Asymmetric lanthanide catalyzed

Metal-catalyzed hydrophosphination has been explored with only a few metals and with a limited array of substrates. Although these reactions usually proceed more quickly and with improved selectivity than their uncatalyzed counterparts, their potential for organic synthesis has not yet been exploited fully because of some drawbacks to the known reactions. The selectivity of Pt-catalyzed reactions is not sufficiently high in many cases, and only activated substrates can be used. Lanthanide-catalyzed reactions have been reported only for intramolecular cases and also sulfer from the formation of by-products. Recent studies of the mechanisms of these reactions may lead to improved selectivity and rate profiles. Further work on asymmetric hydrophosphination can be expected, since it is unlikely that good stereocontrol can be obtained in radical or acid/base-catalyzed processes. 

A tin(II)-catalyzed asymmetric aldol reaction and lanthanide-catalyzed aqueous three-component reaction have been used as the key steps for the synthesis of febrifugine and isofebrifugine (Scheme 8.31).293... 

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

Many other groups further studied the utility of bis(oxazolines) including Ikeda s use of biaryl bis(oxazolines) such as 250 (Fig. 9.75) in the zinc-catalyzed asymmetric alkylation of benzaldehyde with diethylzinc. " This reaction proceeded in yields up to 92% with ee up to 88%. Kodama and co-workers used a biaryl bis(oxazoline) ligand, namely, [l,l -]binaphthalenyl-2,2 -diol (BlNOL)-box 251, in the lanthanide-catalyzed asymmetric 1,3-dipolar cycloaddition of nitrones to alkenes. ... 

Recently, some efficient asymmetric Diels-Alder reactions catalyzed by chiral Lewis acids have been reported [67]. The chiral Lewis acids employed in these reactions are generally based on traditional acids such as titanium, boron, or aluminum reagents, and they are well modified to realize high enantioselectivi-ties. Although lanthanide compounds were expected to be Lewis acid reagents, only a few asymmetric reactions catalyzed by chiral lanthanide Lewis acids were reported. Pioneering work by Danishefsky et al. demonstrated that Eu(hfc)3 (an NMR shift reagent) catalyzed hetero-Diels-Alder reactions of aldehydes with si-loxydienes, but enantiomeric excesses were moderate [68]. 

Kobayashi et aL fulfilled first asymmetric synthesis of febrifugine utilizing tin(II)-catalyzed asymmetric aldol-type reaction and lanthanide-catalyzed... 

To achieve catalytic enantioselective aza Diels-Alder reactions, choice of metal is very important. It has been shown that lanthanide triflates are excellent catalysts for achiral aza Diels-Alder reactions [5]. Although stoichiometric amounts of Lewis acids are often required, a small amount of the triflate effectively catalyzes the reactions. On the basis of these findings chiral lanthanides were used in catalytic asymmetric aza Diels-Alder reactions. The chiral lanthanide Lewis acids were first developed to realize highly enantioselective Diels-Alder reactions of 2-oxazolidin-l-one with dienes [6]. 

In the presence of a catalytic amount of chiral lanthanide triflate 63, the reaction of 3-acyl-l,3-oxazolidin-2-ones with cyclopentadiene produces Diels-Alder adducts in high yields and high ee. The chiral lanthanide triflate 63 can be prepared from ytterbium triflate, (R)-( I )-binaphthol, and a tertiary amine. Both enantiomers of the cycloaddition product can be prepared via this chiral lanthanide (III) complex-catalyzed reaction using the same chiral source [(R)-(+)-binaphthol] and an appropriately selected achiral ligand. This achiral ligand serves as an additive to stabilize the catalyst in the sense of preventing the catalyst from aging. Asymmetric catalytic aza Diels-Alder reactions can also be carried out successfully under these conditions (Scheme 5-21).19... 

Schaus, S. E. Jacobsen, E. N. (2000) Asymmetric ring opening of meso- epoxides with TMSCN catalyzed by (pybox)lanthanide complexes., Org. Lett, 2 1001-1004. 

Bercaw has investigated the application of the 6 2-symmetric, enantiomerically pure lanthanide metallocene derivative (i ,A)-BnBpYH 34 as a catalyst for the asymmetric cyclization/hydrosilylation of 1,5- and 1,6-dienes. Although 34 displayed high activity for the reaction of a number of dienes, asymmetric induction was low. In the best case, reaction of 3,3-dimethyl-1,5-hexadiene with phenylsilane catalyzed by 34 gave silylated cyclopentene 35 in 95% yield with 50% ee (Equation (25)). 

