Lanthanide catalysts, chiral

Because ketones are generally less reactive than aldehydes, cycloaddition reaction of ketones should be expected to be more difficult to achieve. This is well reflected in the few reported catalytic enantioselective cycloaddition reactions of ketones compared with the many successful examples on the enantioselective reaction of aldehydes. Before our investigations of catalytic enantioselective cycloaddition reactions of activated ketones [43] there was probably only one example reported of such a reaction by Jankowski et al. using the menthoxyaluminum catalyst 34 and the chiral lanthanide catalyst 16, where the highest enantiomeric excess of the cycloaddition product 33 was 15% for the reaction of ketomalonate 32 with 1-methoxy-l,3-butadiene 5e catalyzed by 34, as outlined in Scheme 4.26 [16]. 

In 1996, the first example of the catalytic enantioselective aza Diels-Alder reactions of azadienes using a chiral lanthanide catalyst was reported [4], In this article, successful examples of such catalytic reactions are surveyed. 

The most frequently encountered, and most useful, cycloaddition reactions of silyl enol ethers are Diels-Alder reactions involving silyloxybutadicncs (Chapter 18). Danishefsky (30) has reviewed his pioneering work in this area, and has extended his studies to include heterodienophiles, particularly aldehydes. Lewis acid catalysis is required in such cases, and substantial asymmetric induction can be achieved using either a chiral lanthanide catalyst or an a-chiral aldehyde. 

The same group extended this work to a cyclic imine (Scheme 5-47) better results were obtained with heterobimetallic lanthanide catalysts than with chiral titanium alkoxides. 

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. 

Chiral lanthanide catalysts, (R)- and (5 )-Me2Si(Me4Cp)[(—)-menthylCp]SmCH (SiMe3)2, are effective for the asymmetric hydrosilylation of 2-phenyl-1-butene with H3SiPh, giving (R)- and (5 )-2-phenyl-2-silylbutanes with 68 and 65% ee, respectively (Scheme 23)43. 

The metal-catalyzed allylboration is efficient to control both diastereoselectivity and enantioselectivity. The Sc(OTf)3-catalyzed reaction of chiral allyl boronates resulted in 90-98% ee for representative aldehydes (Equation (158)).624 628 629 The first catalytic enantioselective allylboration and crotylboration was achieved by a chiral lanthanide catalyst (Equation (159)).630... 

These woikers have also examined asymmetric induction in the process in some detail. For instance, cycloaddition of sugar aldehyde (167) occurred to afford only adduct (169) at high pressure (equation 80). It was suggested that the Diels-Alder reaction proceeds via diene attack on the aldehyde conformation shown in (168) from the least congested face. Other chiral aldehydes have been investigated by this group, as has the e ect of lanthanide catalysts upon the extent of asymmetric induction. Summaries of this work have recently been published. ... 

Although asymmetric versions of aza Diels-Alder reactions using chiral auxiliaries have been reported, only one example uses a stoichiometric amount of a chiral Lewis acid [44]. The first reported example of a catalytic enantioselective aza Diels-Alder reaction employed a chiral lanthanide catalyst [45]. A chiral ytterbium or scandium catalyst, prepared from Yb(OTf)3 or Sc(OTf)3, (i )-BINOL, and DBU, is effective in the enantioselective aza Diels-Alder reactions. The reaction of A-alkylidene- or N-arylidene-2-hydroxyaniline with cyclopentadiene proceeded in the presence of the chiral catalyst and 2,6-di-rerf-butyl-4-methylpyridine (DTBMP) to afford the corresponding 8-hydroxyquinoline derivatives in good to high yields with good to excellent diastereo- and enantioselectivity (Eq. 15). 

Catalytic Asymmetric Diels-AIder Reactions and Hetero Diels-Alder Reactions Promoted by Chiral Lanthanide Catalysts... 

Inanaga and co-workers developed another type of lanthanide catalyst for asymmetric hetero Diels-Alder reaction (Sch. 3) [36]. Benzaldehyde reacted with 1-meth-oxy-3-(trimethylsiloxy)-l,3-butadiene in the presence of chiral Yb(III) phosphate to afford the corresponding adduct in 77% yield and in 70% ee. Because the reaction mixture was heterogeneous, they tried to make a clear solution by addition of ligands and examined their effects on the reactions. Pyridine and pyridine derivatives dissolved the catalyst and chemical yields and ee were usually improved. The best result (93% ee) was obtained in the reaction of p-anisaldehyde with 2,6-lutidine as additive. 

Mukaiyama aldol reactions are useful means of constructing complex molecules for the total synthesis of natural products. Although catalytic asymmetric Mukaiyama aldol reactions have been achieved by use of a variety of chiral Lewis acids [42], no report of the use of chiral lanthanide catalysts was available until recently, despite the potency of these catalysts. Shibasaki and co-workers reported the first examples of chiral induction with chiral lanthanide complexes (Sch. 7) [43]. Catalysts prepared from lanthanide triflates and a chiral sulfonamide ligand afforded the corresponding aldol products in moderate enantiomeric excess (up to 49% ee). 

Many researchers have refrained from using lanthanide complexes in stereoselective Diels-Alder reactions, perhaps due to large coordination spheres which can accommodate up to a dozen ligands. The rather daunting task of interpreting the identity of active catalysts and substrate-catalyst complexes among the myriad possible options has not hampered the development of some quite useful chiral lanthanide catalysts. 

The use of ZnCh and (3-diketonate complexes of lanthanide metals give predominately endo-type addition products and CF diastereofacial selectivity. The CF stereoselectivity seems to prevail in cases using ZnCh as a catalyst and chiral aldehydes that contain groups that do not chelate to the metal. The selectivity clearly decreases when aldehydes that can form chelates are used. The lanthanide catalysts also exhibit strong CF-type selectivity with nonchelating chiral aldehydes. This trend seems to hold with most chelating aldehydes however, ACF products have been reported when a-alkoxy aldehydes are used in the reaction.56 The stereochemical consequences of using lanthanide catalysts with a-alkoxy aldehydes are therefore not predictable. 

An enantioselective version of the above reactions has been reported. Lewis acids such as Yb(OTf)3 can profoundly affect the stereochemical outcome of the carbonyl ylide 1,3-dipolar cycloadditions [137]. This provided an indication to effect asymmetric carbonyl ylide cycloaddition using a chiral Lewis acid. The first example of such asymmetric induction using the chiral lanthanide catalysts has been reported [138,139]. For example, the reaction of diazoacetophenone 89 with benzyloxyacetaldehyde, benzyl pyruvate and 3-acryloyl-2-oxazoHdinone in the presence of chiral 2,6-bis(oxazolinyl)pyridine ligands and scandium or ytterbium complexes furnished the corresponding cycloadducts 165-167 with high enantioselectivity (Scheme 53). 

Chiral lanthanide catalysts studied for the asymmetric hydrogenation of unfunctionalized olefins. 

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