Transition Structures of Schenck Reaction -On the Substituent Effect-

Abstract

Schenck reaction, which is the oxidation reaction of alkenes having allylic hydrogen (s) with singlet oxygen (¹O₂) to give hydroperoxide (ROOH), is the one of the most useful reactions to introduce oxygen function into the allylic position.¹O₂, which is one of the active oxygen, has been clarified to be involved in the various physiological phenomena. In order to accumulate and analyze the geometrical and energetic information to predict the reactivity and regio- and stereoselectivity (to predict the major stereoisomeric product) for the reaction, the transition structures for Schenck reaction of singlet oxygen (¹O₂) with some alkenes (methyl-, vinyl and formyl substituted alkenes) have been located with ab initio molecular orbital theory. All stationary points, including transition structures were optimized with no geometry constraint at the RHF/ 3-21G basis set.
The methyl substitution makes the transition structure earlier and more stable (the increase of methyl group on alkenes makes the transition structure more stable and the activation energy decreases (≈3 kcal/mol) proportionally with the number of methyl group). The position of methyl subst ituent (s)changes the transition geometry: All cis-substituted alkenes have the transition structures including 3membered ring structure formed by 0², C1 and C² atoms (Scheme 1).
The oxidation by ¹O₂ or OH radical of allylic position can be conside red to be the plausible mechanism of the peroxidation of unsaturated fatty acids or some hormones having X-CH₂-X type allylic methylene group (X=cis substituted double bond), such as prostaglandins. The RHF/ 3-21G transition geometries and activation energies for Schenck reactions of 9a-b having vinyl group and 11a-c having formyl group show that the methyl substitution at C¹ position (see Scheme 2) lowered the activation energy, though the energy did not hardly change by the substituents at C³ position. The small activation energies for the reaction of ¹O₂ with 9 and 11 indicate that the methylene group (CH₂) between two double bonds is easily oxidized by Schenck reaction, suggesting the possibility for the involvement of active oxygen such as ¹O₂ for the peroxidation mechanism of some biomolecules.