Mechanism and regioselectivity of the cycloaddition between nitrone and dirhodium vinylcarbene catalyzed by Rh2(O2CH)4: a computational study

Abstract

The B3 LYP density functional studies on the cycloaddition between nitrone and dirhodium vinylcarbene with subsequent cascade carbenoid aromatic cycloaddition/N – O cleavage and rearrangement revealed the energetics and the geometries of important intermediates and transition states in the catalytic cycle. The reaction initially occurs through a stepwise [3+2]-cycloaddition in which the dirhodium carbene activates the adjacent vinyl group for [3+2]-cycloaddition by the nitrone . Driven by the reactivity of the carbene carbon, intramolecular cyclopropanation takes place to release the catalyst, followed by the electrocyclic opening of the cyclopropane ring to form a seven-membered ring intermediate. N – O cleavage and [1,7]-oxygen migration to fi nal product proceed in a single step with a high activation energy. With meta -substituent ( Cl ) in the N -aryl group, the charge density of chlorine ’ s ortho -site decreases due to the negative inductive effect of chlorine. Therefore, as compared to the electrophilic addition on the C 8 atom, the electrophilic addition on the C 9 site owns a signifi cantly incensement on the energy barrier of cyclopropane ( TSIII and TSIX ). In addition, according to the calculations, the basicity of the product 4 is stronger than that of the product 5 . So an acidic compound 1,1,1,3,3,3-hexafl uoro-2-propanol added in the experimental study (Wang et al. in Angew Chem Int Ed 51:5907 – 5910, 2012 ) more easily reacts with 4 than 5 and thus promotes the formation of product 4 .