Tandem [4+2]/retro[3+2]/[3+2] cycloaddition reactions of fluorinated-oxadiazoles with conjugated, unconjugated, cyclic, and acyclic dienes

Abstract

The tandem reactions of 2,5-bis(trifluoromethyl)-1,3,4-oxadiazole with conjugated, unconjugated, acyclic, and cyclic dienes have been studied at the M06-2X/6-311++G(d,p) level of theory. The rate-determining step is the initial [4+2] cycloaddition in the tandem reaction of 2,5-bis(trifluoromethyl)-1,3,4-oxadiazole with acyclic, cyclic, conjugated, and unconjugated dienes, whereas the stereochemistry of the tandem adduct is determined by the [3+2] step. The exo-coupling is kinetically favored over the endo-coupling in the initial [4+2] reaction of 2,5-(bis-trifluoromethyl)-1,3,4-oxadiazole with all the considered dienes. In the retro [3+2] step (N₂ extrusion), higher activation energy is required to furnish the carbonyl ylide in the reaction of 2,5-(bis-trifluoromethyl)-1,3,4-oxadiazole with conjugated and unconjugated cyclic dienes as compared with the reaction with unconjugated acyclic dienes. At the stereochemistry [3+2] step, the intermolecular addition is kinetically favored over the intramolecular addition in the [3+2] reaction of 2,5-(bis-trifluoromethyl)-1,3,4-oxadiazole with both conjugated or unconjugated cyclic dienes and unconjugated acyclic dienes. The reaction proceeds with low activation energies when conjugated and unconjugated cyclic dienes are participating, compared with those of unconjugated acyclic dienes. Overall, the tandem process proceeds via an asynchronous one-step mechanism [4+2] coupling in an exo- or endo-cycloaddition fashion, followed by a retro [3+2], which extrudes the N₂, and then the stereo-determining intermolecular or intramolecular [3+2] cycloaddition step, which leads to the tandem products. The polarity of both inter- and intra-molecular cycloaddition steps can be influenced by two factors: the nature of the heteroatom present on the diene molecule and the size of the cyclic diene. These factors play a role in determining the reactivity and electron distribution within the diene, thereby impacting the overall polarity of the cycloaddition reactions.