Karas Lucas J, Campbell Adam T, Alabugin Igor V, Wu Judy I
Department of Chemistry, University of Houston, Houston, Texas 77204, United States.
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States.
Org Lett. 2020 Sep 18;22(18):7083-7087. doi: 10.1021/acs.orglett.0c02343. Epub 2020 Aug 28.
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 [4+2] π-aromaticity. Conversion of the carbonyl group into a hydrazone ion (═N-NR) reverses the polarity of the exocyclic double bond, increases the [4] 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 SOCH). Other nonbenzenoid aromatics can be activated as normal-electron-demand Diels-Alder dienes in the same way.
我们提出了一种羰基极性反转策略，用于将环庚三烯酮作为正常电子需求的狄尔斯-阿尔德二烯进行活化。由于其[4+2]π芳香性，环庚三烯酮对狄尔斯-阿尔德反应的反应活性较低。将羰基转化为腙离子（═N-NR）可反转外环双键的极性，增加[4]环π反芳香性，并提高最高占据分子轨道（HOMO）能量。与马来酰亚胺发生狄尔斯-阿尔德反应的计算气相活化自由能表明，当环庚三烯酮的C═O被═N-NR（R = H或SOCH）取代时，反应速率提高了十亿倍。其他非苯型芳烃也可以通过同样的方式作为正常电子需求的狄尔斯-阿尔德二烯被活化。
