Cobalt(II)‐Nitromethane Complex as an Efficient Catalyst Of Demanding Diels‐Alder Reactions on the Example of an Anthracene‐Chalcone System.

Diels‐Alder (DA) cycloaddition of chalcones to anthracene and its 9‐substituted derivatives is an example of a demanding reaction that is not effective even under increased pressure. In this article we show that an easily accessible complex cation [Co(CH3NO2)6]2+ stabilized by weakly coordinating anion [Al(OC(CF3)3)4]− exhibits good catalytic activity in the reaction. Our screening covering chalcone and anthracene derivatives with either activating or deactivating effects on reactivity in DA cycloadditions show that the catalyst allows to obtain [4+2] cycloadducts with yields up to 99 % under mild conditions, even for 9‐bromoanthracene – a very poor diene in DA reactions. The reaction proceeds best in liquid SO2, which is an unusual solvent for this type of reaction. 9‐methylanthracene ‐ the most reactive diene among those tested in the study ‐ gives two regioisomeric cycloadducts and their ratio depends on the solvent used. Modeling of the reaction with DFT methods shows that Co(II) significantly lowers the activation energy of the cycloaddition. In the case of 9‐bromoanthracene the effect is much more pronounced than with e. g. recently reported redox‐activation of the diene, which is in agreement with much better yields obtained with the use of the cobalt catalyst. [ABSTRACT FROM AUTHOR]