Chiral scandium catalyst

Table 3. Enantioselective Diels-Alder reactions with a chiral scandium catalyst,...

Catalytic asymmetric 1,3-dipolar cycloaddition of a nitrone with a dipolarophile has been performed using a chiral scandium catalyst [31]. The chiral catalyst, which was effective in asymmetric Diels-Alder reactions, was readily prepared from Sc(OTf)3, (7 )-(-i-)-BINOL, and d5 -l,2,6-trimethylpiperidine. The reaction of benzylbenzylide-neamine A-oxide with 3-(2-butenoyl)-l,3-oxazolidin-2-one was performed in the presence of the chiral catalyst to yield the desired isoxazolidine in 69 % ee with perfect diastereoselectivity (endolexo = > 99 1) (Sch. 8) [31,46], It was found that reverse enantioselectivity was observed when a chiral Yb catalyst, prepared from Yb(OTf)3, the same (i )-(-i-)-BINOL, and cd-l,2,6-trimethylpiperidine, was used instead of the Sc catalyst under the same reaction conditions. 

In the presence of a chiral zirconium " " or aluminum " catalyst, Bu3SnCN react with imines to give a-cyanoamines enantioselectively. The reaction of an imine and TMSCN gives the cyano amine with good enantioselectivity using a chiral scandium catalyst.Titanium catalysts have been used in the presence of a chiral Schiff base. Treatment of an imine with a chiral 1,4,6- triazabicy-clo[3.3.0]oct-4-ene and then HCN give the a-cyano amine with good enantioselectivity. 

A chiral scandium catalyst prepared from Sc(OTf)3, (/ )-BINOL, and DBU is effective in enantioselective aza Diels-Alder reactions (eq 24). The reaction of iV-alkylidene- or/V-arylidene-2-hydroxyaniline with cyclopentadiene proceeds in the presence of the chiral catalyst and 2,6-di-terf-butyl-4-methylpyridine (DTBMP) to afford the corresponding 8-hydroxyquinoline derivatives in good to high yields with good to excellent diastereo selectivity and enantioselectivity. 

In the presence of the chiral scandium catalyst, the 1,3-dipolar cycloaddition of benzylbenzylideneamine A -oxide with 3-(2-butenoyl)-l,3-oxazolidin-2-one proceeds to yield the endo adduct endolexo 99/1) in 69% ee (eq 25). Chiral scandium complexes prepared from Sc(OTf)3 and 2,2 -bis(oxazolyl)-BESfOL derivatives also serve as efficient catalysts. ... 

Similarly, the catalyst prepared from jdterbium triflate (Yb(OTf)3) and Bl-NOL acts as efficient enantioselective catalyst in the reaction of the previous oxazolidine derivative and cyclopentadiene. The role of the amine addition is determined (113). The analogous chiral scandium catalysts are prepared from Sc(OTf)3, BINOL, and tertiary amine in the same manner (114). As a major difference between Yb(lll) and Sc(lll) chiral catalysts, their coordination numbers are thought although Sc(III) is known to coordinate up to seven ligands, the specific coordination of Yb(III) allows up to 12 ligands (115). 

Two reports of enantioselective Danheiser annulations have appeared in the literature. Evans et al. have reported the annulation of ethyl glyoxylate (104) to form dihydrofuran products in good to excellent enantioselectivity using a chiral scandium catalyst. For example, reaction of ethyl glyoxylate (104) with tert-butyldiphenylsilylallene 105 in the presence of the chiral scandium catalyst 106 led to dihydrofuran 107 in 91% yield and 92% ee. As... 

Highly enantioselective Michael reactions of indanone and tetralone -ketoesters derivatives with a,P-unsaturated ketones promoted by a chiral scandium catalyst have been developed [123]. In the presence of Sc(OTf)3 and bipyridine (3), the reactions proceeded smoothly in dichloroethane at 40 °C to give the corresponding adducts in moderate to excellent yields with excellent enantioselectivities in most cases (Scheme 12.47). It was found that a lower concentration of the reaction mixture was key to attaining high selectivity. 

Allenylsilanes also undergo formal [3 + 2] cycloaddition reactions with enones, aldehydes, iminium species generated in situy and acid chlorides to give a variety of five-membered products (Scheme 3-111). The reaction is considered to proceed through stepwise mechanisms. The [3 + 2] cycloaddition reaction can be performed in an enatioselective manner by using the chiral scandium 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. 

Jorgensen et al. reported that C2-symmetric bis(oxazoline)-copper(II) complex 25 also acts as chiral Lewis acid catalyst for a reaction of allylic stannane with ethyl glyoxylate [37]. Meanwhile, p-Tol-BINAP-CuCl complex 26 was shown to be a promising chiral catalyst for a catalytic enantioselective allylation of ketones with allyltrimethoxysilane under the influence of the TBAT catalyst [38]. Evans and coworkers have developed (S,S)-Ph-pybox-Sc(OTf)3 complex 27 as a new chiral Lewis acid catalyst and shown that this scandium catalyst promotes enantioselective addition reactions of allenyltrimethylsilanes to ethyl glyoxylate [39]. But, when the silicon substituents become bulkier, nonracemic dihydrofurans are predominantly obtained as products of [3+2] cycloaddition. 

Although Lewis add-catalyzed carbon-carbon bond-forming reactions are now of great interest in organic synthesis, these reactions must be conducted under strictly anhydrous conditions, because most Lewis adds react immediately with water rather than the substrates, and are decomposed or deactivated. Sc(OTf)3, however, was found to be stable in water, and effectively activated carbonyl and related compounds as a Lewis add in water. Although it had already been found that lanthanide triflates (Ln(OTf)3 Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and yttrium triflate (Y(OTf)3) are stable in water and can act as Lewis acid catalysts in aqueous media [3], Sc(OTf)3 occasionally has even better properties even than Ln(OTf)3. Sc(OTf)3, moreover, worked well as a Lewis acid catalyst in several organic solvents, and chiral scandium triflates have also been developed. 

In the presence of a scandium catalyst, chiral allylic boranes open epoxides at the less substituted position to generate chiral, homoallylic alcohols. 

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... 

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). 

The first example of the use of rare earth metal complexes for asymmetric catalysis in organic solvents was reported in 1983 in chiral europium-catalyzed hetero Diels-Alder reactions. As for scandium catalysts, the first chiral catalyst was reported in 1994. Diels-Alder reactions using a chiral catalyst prepared from Sc(OTf)3, (/ )-BINOL, and an amine afforded the desired products in up to 97% ee. Following these results, many chiral rare earth metal catalysts have been developed. 

A chiral scandium complex with a new ferrocenyl-based chiral diol ligand, FER-RODIOL, was reported to be an effective catalyst for enantioselective Diels-Alder... 

It has been shown that the use of a chiral Scandium(III) catalyst in the reaction of 0 3-substituted-oxindoles with diaryliodonium salts may result in the enantioselective formation of a-arylated derivatives (76). 

Chiral titanium- and scandium-based catalysts (61 and 62, Figure 3.11) were used to accelerate the cycloadditions of acyl-l,3-oxazolidin-2-ones 60 (Scheme 3.14) with butadiene, isoprene and cyclopentadiene. The cycloadditions... 

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