Sulfonium yhdes formation

The preformed benzyl sulfonium salt produced the oxirane in nearly the same yield and optical purity as the direct use of the sulfide, indicating that the one-pot reaction proceeds via initial formation of the sulfonium salt, which is subsequently deprotonated to generate the yhde in situ (Scheme 20.4). The sulfide is finally released upon epoxide ring closure. 

Selenium and tellurium ylides other than sulfonium have also been reported to be useful for the cyclopropanation. Selenium ylides were smdied by Kataoka and coworkers, and cyclopropane formation was observed. Diphenyl vinyl or allenyl selenium triflate serves as an electron-deficient alkene, and selenium ylides can be generated by the conjugate addition. The resulting yhdes underwent cyclopropane formation by an MIRC reaction (Scheme 1.31) [54]. 

Sulfide 2a was later employed by Tang and coworkers in the reaction of 4-chlorobenzaldehyde with 3-trimethylsilylallyl bromide for the synthesis of viny-loxiranes. In the presence of 20mol% of 2a, the trans vinyl epoxide was obtained as the major diastereoisomer in 40% yield and 37% ee [21]. Higher concentrations of both aldehyde and aUyUc bromide were found to be beneficial to the yields, because the high concentration of aUyHc bromide favors the formation of the sulfonium salt and the high concentration of aldehyde could probably allow capture of the sulfur yhde before the [2,3]-sigmatropic rearrangement (Scheme 20.6). This reaction with thiolane (THT) as a catalyst could also be carried out without solvent, but the yield was lower than that in t-BuOH. 

When compared with phosphonium and sulfonium ylides, the corresponding selenonium and telluronium yhdes frequently exhibit better reactivity and nucle-ophilicity, while the use of selenides and tellurides in catalytic yUde reactions is far less reported. In 2001, Metzner and coworkers reported an asymmetric epoxi-dation reaction of aldehydes using a C2-symmetric selenide 42 resembling their sulfide catalyst (Scheme 20.32). High yields (65-97%) and enantioselectivity (76-94% ee) were obtained with a range of aromatic aldehydes and cinnamalde-hydes. However, no diastereoselectivity (trans cis = 1 1) was observed in these reactions, while the corresponding sulfide gave around 80% diastereoselectivities, which could be rationalized as the formation of a less diastereoselective early transihon state with the more reactive selenonium ylides. The reactions of more electron-deficient aryl aldehydes (p-Cl and p-CFs) were less enantioselective (76% ee and 83% ee, respectively) [58]. 

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