Back to Search Start Over

Concerted vs stepwise mechanisms in dehydro-Diels-Alder reactions.

Authors :
Ajaz A
Bradley AZ
Burrell RC
Li WH
Daoust KJ
Bovee LB
DiRico KJ
Johnson RP
Source :
The Journal of organic chemistry [J Org Chem] 2011 Nov 18; Vol. 76 (22), pp. 9320-8. Date of Electronic Publication: 2011 Oct 18.
Publication Year :
2011

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 dehydropericyclic 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-1,3-cyclobutadiene. This potential energy surface appears to be flat, with only a minute advantage for a concerted process; many diyne cycloadditions or aryne cycloreversions will proceed by a stepwise mechanism.

Details

Language :
English
ISSN :
1520-6904
Volume :
76
Issue :
22
Database :
MEDLINE
Journal :
The Journal of organic chemistry
Publication Type :
Academic Journal
Accession number :
21977993
Full Text :
https://doi.org/10.1021/jo201567d