DFT Calculation on the Stereochemistry of the Allylic Oxidation. Selenium Dioxide-Mediated Oxidation of an Exocyclic Olefinic Hydrindane Compound

Abstract

Selenium dioxide-mediated oxidation is among the most important transformations for introducing an hydroxy group into the allylic position in the substituted alkenes in organic and biochemical pathways. A unique mode of highly stereoselective interaction of selenium dioxide with olefins, resulting in allylic alcohols, is well exemplified for the preparation of a steroid compound as shown Scheme 1. Since Sharpless and Lauer proposed a mechanism, generally accepted, for this process in 1972 as resulting from an initial ene reaction followed by a [2,3]-sigmatropic rearrangement, there have been considerable efforts on particularly clarifying the stereo-selectivity of this reaction. For example, Singleton and Hang clarified a concerted aspect of the ene step in the allylic oxidation of 2-methyl-2-butene with selenium dioxide (1 → 2 in Scheme 2). Recently we have elucidated the overall pathways in detail by treating theoretically both an ‘ene’ reaction and the 2,3-sigmatropic rearrangement involved in this selenium dioxide-mediated oxidation of 1: the allylic oxidation occurs by complex mechanisms via two competing paths such as anti-approach and syn-approach in the ‘ene’ reaction to produce the (E)allylic alcohol 2, where it has been first noticed the Cα-Cβ bond rotation in 2,3-rearrangement step plays an important part for the stereo-selectivity of the reaction. Now our attention is brought to understand the consequence of the Cα-Cβ bond rotation in the selenic acid intermediate produced from the ene step as to whether it may affects the stereochemical course of the allylic oxidation of a hydrindane compound 5, where an exocyclic olefinic structure is crafted (Scheme 3). Here, we report the results of ab initio studies of the allylic oxidation of 5, focusing on analyzing whether the conformational change via bond rotation in selenic acid intermediate 6 may give rise to produce 9. To study the reaction pathway with relative stabilities for possible transition states during the allylic oxidation of model compound 5 (Scheme 4 and Table 1), we have investigated the energy of possible transition states in SeO2 oxidation of 5 by b3lyp/6-31G* calculation. In ene reaction step, the anti approach of selenium dioxide is energetically favored than the syn one by 1.82 kcal/mol (entry 2 and 3 in Table 1). In the following 1,3-rearrangement step, the O=Segroup approach to the α face of double bond is more favorable than the β face by 2.28 kcal/mol (entry 4 and 5 in Table 1). The compound 7 as a major product should be obtained by the pathway of ts-1a and ts-1a-2α. The other isomer 9 can be formed through the pathway of ts-1a and ts1a-2β, where a Cα-Cβ bond rotation of the ‘ene’ reaction