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{{Short description|Chemical rearrangement reaction involving migration of a carbon or heteroatom group within a molecule}} |
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The '''semipinacol rearrangement''' is a [[rearrangement reaction]] in [[organic chemistry]] involving a heterosubstituted alcohol of the type R<sub>1</sub>R<sub>2</sub>(HO)C–C(X)R<sub>3</sub>R<sub>4</sub>. 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]].<ref>''Strategic applications of named reactions in organic synthesis: background and detailed mechanisms'' László Kürti, Barbara Czakó 2005</ref> 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'' ".<ref>''Semipinacol Rearrangement in Natural Product Synthesis'' Zhen-Lei Song, Chun-An Fan, Yong-Qiang Tu Chemical Reviews {{doi|10.1021/cr200055g}}</ref> |
The '''semipinacol rearrangement''' is a [[rearrangement reaction]] in [[organic chemistry]] involving a heterosubstituted alcohol of the type R<sub>1</sub>R<sub>2</sub>(HO)C–C(X)R<sub>3</sub>R<sub>4</sub>. 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]].<ref>''Strategic applications of named reactions in organic synthesis: background and detailed mechanisms'' László Kürti, Barbara Czakó 2005</ref> 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'' ".<ref>''Semipinacol Rearrangement in Natural Product Synthesis'' Zhen-Lei Song, Chun-An Fan, Yong-Qiang Tu Chemical Reviews {{doi|10.1021/cr200055g}}</ref> |
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The semipinacol rearrangement is a rearrangement reactioninorganic 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 ketoneoraldehyde.[1] 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 ".[2]
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.
