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Concerted vs stepwise mechanisms in dehydro-Diels-Alder reactions
Aida Ajaz1, Alexander Z Bradley, Richard C Burrell
1Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824, USA.
The Journal of Organic Chemistry
|October 8, 2011
Summary
Dehydro-Diels-Alder reactions involving enynes and diynes are computationally studied. Concerted pathways are generally favored, though stepwise mechanisms become competitive for diynes.
Area of Science:
- Organic Chemistry
- Computational Chemistry
Background:
- The Diels-Alder reaction is a fundamental organic transformation.
- Dehydro-Diels-Alder reactions extend this to unsaturated systems like enynes and diynes.
- These reactions can generate strained cyclic allenes and novel molecular architectures.
Purpose of the Study:
- To computationally investigate the cycloaddition reactions of vinylacetylene and butadiyne with ethylene and acetylene.
- To explore both concerted and stepwise diradical reaction pathways.
- To determine the energetic favorability of different mechanisms and the impact of unsaturation on activation barriers.
Main Methods:
- High-level ab initio electronic structure calculations using CCSD(T)//M05-2X.
- Identification and characterization of stationary points on the potential energy surface.
- Analysis of activation barriers and reaction exothermicity.
Main Results:
- Replacement of a double bond with a triple bond in Diels-Alder reactions increases activation barriers.
- Concerted reaction pathways are energetically favored over stepwise diradical pathways for both enynes and diynes.
- The energetic difference between concerted and stepwise mechanisms decreases significantly for diynes compared to enynes.
Conclusions:
- Dehydro-Diels-Alder cycloadditions are influenced by the degree of unsaturation.
- While concerted mechanisms are generally preferred, stepwise pathways gain importance for diyne reactions.
- Computational findings align with experimental evidence suggesting competing mechanisms in certain diyne cycloadditions.
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