Control of Chemoselectivity by Coordinated Water and Relative Size of Ligands to Metal Cations of Lewis Acid Catalysts for Cycloaddition of an Oxirane Derivative to an Aldehyde: Theoretical and Experimental Study

The role played by Lewis acid catalysts in the selective cleavages of C–O and C–C bonds of oxirane derivatives with aldehydes is investigated both theoretically and experimentally. According to the different chemoselectivities, various catalysts are divided into four series: C–O selectivity, both, C–C selectivity, and none, respectively. The involvement of coordinated water molecules is crucial to rationalize the experimental observation of C–C selectivity for the Ni­(ClO<sub>4</sub>)<sub>2</sub>·6H<sub>2</sub>O-catalyzed reaction, which is supported by experiment on changing originally unreactive Ni­(OTf)<sub>2</sub> to be an effective catalyst by mixing with water. Furthermore, the steric hindrance from the anion in Lewis acid and the water molecule have significant influence on the efficiency of catalysts. A steric parameter, α, defined as the relative ratio of ligand size to radius of the center metal cation, gives a general picture to understand the selectivities of various Lewis acid catalysts. The ineffective M­(OTf)<sub>2</sub> type catalysts have remarkable steric hindrance with α > 4.5. Large cations (<i>R</i><sub>M</sub> > 74 pm) relative to their surrounding ligands with α < 4 prefer the C–O bond cleavage path, while small cations (<i>R</i><sub>M</sub> < 70 pm) with α < 4.5 lead to C–C bond breaking. An understanding of the relationship between selectivity and Lewis acid catalysts may guide the design of more selective and versatile Lewis acid catalysts for organic synthesis.