High Level Computational Study of the Stereoelectronic Effects of Substituents on Alkene Epoxidations with Peroxyformic Acid

Published

September 1, 1998

Author(s)

R D. Bach, M N. Glukhovtsev, Carlos A. Gonzalez

Abstract

The epoxidations of propene and isobutene with peroxyformic acid proceed in a concerted way via Markovnikov-type slightly unsymmetrical transition structures where the differences in the bond distances between the double bond carbons and the spiro-oxygen are only at 0.021 and 0.044 at the QCISD level. In contrast, the more polarizable nature of the carbon-carbon double bond of α,Β-unsaturated systems results in a higher unsymmetrical transition structure for the epoxidation of 1,3-butadiene with an order of magnitude difference in the carbon-oxygen bond distances of 0.487 at the QCISD/6-31G* level. A highly unsymmetrical transition structure has been also found for the epoxidation of acrylonitrile. Notwithstanding the difference in the extent of asymmetry of the transition structures, both epoxidations of methyl substituted alkenes and such α,Β-unsaturated systems as 1,3-butadiene and acrylonitrile with peroxyformic acid follow a concerted asynchronous pathway. An unsymmetrical type of the transition structure for 1,3-butadiene epoxidation and a concerted nature of the mechanism has been confirmed by means of kinetic isotope effects. The closeness of the barriers for propene and 1,3-butadiene epoxidations supports the conclusion that both reactions have similar mechanisms albeit they differ in the asynchronous character of their transition structures. Methyl substitution leads to a decrease in the epoxidation barriers from 4.5 kJ/mol for ethylene to 3.3 kJ/mol for isobutene at the QCISD(T)/6-31G*//QCISD/6-31G* level. While the activation barrier for the epoxidation of 1,3-butadiene with peroxyformic acid (3.9 and 2.9 kJ/mol at the QCISD and B3LYP levels, respectively) is close to that for the propene epoxidation (3.8 and 2.9 kJ/mol at the QCISD and B3LYP levels, respectively), the barrier for acrylonitrile epoxidation is higher (4.1 kJ/mol at the B3LYP level). This increase of the barrier height reflects the decreased nucleophilicity of double bonds bearing electron withdrawing substituents. The energy differences between syn and anti configurations of the transition structures for the epoxidations of 1,3-butadiene and acrylonitrile with peroxyformic acid are very small (0.02-0.07 kJ/mol).

Citation

Journal of the American Chemical Society

Volume

120

Issue

No. 38

Pub Type

Journals

Keywords

ab initio, barriers, epoxidation, secondary isotope effects

Citation

Bach, R. , Glukhovtsev, M. and Gonzalez, C. (1998), High Level Computational Study of the Stereoelectronic Effects of Substituents on Alkene Epoxidations with Peroxyformic Acid, Journal of the American Chemical Society (Accessed April 26, 2024)