Rotation of the Allylselenic Intermediate in Moderating the Stereochemical Course of the Selenium Dioxide-Mediated Allylic Oxidation

Allylic oxidation adjacent to C = C bonds to produce allylic alcohols remains a reaction of considerable interest in organic chemistry and various methods for this purpose have been developed. Among those the selenium dioxide-mediated oxidation is regarded as the most reliable and predictable method to introduce a hydroxy group into the allylic position for substituted alkenes regioand stereo-selectively. The key mechanistic basis to induce a high regioand stereo-selectivity in the allylic oxidation is the concerted and pericyclic process through two consecutive steps (an electrophilic ‘ene’ reaction followed by the [2,3]-sigmatropic rearrangement) (Scheme 1) as in a theoretical mechanism proposed by Sharpless and a recent account concerning theoretical and experimental data. We reported the overall stereochemical pathways by elucidating both an ‘ene’ reaction and the [2,3]-sigmatropic rearrangement of the selenium dioxide-mediated oxidation of 2methyl-2-butene. Those efforts seem successful to explain regioand stereo-selective aspects experimentally observed in the selenium dioxide-mediated allylic oxidation of several trisubstituted olefin compounds, particularly those where the concerted nature can be maintained during the whole process. However, modes of selectivity in the allylic oxidation of substituted olefin compounds are found to depend strongly on the geometric features of the alkene substrates. In the case of the allylic oxidation of more general cyclic systems such as 1-tert-butyl-4-alkylidene cyclohexanes, where the system can avoid the concerted mode throughout two steps, an unexpected isomeric compound, cis-5-tert-butyl-2-ethylidenecyclohexanol was obtained. There should be involved a rotation of the allylselenic intermediate between two pericyclic steps. The purpose of this note is to provide a clearer explanation for the mechanism of the selenium dioxide-mediated oxidation in the substituted alkenes how the rotation of the allylselenic intermediate affects the stereochemical mechanism. The Rotation of the Allylselenic Intermediate in the Allylic Oxidation of 1-tert-Butyl-4-alkylidene Cyclohexanes. In the allylic oxidation of 1-tert-butyl-4-alkylidene cyclohexanes, we observed a different stereo-selectivity mode compared to 2-methyl-2-butene. Two important approaches of selenium dioxide toward the olefin function in the pericyclic ‘ene’ step of the allylic oxidation of 1-tert-butyl-4-ethylidene cyclohexane (1) are trans-anti approach (A) and trans-syn (B) approach (Figure 1). Thus, theoretical calculations show significant products obtained from the allylic oxidation of 1-tertbutyl-4-ethylidene cyclohexane should be ones via two approaches (A and B). The relative transition state energy of as shown in Figure 1, the trans-anti approach (A) is predicted moderately preferred than trans-syn approach (B), The TS energy of A has been calculated to be more stable than B by 0.633 and 0.577 kcal/mol in HF/3-21G and B3LYP/6-31G*, respectively. According to the theoretical study of the reaction using HF/3-21G method, the trans-isomer 2a is predicted as a major product (73%). The trans-isomer can be provided from the initial trans-anti approach (A) in the ‘ene’ step and in situ [2,3]-rearrangement. The allylselenic intermediate generated after trans-anti approach proceeds with a concerted mode, i.e., without C3-C4 bond rotation into a [2,3]-sigmatropic rearrangement. In this case, the position where the ‘anti’ hydrogen (Hanti in Scheme 2) originally located is substituted by the OSeOH group after two concerted steps. The dominant option after trans-syn ‘ene’ approach is to