BINOL heterobimetallic complex

Moreover, these rare earth heterobimetallic complexes can be utilized for a variety of efficient catalytic asymmetric reactions as shown in Scheme 7 Next we began with the development of an amphoteric asymmetric catalyst assembled from aluminum and an alkali metal.1171 The new asymmetric catalyst could be prepared efficiently from LiAlH4 and 2 mol equiv of (R)-BINOL, and the structure was unequivocally determined by X-ray crystallographic analysis (Scheme 8). This aluminum-lithium-BINOL complex (ALB) was highly effective in the Michael reaction of cyclohexenone 75 with dibenzyl malonate 77, giving 82 with 99% ee and 88 % yield at room temperature. Although LLB and... 

Analogous addition with scandium-(5, 5 -2,6-bis(oxazolinyl)pyridine) complex resulted in lower enantioselectivity. A chiral heterobimetallic complex of Y(OTf)3 and Li-BINOL provided high ee in addition of 0-methylhydroxylamine to enones of type 55 (equation 36) . ... 

The second part of the chapter deals with several kinds of asymmetric reactions catalyzed by unique heterobimetallic complexes. These reagents are lanthanoid-alkali metal hybrids which form BINOL derivative complexes (LnMB, where Ln = lanthanoid, M = alkali metal, and B = BINOL derivative). These complexes efficiently promote asymmetric aldol-type reactions as well as asymmetric hydrophosphonylations of aldehydes (catalyzed by LnLB, where L = lithium), asymmetric Michael reactions (catalyzed by LnSB, where S = sodium), and asymmetric hydrophosphonylations of imines (catalyzed by LnPB, where P = potassium) to give the corresponding desired products in up to 98% ee. Spectroscopic analysis and computer simulations of these asymmetric reactions have revealed the synergistic cooperation of the two different metals in the complexes. These complexes are believed to function as both Brpnsted bases and as Lewis acids may prove to be applicable to a variety of new asymmetric catalytic reactions.1,2... 

The Ln-BINOL derivative complexes are efficient asymmetric catalysts for Michael reactions and the epoxidations of enones. However, as was mentioned above, almost racemic products are obtained in the case of the asymmetric nitroaldol reaction of 2 with 12. For this transformation, a new class of catalysts, heterobimetallic species, have been developed. 

LDI-TOF mass spectral analysis of 49 revealed that the structure was a heterobimetallic complex consisting of one lanthanum, three lithiums, and three BINOL moieties.24 The relevant spectra are shown in Figure 10. LDI-TOF mass... 

Conceptually new multifunctional asymmetric two-center catalysts, such as the Ln-BINOL derivative, LnMB, AMB, and GaMB have been developed. These catalysts function both as Brpnsted bases and as Lewis acids, making possible various catalytic, asymmetric reactions in a manner analogous to enzyme catalysis. Several such catalytic asymmetric reactions are now being investigated for potential industrial applications. Recently, the catalytic enantioselective opening of meso epoxides with thiols in the presence of a heterobimetallic complex has... 

The first chiral aluminum catalyst for effecting asymmetric Michael addition reactions was reported by Shibasaki and coworkers in 1986 [82], The catalyst was prepared by addition of two equivalents of (i )-BINOL to lithium aluminum hydride which gave the heterobimetallic complex 394. The structure of 394 was supported by X-ray structure analysis of its complex with cyclohexenone in which it was found that the carbonyl oxygen of the enone is coordinated to the lithium. This catalyst was found to result in excellent induction in the Michael addition of malonic esters to cyclic enones, as indicated in Sch. 51. It had previously been reported that a heterobimetallic catalyst prepared from (i )-BINOL and sodium and lanthanum was also effective in similar Michael additions [83-85]. Although the LaNaBINOL catalyst was faster, the LiAlBINOL catalyst 394 (ALB) led to higher asymmetric induction. 

