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.