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Published on: June 21, 2017
Decorating Enediynes with Donor-Acceptor Aromatics: The Role of Through-Bond Charge Transfer in Thermal Reactivity
Subhendu Sekhar Bag1,2,3, Suravi Paul1, Epsheeta Baruah1
1Chemical Biology/Genomics Laboratory, Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India.
Abstract:
The decoration of designer enediynes with donor-acceptor aromatics has been devised. The effect of electron donor-acceptor substituents on the two acetylenic termini of the enediynes on the thermal reactivity toward Bergman cyclization (BC) has been investigated. The thermal reactivity in terms of onset temperature from differential scanning calorimetry (DSC) among the various classes of enediynes follows the order: acceptor-acceptor > acceptor-donor > donor-donor. The reactivity variances among the designer enediynes were explained in terms of possible differences in through-bond charge transfer/withdrawal delocalization interactions, leading to stabilization of the TS of a possible cycloaromatization reaction. Further, the repulsion of the in-plane π-orbital is decreased while enediyne proceeds toward the TS in the case of acceptor-acceptor enediynes. This is in contrast to the effect of the presence of the donor-acceptor or donor-donor substituents. A UV-visible spectroscopic study and an NBO analysis support the charge delocalization stabilization of the activated complex of enediynes containing acceptor substituents in both arms. Overall, an electron-withdrawing substituent offers an accelerating effect on the thermal cyclization, resulting in the observed reactivity order.
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