For example, an effective procedure for the synthesis of LLB (where LL = lanthanum and lithium) is treatment of LaCls 7H2O with 2.7 mol equiv. BINOL dilithium salt, and NaO-t-Bu (0.3 mol equiv.) in THF at 50 °C for 50 h. Another efficient procedure for the preparation of LLB starts from La(0-/-Pr)3 [54], the exposure of which to 3 mol equiv. BINOL in THF is followed by addition of butyllithium (3 mol equiv.) at 0 C. It is worthy of note that heterobimetallic asymmetric complexes which include LLB are stable in organic solvents such as THF, CH2CI2 and toluene which contain small amounts of water, and are also insensitive to oxygen. These heterobimetallic complexes can, by choice of suitable rare earth and alkali metals, be used to promote a variety of efficient asymmetric reactions, for example nitroaldol, aldol, Michael, nitro-Mannich-type, hydrophosphonylation, hydrophosphination, protonation and Diels-Alder reactions. A catalytic asymmetric nitroaldol reaction, a direct catalytic asymmetric aldol reaction, and a catalytic asymmetric nitro-Mannich-type reaction are discussed in detail below. 

Heterobimetallic complexes 2 and 22 catalyze asymmetric Diels-Alder reactions of some dienophiles with cyclopentadiene [110]. Use of 6,6 -dibromo-substituted BINOL ligands led to significantly improved yield, endo.exo ratio, and enantioselectivity. 

The first catalytic asymmetric tandem Michael-aldol reactions were also achieved by the Al-Li-binol complex (ALB), which was prepared from LiAlH4 and binol. The ALB catalysts gave the Michael adducts in up to 99% ee (Eq. (12.2)) [12J. Mechanistic and calculation studies on ALB revealed that ALB is a heterobimetallic complex which acts as a multifunctional catalyst. 

Addition to imines. The heterobimetallic complex derived from BINOL, (i-PrOljYb and t-BuOK catalyzes the asymmetric reaction between nitroalkanes and. V-phosphonimines. 

By virtue of a deep understanding of his LnM3tris(BINOLate)3 complexes (Ln = rare-earth metal, M = alkali metal) based on evidence from X-ray analysis and other experiments, Shibasaki developed chiral heterobimetallic yttrium(in) lithium(i) tris(binaphtholate) complex 22, which can promote the catal) ic enantioselective aza-Michael reaction of metho g lamine to enones in excellent yields with up to 97% ee as a Lewis-acid-Lewis-acid cooperative catalyst (Scheme 2.17). Transformation of the 1,4-adducts 23 afforded the corresponding optically active aziridines 24 in high yields. 

The first catalytic enantioselective nitro-Mannich-type reaction was developed by Shibasaki by the use of chiral BINOL-derived heterobimetallic complex 90 (Scheme 2.51). The catalyst prepared from Yb(0/-Pr)3, KOt-Bu, and (/ )-BINOL in a ratio of 1 1 3 gave the best result (82-91% ee) in the reaction of aldimines with nitromethane, while the conventional ratio (Yb K BINOL = 1 3 3) was much less effective (52% ee) in the same reaction. Moreover, the complex with a ratio of Yb K BINOL=l l 2 did not promote the reaction. Therefore, the complex YbK((I )-BINOLate)2 itself or its aggregated complex [YbK((I )-BINOLate)2] was not likely to be tbe actual catalyst. The active catalyst might be 90, a complex of YbK((I )-BINOLate)2... 

Sc(BINOL)2Li, a new chiral heterobimetallic complex, was shown to catalyze addition of a cyanide source (hydrogen cyanide (HCN) and trimethylsilyl cyanide (TMSCN)) to various imines enantioselectively [128]. Moderate to high conversions and enantiomeric excesses were obtained in this Strecker reaction using 10 mol% of the catalyst. High enantioselectivity (84% ee) and quantitative yield were also obtained by the addition of TMSCN to benzaldehyde. 

Fig. 13.11 Structure of M3[Ln(binol)3)-type heterobimetallic complexes (LnMB).

Aspinall and coworkers reported detailed structural studies of rare earth-alkali metal heterobimetallic complexes [131]. Whereas the crystal structures provided by Shibasaki and coworkers included one molecule of water coordinating to the central metal, Aspinall and coworkers succeeded in preparing anhydrous crystals of M3[Ln(binol)3] (LnMB). Li3[Ln(binol)3] complexes were obtained from Ln(N(SiMe3)2)3 and Li(Hbinol) in THE or Et20. The resulting HN(SiMe3)2 was removed under reduced pressure, and the crystals were obtained from... 

