Lanthanide binaphtholate, chiral

Kobayashi et al. have reported the use of a chiral lanthanide(III) catalyst for the Diels-Alder reaction [51] (Scheme 1.63, Table 1.26). Catalyst 33 was prepared from bi-naphthol, lanthanide triflate, and ds-l,2,6-trimethylpiperidine (Scheme 1.62). When the chiral catalyst prepared from ytterbium triflate (Yb(OTf)3) and the lithium or sodium salt of binaphthol was used, less than 10% ee was obtained, so the amine exerts a great effect on the enantioselectivity. After extensive screening of amines, ds-1,2,6-... 

The synthesis of a series of chiral organophosphine oxide/sulfide-substituted binaphtholate ligands has recently been reported by Marks and Yu and their corresponding lanthanide complexes characterized. These complexes, generated in situ from Ln[N(TMS)2]3, cleanly catalysed enantioselective intramolecular hydroamination/cyclisation of 1-amino-2,2-dimethyl-4-pentene albeit with a low enantioselectivity of 7% ee (Scheme 10.82). 

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... 

Several chiral lanthanide(III) Lewis acid catalysts, derived from chiral binaphthols, have been used in the cycloaddition reactions of cyclopentadiene with substituted iV-acryloyl-1,3-oxazolidin-2-ones. A catalyst derived from ytterbium triflate, (R)-binaphthol... 

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). 

Shibasaki has developed a family of heterobimetallic complexes 274 to 279 derived from the alkali metal diaryloxy salt of chiral binaphthols and lanthanide alkoxides these complexes function as catalysts in a variety of useful enantiose-lective addition reactions [127,128,13j]. 

A conceptually different [4-1-2] cycloaddition catalyzed by a chiral lanthanide complex has been disclosed. The inverse electron demand Diels Alder reaction of 3-methoxycarbonyl-2-pyrone (67) and enol ethers or sulfides [135] was catalyzed by a chiral ytterbium(III) triflate-binaphthol complex in the presence of diisopropylethylamine (Scheme 51) [136]. Thermal decarboxylations of bicyclic lactones such as 68 are known to yield dienes which may undergo subsequent pericyclic reactions [137] thus, the adducts of this process are potentially useful chiral building blocks. The nature of the substituent on the 2k component was found to be crucial for the realization of high enantioselectivity. 

Although Sc(OTf)3 has slightly different properties compared with other lanthanide triflates,the chiral Sc catalyst could be prepared from Sc(OTf)3, (R)-(+)-binaphthol, and a tertiary amine in dichloromethane [76]. The catalyst was also found to be effective in the Diels-Alder reactions of acyl-1,3-oxazolidin-2-ones with dienes. The amines employed in the preparation of the catalyst influenced the enantioselectivities strongly. For example, in the Diels-Alder reaction of 3-(2-butenoyl)-l,3-oxazolidin-2-one with cyclopentadiene (CH2C12, 0°C), the enantiomeric excesses of the endo adduct depended crucially on the amines employed aniline, 14% ee lutidine, 46% ee triethylamine, 51% ee 2,2,6,6-tetram-ethylpiperidine, 51% ee diisopropylethylamine, 69% ee 2,6-dimethylpiperid-ine, 69% ee 1,2,2,6,6-pentamethylpiperidine, 72% ee and cis-1,2,6-trimethyl-piperidine, 84% ee. 

First, Yb(OTf)3 was chosen as a representative of the lanthanide triflates. The chiral Yb triflate was prepared in situ from Yb(OTf)3, (R)-binaphthol, and a tertiary amine ... 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

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,... 

Obviously, catalyst structure is the key to determining enantiocontrol in the phospho-aldol reaction. In the lanthanide heterobimetaUic systems, backbone chirality within the binaphthol leads to generation of a stereocentre at the metal itself and does so with complete diastereospecificity. This is yet another example of the concept of chirality tra s/er, wherein fixed stereochemistry at one site leads to control over stereochemistry at a more remote site and is one of the most powerful concepts in catalyst design. It is the presence of such a highly... 

These intramolecular hydroaminations lead to the formation of chiral products. Thus, several studies have been devoted to developing ligands for enantioselective, intramolecular hydroaminations of olefins catalyzed by lanthanide complexes. ° Selectivities of these reactions with chiral cyclopentadienyl derivatives have been modest. Selectivities have been higher with catalysts containing non-cyclopentadienyl ligands. Some of tlie most selective catalysts are the yttrium complexes of the bis(thiolate) ligands reported by Livinghouse, the scandium and lutetium complexes of 33 -disubstituted binaphtholates reported by Hultzsch, and the BINAM-based bisamidates of Schafer.- A representative cyclization catalyzed by a member of each of these classes of catalyst are shown in Equation 16.64. 

---