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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Semipinacol rearrangement ： ウィキペディア英語版 Semipinacol rearrangement The semipinacol rearrangement is a rearrangement reaction in organic chemistry involving a heterosubstituted alcohol of the type R1R2(HO)C–C(X)R3R4. The hetero substituent can be a halogen (Cl, Br, I), a tosylate, a mesylate or a thiol group. This reaction proceeds by removal of the leaving group X forming a carbocation as electron deficient center. One of the adjacent alkyl groups then migrates to the positive carbon in a 1,2-shift. Simultaneously with the shift, a pi bond forms from the oxygen to carbon, assisting in driving the migrating group off its position. The result is a ketone or aldehyde.〔''Strategic applications of named reactions in organic synthesis: background and detailed mechanisms'' László Kürti, Barbara Czakó 2005〕 In another definition all semipinacol rearrangements "''share a common reactive species in which an electrophilic carbon center, including but not limited to carbocations, is vicinal to an oxygen-containing carbon and can drive the 1,2-migration of a C–C or C–H bond to terminate the process, generating a carbonyl group'' ".〔''Semipinacol Rearrangement in Natural Product Synthesis'' Zhen-Lei Song, Chun-An Fan, Yong-Qiang Tu Chemical Reviews 〕 The rearrangement reaction can be classified into 4 types. Type 1 concerns all 2-heterosubstituted alcohols. Substrates in type 2 rearrangements are allyl alcohols. The carbocation is formed by electrophilic addition to the alkene group with electrophiles such as halonium ions, Brønsted acids and Lewis acids. In type 3 the substrates are epoxides, notably 2,3-epoxy-alcohols and type 4 concerns the reactions of alpha hydroxyketones and alpha hydroxy imines. Reactions of type 4 are also called acyloin rearrangements. While similar to the pinacol rearrangement, the semipinacol rearrangement differs from the pinacol rearrangement in that the cation is not formed from a vicinal 1,2-diol. With diazoalcohols the reaction is known as the Tiffeneau–Demjanov rearrangement. ==References== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Semipinacol rearrangement」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.