Mechanisms and origin of regioselectivity on N-heterocyclic carbene-catalyzed [3+2]/[4+2] annulations of C60 with α,β-unsaturated aldehydes

Abstract

The N-Heterocyclic carbenes (NHC)-catalyzed annulations of C60 with α,β-unsaturated aldehydes represent as an attractive method for C60 functionalization, but the detailed reaction mechanism and chemoselectivity- as well as regioselectivity-determining factors remain elusive. In this study, the reactions were investigated using DFT method, which show that the homoenolate intermediates can undergo [3+2]/[4+2] annulations with C60 depending on the substituent groups on it. The homoenolate intermediate devoid of β-methylene substituent group can react with C60 forming a fullerenyl anion species, which then undergo tautomerization followed by intramolecular cyclization and catalyst elimination to afford the [3+2] cycloadducts. The tautomerization step was demonstrated to be rate-determining, and EtOH in combination with 1,4-benzoquinone (BQ) can lower the free energy barrier for this reaction step. In contrast, the homoenolate intermediates with β-methylene substituent groups can preferentially undergo oxidation by 3,3ʹ,5,5′-tetra-tert-butyldiphenoquinone (DQ) followed by deprotonation to generate the azolium dienolate, which can then react with C60 through [4+2] rather than [3+2] annulations. For both the two kinds of annulations, [6,6]-regioselectivities can be successfully predicted by Parr function analyses on the first CC bond formation intermediates. The computational results open a convenient door for prediction and rational design of NHC-catalyzed annulation reactions involving C60 with special regioselectivities.