Ytterbium catalytic asymmetric Diels-Alder reaction

Kobayashi and co-workers exploited the use of lanthanide in a variety of achiral reactions and extended them into several catalytic asymmetric reactions. Their work commenced with catalytic asymmetric Diels-Alder reactions [32], The reaction was performed with a chiral ytterbium catalyst prepared from Yb(OTf)3, binaphthol and a tertiary amine. The amine significantly influenced reaction selectivity. When triethyl-amine was used in the preparation of the catalyst, the desired product was obtained in moderate ee (33%) (Table 8, entry 1). After screening several reaction conditions, they found that, in general, bulky amines gave better results (entries 2-6). They suggested interesting explanations of this experimental result on the basis of investigations into catalyst structure. Consequently, the use of cw-l,2,6-trimethylpiperidine combined with 4 A molecular sieves (4A MS) was found to produce the best result (yield 77%, endolexo = 89/11, endo = 95% ee) (entry 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... 

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 Aza Diels-Alder Reactions Promoted by Chiral Ytterbium Catalysts... 

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