura Boddington1, DiRico, Kenneth J.1, Johnson, Richard P.1 Richard.Johnson@unh.edu
Source: Journal of Organic Chemistry. 11/28/2011, Vol. 76 Issue 22, p9320-9328. 5p.
Subjects: Diels-Alder reaction kinetics, Chemical reactions, Ring formation (Chemistry), Cyclic compounds, Allene, Chemical bonds
Abstract: The Diels-Alder reaction is not limited to 1,3- dienes. Many cycloadditions of enynes and a smaller number of examples with 1,3-diynes have been reported. These "dehydro". Diels-Alder cycloadditions are one class of dehydropericydic reactions which have long been used to generate strained cyclic allenes and other novel structures. CCSD(T)//M05-2X computational results are reported for the cycloadditions of vinylacetylene and butadiyne with ethylene and acetylene. Both concerted and stepwise diradical routes have been explored for each reaction, with location of relevant stationary points. Relative to 1,3-dienes, replacement of one double bond by a triple bond adds 6-6.5 kcal/mol to the activation barrier; a second triple bond adds 4.3-4.5 kcal/mol to the barrier. Product strain decreases the predicted exothermicity. In every case, a concerted reaction is favored energetically. The difference between concerted and stepwise reactions is 5.2-6.6 kcal/mol for enynes but diminishes to 0.5-2 kcal/mol for diynes. Experimental studies on intramolecular diyne + ene cycloadditions show two distinct reaction pathways, providing evidence for competing concerted and stepwise mechanisms. Diyne + yne cycloadditions connect with arynes and ethynyl-l,3-cyclobutadiene. This potential energy surface appears to be fiat, with only a minute advantage for a concerted process; many diyne cycloadditions or aryne cydoreversions will proceed by a stepwise mechanism. [ABSTRACT FROM AUTHOR]
Copyright of Journal of Organic Chemistry is the property of American Chemical Society and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Concerted vs Stepwise Mechanisms in Dehydro-Diels-Alder Reactions.
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  Data: <searchLink fieldCode="DE" term="%22Diels-Alder+reaction+kinetics%22">Diels-Alder reaction kinetics</searchLink><br /><searchLink fieldCode="DE" term="%22Chemical+reactions%22">Chemical reactions</searchLink><br /><searchLink fieldCode="DE" term="%22Ring+formation+%28Chemistry%29%22">Ring formation (Chemistry)</searchLink><br /><searchLink fieldCode="DE" term="%22Cyclic+compounds%22">Cyclic compounds</searchLink><br /><searchLink fieldCode="DE" term="%22Allene%22">Allene</searchLink><br /><searchLink fieldCode="DE" term="%22Chemical+bonds%22">Chemical bonds</searchLink>
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  Data: The Diels-Alder reaction is not limited to 1,3- dienes. Many cycloadditions of enynes and a smaller number of examples with 1,3-diynes have been reported. These "dehydro". Diels-Alder cycloadditions are one class of dehydropericydic reactions which have long been used to generate strained cyclic allenes and other novel structures. CCSD(T)//M05-2X computational results are reported for the cycloadditions of vinylacetylene and butadiyne with ethylene and acetylene. Both concerted and stepwise diradical routes have been explored for each reaction, with location of relevant stationary points. Relative to 1,3-dienes, replacement of one double bond by a triple bond adds 6-6.5 kcal/mol to the activation barrier; a second triple bond adds 4.3-4.5 kcal/mol to the barrier. Product strain decreases the predicted exothermicity. In every case, a concerted reaction is favored energetically. The difference between concerted and stepwise reactions is 5.2-6.6 kcal/mol for enynes but diminishes to 0.5-2 kcal/mol for diynes. Experimental studies on intramolecular diyne + ene cycloadditions show two distinct reaction pathways, providing evidence for competing concerted and stepwise mechanisms. Diyne + yne cycloadditions connect with arynes and ethynyl-l,3-cyclobutadiene. This potential energy surface appears to be fiat, with only a minute advantage for a concerted process; many diyne cycloadditions or aryne cydoreversions will proceed by a stepwise mechanism. [ABSTRACT FROM AUTHOR]
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  Data: <i>Copyright of Journal of Organic Chemistry is the property of American Chemical Society and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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      – SubjectFull: Chemical reactions
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      – SubjectFull: Ring formation (Chemistry)
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              Text: 11/28/2011
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