Enantioselective catalysts, Claisen rearrangements

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]. 

Moyano A, Rosol M, Moreno RM, Lopez C, Maestro MA (2005) Oxazoline-mediated interannular cyclopalladation of ferrocene chiral palladium(II) catalysts for the enantioselective Azti-Claisen rearrangement. Angew Chem Int Ed 44 1865-1869... 

Scheme 16.13 Enantioselective aza-Claisen rearrangement with chiral palladium bis(ADC) catalysts.

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. 

Weiss ME, Fischer DF, Xin ZQ, Jautze S, Schweizer WB, Peters R (2006) Practical, highly active, and enantioselective ferrocenyl-imidazoline palladacycle catalysts (FIPs) for the Aza-Claisen rearrangement of A-para-methoxyphenyl trifluoroacetimidates. Angew Chem Int Ed 45 5694-5698... 

The enantioselective a-allylation of a ketone has become increasingly important. Martin Hiersmann of the Technische Universitat Dresden reports (Tetrahedron Lett. 2004,45, 3647) that a-ketoesters readily form enol ethers, such as 6. On exposure to an enantiomerically-purc Cu catalyst, the enol ethers undergo facile Claisen rearrangement, leading to the allylated ketone (e.g. 7) with high enantiomeric... 

In a follow-up paper, Jacobsen showed that the chiral guanidinium 80 provides catalytic power and enantioselectivity in the Claisen rearrangement systems such as Reaction 6.36. Here the reaction proceeded in 81 percent yield with an ee of 84 percent. The reaction is also diasteroselective for Reaction 6.37, the diastere-oselectivity increases from 11 1 without catalyst to 16 1 with 80. 

Uyeda, C. Jacobsen, E. N. Enantioselective Claisen rearrangements with a hydrogen-bond donor catalyst, 7. Am. Chem. Soc. 2008,130,9228-9229. 

Several chiral organoaluminum Lewis acids catalyze the Claisen rearrangement of achiral allyl vinyl ethers to furnish chiral (3, y-unsaturated aldehydes with good enantioselectivity. Among the most effective catalysts is ATBN-F, a chiral aluminum tris(P-naphthoxide) species prepared from enantiomerically pure binapthol. ... 

Rearrangements. By applying Corey s diazaborolidine catalyst to aromatic Claisen rearrangement, a chiral side chain is introduced into the ortho position of catechols. Highly enantioselective rearrangement of 0-allylimidates 111 containing a methoxybinaphthyl substituent has been witnessed. ... 

Linton and Kozlowski have installed quaternary centers at oxindole C3 in enantioselective fashion via the Pd-catalyzed rearrangement of 2-allyloxy indoles (Scheme 2) [16]. For example, indole 7 underwent an enantioselective Meerwein-Eschenmoser-Claisen rearrangement in the presence of Pd(SbF6)2 and the chiral phosphinooxazoline ligand 8 to afford oxindole 9 in 89% yield and 89% ee. A two-point coordination of the chiral palladium catalyst to the C3 carbonyl and C2 oxygen (6-membered coordination system) has been proposed to rationalize the enantioselectivity of the transformation. Modest to good enantioselectivities were also observed for a series of bisphosphine chiral ligands. 

The Claisen rearrangement can be catalyzed by Lewis acid catalysts (Figure 11.54), and the use of chiral Lewis catalysts provides a path to diastereoselective and enantioselective Claisen rearrangements. ... 

The enantioselectivity of [Cu(S,S)-tBu-box](OTf)2 (13b)Claisen rearrangement is explained as follows (Fig. 2.4). The alkoxycarbonyl and ether oxygens coordinate in a bidentate fashion to the Cu (box) complexes. The square planer geometry around the copper(II) cation has been proposed and a chair-hke transition-state model is suggested. The aUyUc ether moiety should approach the vinyl ether moiety from the opposite direction of the t-Bu substituents on the box-hgand. The Cu"(box) catalyst differentiates between two enantiomeric chair-hke transition state by selective coordination of enantiotopic lone pairs on oxygen to form (S,S,pro-S)-14a. 

The catalyzed version of this rearrangement is useful for the synthesis of medium and large-sized carbocycles [84]. For example, the enantioselective synthesis of carbocyclic natural product, (-)-9,10-dihydroecklonialactone B 121 was done successfully by catalytic asymmetric Claisen rearrangement of a GosteU-type aUyl vinyl ether 122 in the presence of (S,S)-Cu (box)-catalyst A to produce a chiral a-ketoester 123, as a building block unit [85]. 

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