Characterization of Diels-Alder Transition Structures

The TS of D-A reactions can also be characterized with respect to synchronicity. If both new bonds are formed to the same extent the reaction is synchronous, but if they differ it is asynchronous. Synchronicity has been numerically defined in terms of Wiberg bond order indices.  

Computations have also been applied to the analysis of exo endo ratios. The computed differences in energies of the exo and endo TS are often small and are subject to adjustments when solution models are used. Cyclopentadiene has been a common subject, since there is more experimental data for this compound than for any other. MP3/6-31G -level computations were used to compare the exo and endo TS for the reactions with acrylonitrile and but-2-en-3-one (methyl vinyl ketone), and ZPE and thermal corrections were included in the calculations. Good qualitative agreement was achieved with the experimental results, which is little stereoselectivity for acrylonitrile and endo stereoselectivity for but-3-en-2-one. 

Visual models, additional information and exercises on the Diels-Alder Reaction can be found in the Digital Resource available at Springer.com/carey-sundberg. 

Similar calculations have been done for propenal. For the uncatalyzed reaction, the endo-cis TS is slightly favored over the exo-cis the two trans TSs are more than 1 kcal/mol higher. The order is the same for the catalyzed reaction, but the differences are accentuated. The TSs for the catalyzed reactions are considerably more asynchronous than those for the uncatalyzed reactions. For example, for the reaction of butadiene and acrolein, the asynchronicity was measured as the difference in bond length of the two forming bonds. 

The value of A f increases from 0.617 to 0.894 going from the uncatalyzed to the BFj-catalyzed reaction. 

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Computational Characterization of Diels-Alder Transition Structures... 

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