Abstract
The stereochemical aspects of epoxidation of norbornene and its 7- syn -substituted derivatives by performic acid were investigated. Geometry and thermodynamic parameters of transition states and prereactive complexes were computed at the UBHandHLYP/6-31G(d) level of theory. It is shown that the transition states have a pronounced biradical character and a nearly coplanar orientation of the C=C bond and the molecule of performic acid. Transition state analysis revealed that, in the case of the syn -7-hydroxy derivative, the preference for the exo -approach of the oxidant can be explained by the stabilization of transition state with hydrogen bonding. In contrast, a chlorine atom or a methyl group at the 7- syn position facilitated the formation of endo -epoxides due to steric repulsion between the substituent and the oxidant.
Highlights
Norbornene is a unique representative of cyclic olefins
Among the noteworthy structural alterations in the TS1n are the change in the H-C2C3-C4 dihedral angle and the more planar arrangement of the six-membered ring, quantified by the β parameter – an angle between the planes C1C2C3 and C2C6C5 (Fig. 1, Table 3)
Attachment of a chlorine atom or a methyl group to the C7 leads to a significant increase in the energy of activation of exo-epoxidation compared to unsubstituted norbornene
Summary
Norbornene (bicyclo[2.2.1]hept-2-ene, 1) is a unique representative of cyclic olefins. Epoxidation of norbornene and its derivatives attracted significant attention, since it allows to investigate the influence of strain and steric factors upon reactivity [9; 11; 15], regio- and stereoselectivity [10; 16; 17]. Formation of endo-epoxides can be preferred when the exo-face of norbornene is shielded by bulky groups.
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