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Distortion/Interaction model and Energy Decomposition Analyses Reveal the Catalyst and Fluorine Effects on Enantioselectivity in Shi Epoxidation

Goal: Explore Enantioselectivity, catalyst effects, fluorine effects

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Yingzi Li
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Explore Enantioselectivity, catalyst effects, fluorine effects
 
Yingzi Li
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The Rh(III)-catalyzed reactions of α,β-unsaturated oximes with alkenes are versatile methods for the synthesis of pyri-dines. Density functional theory (DFT) calculations reported here reveal the detailed mechanism and origins of selec-tivity in this reaction. The Rh(III)/Rh(V)/Rh(I) catalytic cycle was found to be more favorable than the previously pro-posed Rh(III)/Rh(I)/Rh(III) catalytic cycle. The Rh(III)/Rh(V)/Rh(I) catalytic cycle involves C–H activation, alkene in-sertion, deprotonation, oxime migratory oxidative addition, nitrene insertion, 1,5-hydrogen shift, and β-hydride elimi-nation to give the pyridine product and form a Rh(I) species. Subsequent oxidation by Ag+ regenerates the Rh(III) cata-lyst. Reductive elimination from a alkyl-Rh(III) species is predicted to be difficult, so that the Rh(III)/Rh(I)/Rh(III) cata-lytic cycle can be excluded. The reactivities of oxime ethers and oxime esters are compared. The oxime ester acts as both a directing group and an internal oxidant. In this reaction, the N–O bond is activated by the pivalate, and migratory oxidative addition onto the Rh(III) species generates the corresponding Rh(V) nitrene complex. However, in the ab-sence of the pivalate on the oxime ether, the activation energy for oxidative addition is much higher. The reactivity was analyzed by NPA charge calculations, comparison of the N–O bond orders, and the bond dissociation energies. The calculations also explain the regioselectivity of alkene insertion, which is shown to be an electronic effect rather than a steric effect.