The lanthanide complex-catalyzed asymmetric glyoxylate-ene reaction has also been reported (Scheme 8C.5) [17,18], Although the reaction of glyoxylate and a-methylstyrene... 

Scheme 8C.5. Asymmetric carbonyl-ene reaction catalyzed by chiral lanthanide complexes.

Complex LSB 9 is readily prepared either by the reaction of La(0 Pr)3 with 3 equiv. of B1NOL followed by the addition of NaO Bu (3 equiv.) or by the reaction of LaCl nfLO with sodium binaphthoxide. The complex 9 is stable to oxygen and moisture and has been proven to be effective in the catalytic Michael reaction of various enones with either malonates or p-keto esters. The Michael adducts with up to 92% ee were obtained in almost quantitative yield. Typical results with malonates are summarized in Table 8D.1 (Ln = lanthanide) [18], In general, the use of THF as solvent gave the best results except for the case of the LSB-catalyzed reaction of rmns-chalcone with dimethyl malonate, wherein the use of toluene was essential to give the adduct with good enantiomeric excess. The effects of the central metal (La, Pr, and Gd) on asymmetric induction were also examined in the same reaction, and LSB was found to be the best catalyst. 

Kobayashi reported an asymmetric Diels-Alder reaction catalyzed by a chiral lanthanide(III) complex 24, prepared from ytterbium or scandium triflate [ Yb(OTf)3 or Sc(OTf)3], (Zf)-BINOL and tertiary amine (ex. 1,2,6-trimethylpiperidine) [30], A highly enantioselective and endose-lective Diels-Alder reaction of 3-(2-butenoyl)-l,3-oxazolidin-2-one (23) with cyclopentadiene (Scheme 9.13) takes place in the presence of 24. When chiral Sc catalyst 24a was used, asymmetric amplification was observed with regard to the enantiopurity of (/ )-BINOL and that of the endoadduct [31 ]. On the other hand, in the case of chiral Yb catalyst 24b, NLE was affected by additives, that is, when 3-acetyl-l,3-oxazolidin-2-one was added, almost no deviation was observed from linearity, whereas a negative NLE was observed with the addition of 3-pheny-lacetylacetone. 

Insofar as asymmetric synthesis is concerned, some pioneering work was conducted by Marks and co-workers through their demonstration of enantioselectivity in the samarium or lanthanide complex-catalyzed cyclization of aminoalkenes, an analogy of the simple reaction exemplified in Scheme 36. As before, the reaction works best for Se-unsaturated amines where the product of an exocyclic ring closure pathway is a cyclopentylamine. In the most favorable cases, high turnover to the desired product is observed [110] (Scheme 37). 

The reaction is catalyzed by lanthanide complexes CpjfLnR,41 although noble metal catalysts, notably rhodium, are most widely applied, particularly in asymmetric hydroboration,42 The mechanism is likely to be similar to hydrosilylation. The products may be oxidized with H202 and converted to alcohols or amines. 

Figure 45 Asymmetric hetero-Diels-Alder reactions catalyzed by chiral lanthanide shift reagents...

LnCpa and Cp2LnCl complexes initiate the dehalogenation of aiyl and vinyl halides by NaH. Chemoselectivity is observed in the reduction of m-bromochloro-benzene and p-iodochlorobenzene to chlorobenzene [192], The asymmetric reduction of methylphenyl glyoxylate to methyl mandelate by NADH models is catalyzed by chiral lanthanide )6-diketonates [193],... 

We [23] and others [24] have also reported the lanthanide complex-catalyzed asymmetric glyoxylate-ene reaction (Scheme 5). Although the reaction of glyox-ylate and a-methylstyrene proceeds catalytically under the influence of the lanthanide Ln(NTf2)3 or Ln(OTf)3 [25] complexes with chiral ligands, the enanti-oselectivity is low-to-moderate. 

Although the transition states of the chiral lanthanide(III)-catalyzed reactions are rather complicated due to the specific coordination number and stereochemistry of lanthanide(III), the sense of asymmetric induction in the chiral... 

Although the transition states of the chiral lanthanide(III)-catalyzed reactions are rather complicated due to the specific coordination number and stereochemistry of the lanthanide(III), the sense of asymmetric induction in the chiral scandium-catalyzed reactions can be rationalized by assuming an intermediate octahedral Sc(III)-dienophile complex (scheme 14). The axial chirality of (R)-binaphthol is transferred to the amine,... 

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