Reaction and Characterization of Hypervalent Organo-λ^<3->bromanes

(1)二氟-λ^3-溴代烷在BF_3 - Et_2O存在下与炔基锡烷（2.2当量）在二氯甲烷中发生氧化均偶联，生成对称的1,3-二炔。该反应包括在初始形成的炔基-λ^3-溴基上加成炔基锡烷。炔基-λ^3-溴炔与炔基锡烷交叉偶联得到不对称的1,3-二炔。（二）以硼酸乙烯基为原料，通过硼烷-溴（III）交换，与二氟-k^3-溴合成环己烯基和环戊基-λ^3-溴。环戊基-λ^3-溴在水溶液醇中溶解生成环戊基阳离子，得到环戊酮的收率较高。我们测量了溶剂溶解的速率，结果表明芳基-λ^3-溴基具有很高的离去基能力。在热催化（醋酸铑）条件下，开发了卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤代卤。与碘化ylide相比，氯鎓和溴化ylide是生成羰基（或类羰基）的更好的前体，并且在非催化的热条件下可以有效地进行烯烃（如环二烯）的环丙烷化。与λ^3-碘基相比，λ^3-氯基和λ^3-溴基的离去基能力更强，这似乎是造成这些卤代烷之间观察到的反应性差异的原因。

(I) Reaction of difluoro-λ^3-bromane with alkynylstannanes (2.2 equiv) in the presence of BF_3 Et_2O in dichloromethane results in oxidative homocoupling to give symmetrical 1,3-diynes. The reaction involves a Michael addition of alkynylstannanes to the initial formed alkynyl-λ^3-bromanes. Cross-coupling of alkynyl-λ^3-bromanes with alkynylstannanes affords unsymmetrical 1,3-diynes.(II) Cyclohexenyl- and cyclopentenyl-λ^3-bromanes were prepared from vinyl borates by the reaction with difluoro-k^3-bromane via borane-bromane (III) exchange. Solvolysis of cyclopentenyl-λ^3-bromane in aqueous alcohols afforded a good yield of cyclopentanone via generation of cyclopentenyl cation. We have measured the rates of solvolysis, which show very high leaving group ability of aryl-λ^3-bromanyl groups.Intermolecular transylidation between halonium ylides under thermal and catalytic (rhodium (II) acetate) conditions, which makes it possible to synthesize a hitherto unknown kind of aliphatic chloronium ylides as well as a variety of bromonium and iodonium ylides, was developed. Compared to the iodonium ylide, the chloronium and bromonium ylides serve as a better progenitor for generation of carbenes (or carbenoids) and efficiently undergo cyclopropanation of olefins such as cyclooctadiene under uncatalyzed thermal conditions. A greater leaving group ability of the λ^3-chroranyl and λ^3-bromanyl groups compared to that of the λ^3-iodanyl group seems to be responsible for the observed differences in reactivity between these halonium ylides.