Enones chiral metal complexes

Lewis acid-catalyzed cycloaddihon is also a powerful synthehc method, and various types of cycloaddihon have been reported. In parhcular, enantioselective variants using chiral Lewis acids have been comprehensively studied some of these were used as key reactions for natural product syntheses [5]. However, they generally require one or more heteroatoms in the substrates, such as enones or enoates, to which (chiral) Lewis acids can coordinate. In conhast, in the case of transition-metal-catalyzed cycloadditions, the metals coordinate direchy to the tt-electron and activate unsaturated motifs, which means that the heteroatom(s) are unnecessary. Moreover, the direct coordinahon to the reachon site can realize highly enantioselechve reachon using chiral transihon-metal complexes. 

The enantioselective alkylation of indoles catalyzed by C2-symmetric chiral bisoxazoline-metal complexes 90 encouraged many groups to develop superior asymmetric catalysts which are cheap, accessible, air-stable and water-tolerant. Other analogs of the bisoxazoline-metal complex 90 as chiral catalysts and new Michael acceptors have also been studied. The enantioselective alkylations of indole derivatives with of-hydroxy enones using Cu(II)-bis(oxazoline) catalysts 93 and 94 provided the adducts in good yields... 

Initial screening reactions carried out with enone 460 and A -methyl pyrrole in the presence of 10mol% of a series of chiral bis(oxazoline)/metal complexes in CH2CI2 as solvent, revealed complexes 458 and 459 as the most effective (Equation 110) <2005JA4154>. Using these catalysts Eriedel-Crafts adduct 461 (R = BnCH2) was formed in yields of 86% and 80% and most notably, with ee 92% and 91%, respectively. Indole derivatives 462 worked as efficiently as pyrroles and provided adducts 463 in good to excellent yields and enantiomeric excess (Equation 111). 

The reaction is catalyzed by transition metal complexes coordinated with phosphine ligands. Since chiral phosphine ligands are the chiral auxiliaries most extensively studied for transition metal catalyzed asymmetric reactions, one can use the accumulated knowledge of the chiral phosphine ligands for the asymmetric reaction. The asymmetric 1,4-addition of aryl- and 1-alkenylboronic acids to enones proceeded with high enantioselectivity in the presence of a chiral phosphine-rhodium catalyst (Table 2). 

The efficiency of transmetalation from boron to palladium in the catalytic 1,4-addition of aryl or 1-alkenylboronic acids to enones encouraged us to extend the protocol to the addition of aryl- and 1-alkenylboronic adds to aldehydes in an aqueous solution (Eq. 4). The insertion of carbonyl groups into transition metal-carbon bonds has not received much attention, but the catalytic use of transition metals may allow such addition of various organometallics which are inert without a catalyst, the asymmetric addition using a chiral phosphine complex, or the reaction in an aqueous phase. 

Reaction with alkaline peroxide (or hypochlorite) and a chiral catalyst allows the asymmetric epoxidation of enones. Excellent asymmetric induction has been achieved using metal-chiral ligand complexes, such as those derived from lanthanides and (/ )- or (5)-BlNOL. Alternatively, phase-transfer catalysis using ammonium salt derivatives of Cinchona alkaloids, or the use of polyanuno acid... 

The group of Li and Chan [22] reported that metal complexes of bipyridyl-based diphosphane, such as P-Phos, were good catalysts in Rh(I)-catrJyzed 1,4-addition of boronic acids to enones. The rigidity of the bipyridyl backbone allows good transfer of chiral information (Scheme 5.7). The obtained results using a variety of a,p-unsaturated ketones as well as arylboronic acids led the authors to... 

Another impressive example of the transition metal-catalyzed Michael reaction was reported by Sawamura and Ito in 1992 (Scheme 6) [7]. a-Methylcyanoacetate was treated with enones using 1 mol% Rh-TRAP (12) complex, and the corresponding adduct 13 was formed in up to 93 % ee. For this reaction, the trans-coordination mode of the chiral diphosphine 12 was essential for high asymmetric induction. It was proposed that coordination of the nitrile group to Rh, then oxidative addition of the active methine C-H bond gave not the a-C-bound enolate, but the nitrile-coordinating enolate 14, which was considered to be a reactive intermediate. The unique structure of this enolate was supported by X-ray analysis of a similar achiral Ru-cyanoacetate complex [8]. 

The first part of this chapter describes recent advances in the use of novel, chiral, alkali metal free-lanthanoid-BINOL derivative complexes for a variety of efficient, catalytic, asymmetric reactions. For example, using a catalytic amount of chiral Ln-BINOL derivative complexes, asymmetric Michael reactions and asymmetric epoxidations of enones proceed in a highly enantioselective manner. 

Various other examples of nickel-catalyzed asymmetric addition of organozinc compounds to enones have been reported using different chiral auxiliaries111-117. Complexes of other transition metals, such as rhodium118-119, lanthanum120, and titanium121, have also been found to promote asymmetric conjugate addition of enolates to a,/i-unsaturated ketones. 

By virtue of a deep understanding of his LnM3tris(BINOLate)3 complexes (Ln = rare-earth metal, M = alkali metal) based on evidence from X-ray analysis and other experiments, Shibasaki developed chiral heterobimetallic yttrium(in) lithium(i) tris(binaphtholate) complex 22, which can promote the catal) ic enantioselective aza-Michael reaction of metho g lamine to enones in excellent yields with up to 97% ee as a Lewis-acid-Lewis-acid cooperative catalyst (Scheme 2.17). Transformation of the 1,4-adducts 23 afforded the corresponding optically active aziridines 24 in high yields. 

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