Ytterbium triflate yb

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-... 

Ytterbium triflate [Yb(OTf)3] combined with TMSG1 or TMSOTf are excellent reagents for the conversion of a-methyl styrene and tosyl-imines into homoallylic amides 32 (Equation (19)) (TMS = trimethylsilyl).29 These conditions produce the first examples of intermolecular imino-ene reactions with less reactive imines. Typically, glyoxalate imines are necessary. A comprehensive examination of the lanthanoid metal triflates was done and the activity was shown to directly correlate with the oxophilicity scale. The first report used preformed imines, and subsequently it was found that a three-component coupling reaction could be effected, bypassing the isolation of the intermediate imine.30 Particularly noteworthy was the successful participation of aliphatic aldehydes to yield homoallylic amines. 

Readion of anisole (1) with acetic anhydride was chosen as a model, and ytterbium trifluoromethanesulfonate (ytterbium triflate, Yb(OTf)3) was the first RE(OTf)3 representative used. Several reaction conditions were examined the results are summarized in Table 1. When acetic anhydride, acetonitrile, or nitromethane was used as a solvent (entries 4—10), the reaction mixture became homogeneous and the acylation reaction proceeded smoothly. Nitromethane gave the highest yield of4-methoxyaceto-phenone (2) (entries 7-10). On the other hand, in carbon disulfide, dichloroethane, or nitrobenzene (entries 1-3), the reaction mixture was heterogeneous and the yield of 2 was low. It was noted that the acylation proceeded quantitatively when a catalytic amount of Yb(OTf)3 was used (0.2equiv., entry 9). Even when 0.05 equiv. of the catalyst was employed, 2 was obtained in 79 % yield (entry 10). 

Lopez et al. reported the selective activation of armed NPGs (e. g. 136) in the presence of armed glycosyl fluorides (e. g. 137) on treatment with IDCP (O Scheme 18a) [49]. On the other hand, armed and disarmed glycosyl fluorides 139, could he activated in the presence of armed NPGs (e. g. 140) on treatment with ytterbium triflate (Yb(OTf)3) (O Scheme 18b). 

On the other hand, we have found that the Mukaiyama aldol reaction of benzalde-hyde with silyl enol ether 1 was catalysed by ytterbium triflate (Yb(OTf)3) in water-THF (1/4) to give the corresponding aldol adduct in high yield (Equation (1)). When this reaction was carried out in dry THE (without water), the yield of the aldol adduct was very low (ca. 10%). Thus, this catalyst is not only compatible with water but also activated by water probably because of dissociation of the counteranions from the Lewis acidic metal. Furthermore, these catalysts can be easily recovered and reused. 

In the presence of 10 mol% of ytterbium triflate (Yb(OTf)3> a representative lanthanide triflate), N-benzylideneaniline (10a) was treated with 2-trimethylsi-loxy-4-methoxy-1,3-butadiene (Danishefsky s diene, 11) [59] in acetonitrile at room temperature. The imino Diels-Alder reaction proceeded smoothly to afford the corresponding tetrahydropyridine derivative in a 93% yield (Table 19). The adduct was obtained quantitatively when Sc(OTf)3 was used as a catalyst. Imines 10b and 10c also reacted smoothly with 11 to give the corresponding adducts in high yields. The reaction of 10a with cyclopentadiene was performed under the same reaction conditions. It was found that the reaction course changed in this case and that a tetrahydroquinoline derivative was obtained in a 69% yield. In this reaction, the imine worked as an azadiene toward one of the double bonds of cyclopentadiene as a dienophile [43,60]. In the reactions of 2,3-dimethylbutadiene, mixtures of tetrahydropyridine and tetrahydroquinoline derivatives were obtained. 

In the previous section, lanthanide triflates were shown to be excellent catalysts for achiral aza Diels-Alder reactions. While stoichiometric amounts of Lewis acids are required in many cases, a small amount of the triflate effectively catalyzes the reactions. On the other hand, chiral lanthanide Lewis acids have been developed to realize highly enantioselective Diels-Alder reactions of 2-ox-azolidin-l-one with dienes [89]. The reaction of N-benzylideneaniline with cyclop entadiene was first performed under the influence of 20 mol% of a chiral ytterbium Lewis acid prepared from ytterbium triflate (Yb(OTf)3), fR)-(+)-l,l -bi-naphthol (BINOL), and trimethylpiperidine (TMP). The reaction proceeded smoothly at room temperature to afford the desired tetrahydroquinoline derivative in a 53% yield, although no chiral induction was observed. At this stage, it was indicated that bidentate coordination between a substrate and a chiral Lewis acid would be necessary for reasonable chiral induction. N-Benzylidene-2-hydroxy aniline (31a) was then prepared, and the reaction with cyclopentadiene (32a) was examined. It was found that the reaction proceeded smoothly to afford the corresponding 8-hydroxyquinoline derivative (33a) [90] in a high yield. The enantiomeric excess of the cis adduct in the first trial was only 6% however, the selectivity increased when diazabicyclo-[5,4,0]-undec-7-ene (DBU) was used in-... 

Achiral ytterbium Lewis acid was prepared from ytterbium triflate [Yb(OTf)3], ( )-(+)-l,l -bis(2-naphthol) (BINOL) and DBU (1), and subjected to aza Diels-Alder reactions of achiral imines (A-benzylidene-2-hydroxyanilines) and achiral dienophiles [79]. In this reaction the use of a chiral Lewis acid containing 1,3,5-trimethylpiperidine instead of 1 resulted in a lowering of the enantiomeric excess of adduct. Thus, the phenolic hydrogen of the imine interacts with DBU (1) in transition state, as shown in Figure 3.7, to increase the selectivity. 

It was found that the hydroxymethylation of silyl enol ethers with commercial formaldehyde solution proceeded smoothly by using lanthanide triflates as Lewis acid catalysts [7,13], The reactions were first carried out in commercial formaldehyde solution-THF media. The amount of the catalyst was examined by taking the reaction of the silyl enol ether derived from propiophenone with commercial formaldehyde solution as a model, and the reaction was found to be catalyzed even by 1 mol% ytterbium triflate (Yb(OTf)3) [Eq. (1)]. 

This selectivity can also be apphed synthetically as seen in Scheme 12. First, the NPOE diol 74 was smoothly rearranged with ytterbium triflate, Yb(OTf)3, and the resulting NPG diol 75 was doubly glycosylated with NPOE 76 to give trisaccharide 77 [90]. 

The rate of above Michael addition (Scheme 25) was enhanced by the addition of ytterbium triflate [yb(OTf)j]. 

Type of reaction C-N and C-C bond formation Reaction conditions Solvent-free (neat), room temperature Synthetic strategy One-pot multicomponent reaction Cataiyst Ytterbium triflate [Yb(OTf)3]... 

Pioneering work by Lubineau and coworkers showed that the aldol type reaction between a silyl enol ether and an aldehyde (the so-called Mukaiyama aldol reaction) occurred in water at room temperature with high syn stereoselectivity, albeit in low yields.However, the development of water-tolerant Lewis acids for this reaction has led to improved rates and chemical yields. Various lanthanides triflates, such as ytterbium triflate [Yb(OTf)3], scandium triflate [Sc(OTf)3], gadolinium triflate [Gd(OTf)3], or lutetium triflate [Lu(OTf)3], have been found to afford the aldol products between various aldehydes and silyl enol ethers in high yields in aqueous media, with good to moderate syn/anti diastereoselectivi-ties (Scheme 8.3, Table 8.1). ... 

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