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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
journal contribution
posted on 2014-04-14, 00:00 authored by Ziqi Tian, Yuanjing Xiao, Xiangai Yuan, Zuliang Chen, Junliang Zhang, Jing MaThe 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(ClO4)2·6H2O-catalyzed reaction, which is supported
by experiment on changing originally unreactive Ni(OTf)2 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)2 type catalysts have remarkable steric hindrance
with α > 4.5. Large cations (RM > 74 pm) relative to their surrounding ligands with α <
4 prefer the C–O bond cleavage path, while small cations (RM < 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.