Computational studies on [4 + 2] / [3 + 2] tandem sequential cycloaddition reactions of functionalized acetylenes with cyclopentadiene and diazoalkane for the formation of norbornene pyrazolines.

The mechanistic pathways for the sequential tandem [4 + 2] / [3 + 2] versus [3 + 2] / [4 + 2] cycloaddition reaction of functionalized acetylenes with cyclopentadiene and dimethyl diazopropane for the formation of norbornene pyrazolines, employed in the synthesis of pharmaceutically relevant compounds, have been studied computationally with DFT at the M06-2X/6-31G(d) and M06-2X/6-31G(d,p) levels of theory. We have established that, in the reaction of the parent (unsubstituted) acetylene with cyclopentadiene and dimethyl diazopropane, the order of the tandem addition has no substantial effects in product outcomes. The same product is obtained provided the reaction components remain the same for both [4 + 2]/[3 + 2] and [3 + 2]/[4 + 2] tandem addition sequences. The results indicate that the [4 + 2] Diels-Alder addition step is the rate-determining step irrespective of the addition order, while the 1,3-dipolar cycloaddition step has been found to generally proceed rapidly with very low activation energies. It has also been realized that the regio-, stereo-, and chemo-selectivities of the reaction are strictly dictated by the type of substituent on the parent acetylene. For substituted acetylenes, we conclude that the sequence of the tandem addition generally affects the type of isomeric product obtained. The [4 + 2]/[3 + 2] tandem addition sequence has been established to favor the exo stereo-selective isomer over the endo, whereas the [3 + 2]/[4 + 2] tandem addition sequence generally favors the endo product formation. Therefore, it is settled that the mechanistic route taken by any substrate in the [4 + 2]/[3 + 2] versus [3 + 2]/[4 + 2] sequential tandem cycloaddition is greatly affected by both electronic and steric factors. Electrophilicity indices calculations agree with the activation barriers obtained. Perturbation molecular orbital theory was employed to rationalize the results. Global reactivity indices calculations gave a good correlation with the activation energies. Graphical abstract The sequential tandem [4 + 2] / [3 + 2] versus [3 + 2] / [4 + 2] cycloaddition reaction of functionalized acetylenes with cyclopentadiene and dimethyl diazopropane for the formation of norbornene pyrazolines.