The nature of neighbouring group participation by chalcogen substituents

© 2011 Dr. Shea Fern LimSolution phase analysis, crystal structure analysis and theoretical calculations of 2- and 4-alkyl- and aryl-selenomethyl-substituted pyridinium ions 68-72 and 73-77 provide strong evidence for C-Se hyperconjugation (σC-Se-π*) between the C-Se σ-bond and the π-deficient aromatic ring and a through-space interaction (nSe-π*) between the selenium p-type lone pair electrons and the π-deficient aromatic ring. There is also a weak anomeric-type interaction (nSe-σ*C-C) involving the selenium p-type lone pair electrons and the polarised CH2-C(Ar) σ-bond. NBO analysis of calculated cations with varying electron demand (B3LYP/6-311++G**) shows that C-Se hyperconjugation (σC-Se-π*) is the predominant mode of stabilisation in the weakly electron-demanding pyridinium ions (72, 77-79). However, the through-space interaction (nSe-π*) becomes more important as the electron demand of the β-selenium-substituted carbocation increases (80-82). The anomeric interaction (nSe-σ*C-C) is relatively weak in all ions. In the effort to observe these structural effects experimentally, the generation of 7-(phenylselenomethyl)tropylium ion 133 was attempted. Unfortunately, the generation of tropylium ion 133 was unsuccessful under various conditions. Majority of these reactions gave rise to the formation of the more stable 7-(triphenylethyl)tropylium ion 158. The solvolysis of the optically active selenium-substituted trifluoroacetate (+)-182 under various conditions (Solvolysis 1-4) revealed that nucleophilic attack occurs more readily at the selenium atom rather than the carbon atom of the selenium cationic intermediate. The preference for selenophilic attack is the reason for racemisation through the decomplexation-recomplexation process. In addition, observation of crossover products, alcohol 206 and trifluoroethoxy 207 in the solvolysis in the presence of an alkene trap (Solvolysis 2 and 4) may arise by either solvent-facilitated or direct selenenium group PhSe+ transfer to the alkene trap. However, these solvolysis experiments did not reveal conclusive evidence as to the nature of the selenium cationic intermediate species (bridged 183 or open 184). The direct transfer of the phenylselenenium group PhSe+ from seleniranium ion 235 to alkenes has been successfully demonstrated through gas phase ion-molecule reaction. On the other hand, in the same study, the phenylsulfenium group PhS+ from thiiranium ion 236 did not undergo the same direct transfer process. The reaction between thiiranium ion 236 and cyclohexene gave rise to dehydrogenated product 269 and insertion product 270. Through deuterium-labelling experiments, it was postulated that insertion product 270 is formed as a result of an ‘Ene-like’ reaction between the strained C-S bond of thiiranium ion 236 and the C=C double bond of cyclohexene (Schemes 4.26, 4.27). This step is then followed by dehydrogenation to allow for product 269 to form. The possibility of the presence of the open form of seleniranium ion 235 (carbocation 273) and thiiranium ion 236 (carbocation 274) in the gas phase is also considered. The formation of the insertion product may have been the result of a stepwise Prins reaction between the open cation (273, 274) and the various alkenes (Scheme 4.29). Dehydrogenation then takes place in the subsequent step to form the final product

Reactions of Thiiranium and Sulfonium Ions with Alkenes in the Gas Phase

International audienc