Anab initio study of heterodienophiles addition to 2,3-diaza-1,3-butadiene: An example of endo-lone-pair effect on the reaction energy barrier
✍ Scribed by Jursic, Branko S.; Zdravkovski, Zoran
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 701 KB
- Volume
- 17
- Category
- Article
- ISSN
- 0192-8651
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✦ Synopsis
Transition states for the Diels-Alder reactions of 2,3-diaza-1,3-butadiene with ethylene, formaldehyde, formaldimine, cis-and frans-diazene, and nitrosyl hydride were located by ab initio molecular orbital calculations. The bond orders of the new forming bonds have been used to determine the asynchronicity of the reactions. Ab initio calculations show that the energy barrier for the hetero-Diels-Alder reactions is relatively high. The highest energy barrier of 34.76 kcal/mol calculated at the MP4/6-31G*//MP2/6-31G* level was found for the exo-cis-diazene addition to 2,3-diaza-1,3-butadiene. In all cases, when two diastereomeric transition structures are possible, the one with the endo hydrogen, exo lone pair was predicted to have a lower activation barrier. This behavior can be explained by the n -r and n-n loan pair repulsion interaction between the dienophile and diene heteroatoms in the corresponding transition state. The barrier is higher for those reactions which in the transition state have more lone electron pairs. Also, the barrier is higher when the lone pairs are endo oriented than when they are exo oriented in the transition state. 0 1996 by John Wiley & Sons, Inc.
products? These reactions take place through a concerted, synchronous cycloaddition pathway, although a stepwise, zwitterionic mechanism is also possible for polar dienophiles, especially when they are catalyzed by an acid. There are relatively few mechanistic3 and theoretical4 studies of hetero Diels-Alder reactions, contrary to extensive studies of the all-carbon Diels-Alder reactions?r6 We have reactions of hetero dienophiles with dienes that 'See also ref. 6f.