BINOL-ytterbium

The adduct derived from (a-benzyloxyacetaldehyde (97 % ee) is an important intermediate en route to compactin and mevinolin [76]. In contrast, modest enantioselectivity was attained when the cycloadditions were catalyzed by a chiral BINOL-ytterbium-derived catalyst [77]. Pyridines were used as additives, and the best enantioselection (93% ee) was attained only in the case of p-methoxybenzaldehyde using 2,6-lutidine. 

Diels-Alder reactions. The version with inverse electron demand involving a-pyrones and vinyl ethers is subjected to asymmetric induction by a BINOL-ytterbium complex. 

NOL-based systems for addition of (substituted) anilines to meso epoxides. Hou found that a ytterbium-BI NO L complex catalyzed desymmetrization of cyclohexene oxide in up to 80% ee [15], Shibasaki demonstrated that a praseodymium-BINOL complex could promote addition of p-anisidine to several epoxides in moderate yields with modest enantioselectivities (Scheme 7.7) [16]. 

The 2-pyrones can behave as dienes or dienophiles depending on the nature of their reaction partners. 3-Carbomethoxy-2-pyrone (84) underwent inverse Diels-Alder reaction with several vinylethers under lanthanide shift reagent-catalysis [84] (Equation 3.28). The use of strong traditional Lewis acids was precluded because of the sensitivity of the cycloadducts toward decarboxylation. It is noteworthy that whereas Yb(OTf)j does not catalyze the cycloaddition of 84 with enolethers, the addition of (R)-BINOL generates a new active ytterbium catalyst which promotes the reactions with a moderate to good level of enantio selection [85]. 

The authors also investigated the mode of activation of these BINOL-derived catalysts. They proposed an oligomeric structure, in which one Ln-BINOL moiety acts as a Brpnsted base, that deprotonates the hydroperoxide and the other moiety acts as Lewis acid, which activates the enone and controls its orientation towards the oxidant . This model explains the observed chiral amplification effect, that is the ee of the epoxide product exceeds the ee of the catalyst. The stereoselective synthesis of cw-epoxyketones from acyclic cw-enones is difficult due to the tendency of the cw-enones to isomerize to the more stable fraw5-derivatives during the oxidation. In 1998, Shibasaki and coworkers reported that the ytterbium-(f )-3-hydroxymethyl-BINOL system also showed catalytic activity for the oxidation of aliphatic (Z)-enones 129 to cw-epoxides 130 with good yields... 

The effect of additives on Shibasaki s lanthanide-BINOL catalysts has been investigated by Inanaga and coworkers. From a variety of additives, triphenylphosphine oxide turned out to be the best one improving, for example, the obtained ee for the chalcone epoxide from 73% to 96% (Table 16) . The explanation for the positive effect of the additive was the disruption of the oligomeric structure of the catalyst by coordination of the phosphine oxide. As a consequence, epoxidation takes place in the coordination sphere of the ytterbium where the reaction site might become closer to the chiral binaphthyl ring due to the phosphine oxide ligand with suitable steric buUdness. In contrast to the Shibasaki... 

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. 

A chiral ytterbium complex formed from Yb(0-(-Pr)3 and 6,6 -diphenyl-BINOL catalyzed the asymmetric epoxidation of chalcone in 91% yield and 97% ee <2001TL6919>. A related reaction proceeds efficiently using enoates <2005JA8962>. 

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). 

Molybdenum catalysts, Ruthenium porphyrins, Ruthenium(lll) complexes, Iron catalysts, Titanium catalysts. Sharpless epoxidation, Tungsten catalysts, Methyltrioxorhenium, Cobalt, Nickel, Platinum, Aerobic epoxidation, Lanthanum, Ytterbium, Calcium, BINOL-complexes. 2008 Elsevier B.v. 

Since 2000 a few catalysts for asymmetric epoxidation based on polymer-anchored chiral l,l -bi-2-naphthol (BINOL) have been developed. Polystyrene-supported BINOL was prepared by radical copolymerization of styrene with BINOL, bearing 4-vinylbenzyloxy groups in the 3- or 6-position [96]. Immobilization of lanthanum or ytterbium was accomplished by treatment of the polymers... 

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. 

Some of the metal-based catalysts used in the asymmetric hydrophosphonylation of aldehydes (see Section 6.4) can also be applied to the phosphonylation of imines. For instance, Shibasaki s heterobimetallic BINOL complexes work well for the catalytic asymmetric hydrophosphonylation of imines. In this case lanthanum-potassium-BINOL complexes (6.138) have been found to provide the highest enantioselectivities for the hydrophosphonylation of acyclic imines (6.139). The hydrophosphonylation of cyclic imines using heterobimetallic lanthanoid complexes has been reported. Ytterbium and samarium complexes in combination with cyclic phosphites have shown the best results in the cases investigated so far. For example, 3-thiazoline (6.140) is converted into the phosphonate (6.141) with 99% ee using ytterbium complex (6.142) and dimethyl phosphite (6.108). The aluminium(salalen) complex (6.110) developed by Katsuki and coworkers also functions as an effective catalyst for the hydrophosphonylation of both aromatic and aliphatic aldimines providing the resulting a-aminophosphonate with 81-91% ee. ... 

Markd and Evans have used ytterbium triflate complexes of BINOL and found that vinyl sulfide (8.119) provided the highest enantioselectivity in the inverse electron demand Diels-Alder reaction with the diene (8.120). ... 

The first example for catalytic and enantioselective hydrophosphonylation of cyclic imines using cyclic phosphites is the lanthanide BINOL complex catalyzed hydrophosphonylation of 3-thiazolines. The pharmacologically interesting thiazolidinyl phosphonates are synthesized with excellent optical purities of up to 99% ee using the ytterbium-BINOL complex (116). 

Kobayashi and coworkers exploited the use of lanthanide Lewis acid catalysts in various achiral reactions as described in the previous section, and they also successfully extended some of them into asymmetric variants. A series of their works commenced with catalytic asymmetric Diels-Alder reactions [50, 51]. The reaction was performed with a chiral ytterbium catalyst prepared from Yb(OTf)3, (R)-l,l -bi-2-naphthol (BINOL), and tertiary amine. The amine significantly influenced enantioselectivity of the reaction, and cis-l,2,6-trimethylpiperidine combined with 4 A molecular sieves (MS 4 A) aflhrded the best results (endo/exo = 89/11, endo = 95% ee, Yb catalyst A) (Scheme 13.20). Later, Nakagawa and coworkers improved reactivity and selectivity of the Yb catalyst by modification of chiral ligand. Use of l,l -(2,2 -bisacylamino)binaphthalene (Yb catalyst B) gave product in >98%ee [52]. 

Kobayashi has documented that chiral Yb complexes are also effective catalysts for nitrone cycloadditions (Scheme 18.19) [88]. In the presence of the putative ytterbium complex 98, derived from (S)-BINOL and chiral amine 99, the endo-isoxazolidine 100 was obtained in 96% ee. The chiral amine additive proved crucial for the observation of high enantioselectivity. Subsequently, the product 100 was readily converted into -lactam 101. 

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