Practical, Highly Active, and Enantioselective Ferrocenyl-Imidazoline Palladacycle Catalysts (FIPs) for the Aza-Claisen Rearrangement of N-para-Methoxyphenyl Trifluoroacetimidates

摘要

[3,3]-Sigmatropic rearrangements rank among the most fundamental reactions in organic chemistry.[1] Particularly attractive is the PdII-catalyzed aza-Claisen rearrangement, also known as the Overman rearrangement, which allows for the formation of chiral enantioenriched protected allylic amines starting from achiral allylic imidates, which are easily synthesized from allylic alcohols in a single high-yielding step.[2] The resulting allylic amine derivatives are valuable building blocks for the synthesis of important compound classes such as unnatural amino acids.[3] However, most of the catalytic asymmetric aza-Claisen rearrangements that have been investigated have been limited to N-aryl benzimidates, which are of little practical value since cleavage of the amide protecting group is typically very low-yielding. Notable exceptions are the use of allylic trichloroacetimidates[4] and N-para-methoxyphenyl trifluoroacetimidates[5] since the protecting groups can generally be removed in preparatively useful yields.