Lucas J Karas1, Adam T Campbell2, Igor V Alabugin2, Judy I Wu1. 1. Department of Chemistry, University of Houston, Houston, Texas 77204, United States. 2. Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States.
Abstract
We propose a carbonyl umpolung strategy for activating tropone as a normal-electron-demand Diels-Alder diene. Tropone has low reactivity for Diels-Alder reactions because of its [4n+2] π-aromaticity. Conversion of the carbonyl group into a hydrazone ion (═N-NR-) reverses the polarity of the exocyclic double bond, increases the [4n] ring π-antiaromaticity, and raises the HOMO energy. Computed gas-phase activation free energies for a Diels-Alder reaction with maleimide suggest a billion-fold rate increase when the tropone C═O is replaced by ═N-NR- (R = H or SO2CH3). Other nonbenzenoid aromatics can be activated as normal-electron-demand Diels-Alder dienes in the same way.
We propose a carbonyl umpolung strategy for activating n class="Chemical">tropone as a normal-electron-demand Diels-Alder diene. Tropone has low reactivity for Diels-Alder reactions because of its [4n+2] π-aromaticity. Conversion of the carbonyl group into a hydrazone ion (═N-NR-) reverses the polarity of the exocyclic double bond, increases the [4n] ring π-antiaromaticity, and raises the HOMO energy. Computed gas-phase activation free energies for a Diels-Alder reaction with maleimide suggest a billion-fold rate increase when the tropone C═O is replaced by ═N-NR- (R = H or SO2CH3). Other nonbenzenoid aromatics can be activated as normal-electron-demand Diels-Alder dienes in the same way.
Authors: Paul W Peterson; Nikolay Shevchenko; Boris Breiner; Mariappan Manoharan; Forat Lufti; Jess Delaune; Margaret Kingsley; Kirill Kovnir; Igor V Alabugin Journal: J Am Chem Soc Date: 2016-11-28 Impact factor: 15.419
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