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

Metallocyclic Ferrocenyl Ligands

“…Thus the planar chirality is predominantly responsible for the high enantioselectivity observed in the (S,R p ) 2 -90 catalyzed rearrangement. This model of enantioinduction has also been proposed for the rearrangement of trifluoroacetimidates 117 with the catalyst derived from (R p ) 2 -119 [34]. There is the possibility that for the more electron-rich allylic N-arylbenzimidates CÀN bond formation is faster, such that palladium-olefin coordination is then rate-limiting [86].…”

“…The origin of diastereoselection with the related N-sulfonyl-substituted imidazolines 32 is ascribed to the operation of a relay controlling the chirality of the sulfonated pyramidal nitrogen, and thus the position of the equilibrium between 32 and 32 0 [34]. Good diastereoselectivities have been reported for palladation reactions leading to ferrocenyl, pentamethylferrocenyl and pentaphenylferrocenyl derivatives (4R,5R, S p ) 2 -33, and in each case the highest selectivity (20 : 1) was achieved with R 2 ¼ para-tolyl (Scheme 12.10).…”

“…It was noted that use of 2 equiv of silver trifluoroacetate gave a new diamagnetic species that resulted in the rearrangements proceeding extremely slowly. In contrast, addition of 4 equiv gave the active catalyst as a paramagnetic species, presumably due to oxidation of the ferrocene moiety to a ferrocenium cation [34].…”

“…The catalyst precursor (S p ) 2 -119, with either R ¼ H or R ¼ Me, was also applied successfully to the rearrangement of (Z)-117, although a catalyst loading of 5 mol% was required in each case (Table 12.2, entries 5 and 6) [34]. A significant development with (Z)-117 was achieved with the ferrocenyl bispalladacycle (S p ,S p ) 2 -120 which, following activation with silver tosylate, gave excellent yields and enantioselectivities with just 0.1 mol% catalyst loading [35].…”

Molecular Rearrangements: Part 1. Pericyclic Reactions

“…Product distributions observed with electron-rich ortho-substituents have been attributed to competing chair-and boat-like transition states for the initial [ (25) have been shown to undergo nitrosation adjacent to the carbonyl group followed by [3,3]-sigmatropic rearrangement to give oxazine products (26). 23 Stereodefined monoprotected allylic 1,2-diol (27) and its diastereomer have been de converted into stereodefined propenylimidazolidinone (30) and its diastereomer by successive diastereoselective [3,3]-sigmatropic rearrangements of the initially formed allylic cyanates (28), (29), or their diastereomers as shown. 24 The stereoselectivity of the MOM ether-directed, transition metal-catalysed azaClaisen rearrangement of O-(4-methoxymethoxypent-2-en-1-yl) trichloroacetimidate to N -(4-methoxymethoxypent-1-en-3-yl) trichloroacetimidate has been studied under various conditions.…”