Chalcogen vs Halogen Bonding Catalysis in a Water-Bridge-Cocatalyzed Nitro-Michael Reaction

Recently, a tellurium-based chalcogen-bond-catalyzed
nitro-Michael
reaction was reported (Angew.
Chem. Int. Ed. 2019, 58, 16923), taking advantage of the strong Lewis acidity
of the catalyst. This species was found to be more effective than
an analogous iodine-based halogen bond organocatalyst. Herein, we
present a detailed mechanistic and kinetic analysis of these catalytic
cycles including the influence of the solvent (and the performance
of different intrinsic solvation models). While the chalcogen bonding
interaction is fundamental to activate the C–C bond formation,
we found that the presence of a two-water molecular bridge is critical
to allow the following, otherwise high-energy proton transfer step.
Even though the iodine-based halogen bonding interaction is stronger
than the tellurium-based chalcogen bonding one, which makes the former
a stronger Lewis acid and hence in principle a more efficient catalyst,
solvation effects explain the smaller energy span of the latter.