Electronic control of stereoselectivity. 9. The stereochemical course of electrophilic additions to aryl-substituted benzobicyclo[2.2.2]octadienes

Epoxidation of substituted norbornenes. Stereochemical aspects and accompanying intramolecular transformations

“…It was shown that treatment of 7-isopropylidenenorbornene 66 with an equimolar amount of MCPBA results in selective epoxidation of the exocyclic double bond, the ratio of products 67 and 68 being equal to 34 : 66, due to the preferential anti-attack. 139,223 A similar picture has also been observed for 9-isopropylidenebenzonorbornenes; the introduction of chlorine or fluorine atoms into the aromatic ring facilitates formation of the synproduct. Epoxidation of stereoisomeric 7,7-diphenylmethylene-N-methylnorbornene-2,3-dicarboximides 69 and 70 demonstrated that the stereochemistry of the dicarboximide group has crucial influence on the direction of the process.…”

“…13,137 The stereochemistry of epoxidation of the anhydride or esters of endo-bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic acid with trifluoroperoxyacetic acid is also non-stereospecific; the reaction mixture contains both epoxides together with products of lactonisation, whereas in the case of unsaturated lactone 17, only the b-attack is observed, due to steric interactions involving the internal hydrogen atom of the methylene group. 138 The through-space interaction between the double bond and the aromatic fragment in benzobicyclo[2.2.2]octadienes 139 and dibenzobicyclo[2.2.2]octatrienes 140 has been studied by quantum chemistry and photoelectron spectroscopy. The results of the influence of substituents in the benzene ring of dibenzobicyclo[2.2.2]octatrienes 18 on the stereoselectivity of epoxidation are shown in Table 1.…”

Electronic Control of Facial Selection in Additions to Sterically Unbiased Ketones and Olefins

“…The data can then be used to rationalize the observed selectivities. Olefins based on bicyclo[2.2.1]heptane and bicyclo[2.2.2]octane frameworks, 13 − 15 , have been thoroughly examined in this manner. − To reduce the filled orbital repulsions, the exocyclic double bond is polarized away from the etheno bridge. This, in turn, determines the preferred direction of approach of an electrophile.…”

Face Selection in Addition and Elimination in Sterically Unbiased Systems

“…Various other or additional distortions have been mentioned in the literature; pyramidality of the trigonal carbon function, nonplanarity of certain strained olefins, and orbital deformations are prominent among them. Although there is no doubt that such distortions can and do occur, there is no experimental evidence that they play a significant role in the selectivity of the unstrained probe molecules that are the subject of this paper.…”