Enantioselective catalysts, Claisen

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... 

Jautze S, Seiler P, Peters R (2007) Macrocyclic ferrocenyl-bisimidazoline palladacycle dimers as highly active and enantioselective catalysts for the Aza-Claisen rearrangement of Z-conflgured A-para-methoxyphenyl trifluoroacetimidates. Angew Chem Int Ed 46 ... 

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. 

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. 

NAP-MgO acts as a bifunctional heterogeneous catalyst for the Claisen-Schmidt condensation (CSC) of benzaldehydes with acetophenones to yield chalcones, followed by asymmetric epoxidation (AE) to afford chiral epoxy ketones in moderate to good yields and impressive enantioselectivities (ee s). NAP-MgO, in combination with the chiral auxiliary (11 ,21 )-(- -)-1,2-diphenyl-1,2-ethylenediamine (DPED), catalyzed the asymmetric Michael addition of malonates to cyclic and acyclic enones. 

The Bode group disclosed NHC-catalyzed highly enantioselective annulations of a,P,P -trisubstituted enals with cyclic sulfonylimines. Mechanistically, it is proposed that the combination of enal and free NHC leads to the formation of the Breslow intermediate, which is oxidized to form the key a,p-unsaturated acyl azolium. Tautomerization between the imine and the enamine occurs readily in the presence of base, and the enamine is intercepted by the key a,p-unsaturated acyl azolium to form a hemiaminal which further engages in a Stork-Hickmott-Stille-type annulation via a tight-ion-pair/aza-Claisen type transition state. Lactam formation follows the protonation of enolate and brings about catalyst turnover to complete the catalytic cycle (Scheme 7.113). 

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. ... 

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