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Abnormal Claisen Rearrangement

GENERAL DESCRIPTION OF THE REACTION

The first example of abnormal Claisen rearrangements was reported by Lauer and Filbert in 1936.1 In contrast to the regular Claisen rearrangement ([3,3] migration),2 the abnormal Claisen rearrangement3 usually occurs for the allyl aromatic ethers. A similar reaction also occurs for the thermal rearrangement of cyclopropyl ketones to homoallylic ketones.4 The abnormal Claisen rearrangement is believed to proceed via two consecu-tive processes, i.e., the normal ortho Claisen rearrangement of γ-alkylallyl aryl ether to an o-(α-alkylallyl) phenol and the isomerization of the resulting phenol. In general, this type of abnormal Claisen rearrangement does not occur smoothly, except when in the pres-ence of Lewis acids FeCl3, even though other Lewis acids (e.g., HfCl4, GaCl3, ZrCl4) have limited ability to accelerate such reaction.3a It is reported that the abnormal Claisen rearrangement can be prevented by the application of 1,1,1,3,3,3-hexamethyldisilazane and N,O-bis(trimethylsilyl)acetamide.5

GENERAL REACTION SCHEME




PROPOSED MECHANISMS
Two kinds of mechanisms have been proposed for the abnormal Claisen rearrangement:
the concerted process (Scheme 1)6 and the stepwise process consisting of two consecutive steps (Scheme 2).4 However, much experimental evidence is inconsistent with the stepwise mechanism.




author

DESH DEEPAK A P S CHAUHAN

M.Sc. CHEMISTRY!

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