Does the Mechanism of the Garratt-Braverman Cyclization Differ with Substrates? A Computational Study on Bispropargyl...

We studied the variation in mechanism among different bispropargyl substrates-sulfone, sulfide, ether, amine, and methane-toward Garratt-Braverman (GB) cyclization using density functional theory calculations. Isomerization and cycloaddition are the key steps in the GB cyclization. To compare the reactivity among the various substrates, we computed the free energy of activation (ΔG(⧧)) for the cycloaddition and the cyclization steps, whereas we used the theoretically computed pKa values for the isomerization steps. Our results suggest that the sulfones undergo a relatively fast isomerization followed by slower cyclization, while the ethers undergo a slow isomerization followed by easy cyclization. The methanes and amines are similar to the ethers, and the sulfides showed intermediate behavior. We extended our study to unsymmetrical substrates and compare the results with experiments that suggest the isomerization to be the rate-limiting step for bispropargyl ethers, while cyclization through a diradical intermediate is crucial to the rate for the bispropargyl sulfones. On the basis of these findings, we made predictions on the selectivity of unsymmetrical bispropargyl sulfones, amines, methanes, and sulfides. This is the first detailed mechanistic study on the GB cyclization of bispropargyl substrates other than sulfones.