Claisen/Overman rearrangement

Similarly to the mechanism of the Claisen rearrangement, the Overman rearrangement is a suprafacial, concerted, nonsynchronous / 3,37-sigmatropic rearrangement. The reaction is irreversible, which is the result of the significant driving force associated with the formation of the amide functionality. The mechanism of the metal catalyzed reaction is believed to proceed via an iminomercuration-deoxymercuration sequence and it is only formally a [3,3]-sigmatropic shift. 

This reaction is related to the Claisen Rearrangement, Ficini-Claisen Rearrangement, and Overman Rearrangement. 

Cobalt Oxazoline Palladacycles (COPs) are organocobalt-palladium complexes which catalyse the asymmetric rearrangements of non-chiral allylic trichloroacetamidates with very high enantiomeric selectivity (>90%) to provide chiral allylic amines [it is an aza-Claisen rearrangement, The Overman Rearrangement Overman Carpenter Org React 66 2005, Kirsch, Overman and Watson J Org Chem 69 8101 2004] and in the presence of phenols stereospecific cross-couphng also occurs to provide chiral phenoxyallyl ethers with veiy high (>90%) enantiomeric selectivity [Kirsch, Overman and White Org Lett 9 911 2007, Overman Carpenter Org React 66 2005]. 

SYNTHESIS OF THE CHIRAL BUILDING BLOCKS APPLICATIONS OF THE CLAISEN-JOHNSON AND OVERMAN REARRANGEMENTS... 

This last class of peptide-scaffold hybrids (Q) is readily synthesized from the Claisen and Overman rearrangements products. Some examples of peptide-carbohydrate hybrids are shown in Figure 14.5. 

The synthesis of azo sugars has been achieved via an aza-Claisen rearrangement (or Overman rearrangement) of 2-C-hydroxymethyl glycals (Scheme 19). ... 

In 1997 the first asymmetric aza-Claisen rearrangement was reported by Overman et al. [55], which made use of diamines as bidentate ligands for Pd(II), allowing for moderate enantioselectivities. In the same year, Hollis and Overman described the application of the planar chiral ferrocenyl palladacycle 38 as a catalyst for the enantioselective aza-Claisen rearrangement of benzimidates 39 (Fig. 19) [56]. A related ferrocenyl imine palladacycle provided slightly inferior results, while a benzylamine palladacycle lacking the element of planar chirality was not able to provide any enantioselectivity [57]. 

A significant breakthrough was achieved by Overman and Donde in 1999 they reported the first highly selective catalyst 41 for the aza-Claisen rearrangement of benzimidates 39 (Fig. 20) [58]. Enantioselectivities were in most cases good to very good. 

Liebigs Ann Chem 654 180 1962]. It is readily converted into its trichloroacetimidate esters by reaction with ally lie alcohols in CH2CI2, in the presence of DBU at to ambient temperatures [Anderson Overman J Am Chem Soc 125 12412 2003, Kirsch et al. Org Lett 9 911 2007] similarly prepared trichloroacetimidates of allylic alcohols were shown to undergo ether-directed Pd(II)-catalysed aza-Claisen rearrangements [Jamieson Sutherland Tetrahedron 63 2132 2007], and bis-trichloroacetimidates from 2-aminopropane-l,3-diols yielded dihydrooxazines through an acid catalysed cychsation [Rondot et al. Org Lett 9 247 2007],... 

Since its discovery in 1912 [1], the Claisen rearrangement, namely thermal [3,3]-sigmatropic rearrangement of an allyl vinyl ether, has received great attention as one of the most powerful methods for stereoselective carbon-carbon bond formation. Metal catalysis has been introduced to significantly accelerate the Claisen rearrangement even at room temperature. A hterature survey on metal catalysis of Claisen rearrangements is available in excellent reviews by Lutz, Overman, Frauenrath and Hiersemann [2]. 

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