Shibasald s concept of a cooperative effect exhibited by two different metals (usually an alkali with a group 3 metal or a lanthanide) in heterobimetallic BINOL-derived complexes was also fruitful in consecutive Michael-aldol additions. Thus, Al-Li-bis[binaphthoxide] complex R,R)-449, readily accessible from lithium aluminum hydride and 2 equiv. of (S)-BINOL, functions as a highly... 

In 1995, Shibasaki s group disclosed the first example of multifunctional heterobimetallic complex-catalyzed Michael reaction of malonate to enone. The chiral catalyst, lanthanum-sodium-BINOL complex (/ )-LSB, was prepared from La(Of-Pr)3, (/ )-BINOL, and NaOt-Bu. Two different metals indeed play their unique roles to enhance the reactivity of both substrate partners by locating them in designated positions. The Lewis acidic metal (lanthanides or group 13 elements) has been found capable to activate the acceptor, whereas the second metal center (alkali metals bound to a Brpnsted base) assists the coordination of enolate. The proposed catalytic cycle is shown in Scheme 9.5. 

Shibasaki pioneered the use of chiral heterobimetallic complexes of BINOL as catalysts for an impressively wide variety of useful asymmetric transformations (Schemes 12.15 and 12.16) [116-119]. The complexes, typically consisting of a group 3 or a group 13 metal ion in combination with alkali metal ions, have been suggested to exhibit dual function as Lewis acids and Bronsted bases. Such complexes are generated in situ upon mixing of BINOL with metal salts in the appropriate proportions and consequently represent a notable example of macromolecular self-assembly. 

These heterobimetallic M -M -binol complexes consdnite a new class of v/idely applicable chiral catidysts as shown in Scheme 3.16. The new catidysts consist of a centtidmetid ion fe.g.. La" , AT , Sin , Ga , three alkali inetid ions fe.g., LT, Na , K l, and three chirid tliphenol... 

These heterobimetallic M1-M2-binol complexes constitute a new class of widely applicable chiral catalysts as shown in Scheme 3.16. The new catalysts consist of a central metal ion (e.g., La3+, Al3+, Sm3+, Ga3+), three alkali metal ions (e.g., Li+, Na+, K+), and three chiral diphenol... 

A heterobimetallic BINOL-Ga/Li complex 53 has been developed for the enantioselective ARO of meso-cpoxides (BINOL = l,T-bi(2-naphthol)).278 Using />-methoxyphenol as the nucleophile, this etherification reaction was observed to take place with a high level of asymmetric induction. An improved catalyst 54 has also been reported that exhibits greater stability under the reaction conditions and delivers higher yields and ee s (Equation (78)).279 A simple catalyst derived from Sc(OTf)3 and the chiral bipyridine ligand 52 has been shown to be effective for the ARO of aryl-substituted /// -epoxides with aliphatic alcohols to give high ee s (Equation (79)).280... 

This idea was realized very successfully by Shibasaki and Sasai in their heterobimetallic chiral catalysts [17], Two representative well-defined catalysts. LSB 9 (Lanthanum/Sodium/BINOL complex) and ALB 10 (Aluminum/Lithium/BINOL complex), are shown in Figure 8D.2, whose structures were confirmed by X-ray crystallography. In these catalysts, the alkali metal (Na, Li, or K)-naphthoxide works as a Br0nsted base and lanthanum or aluminum works as a Lewis acid. 

Another highly useful heterobimetallic catalyst is the aluminum-lithium-BINOL complex (ALB) prepared from LiAlH4 and 2 equiv. of (/ )-BINOL. The ALB catalyst (10 mol %) is also effective in the Michael reaction of enones with various malonates, giving Michael products generally with excellent enantioselectivity (91-99% ee) and in excellent yields [23]. These results ate summarized in Table 8D.3. Although LLB and LSB complement each other in their ability to catalyze asymmetric nitroaldol and Michael reactions, respectively, the Al-M-(/ )-BINOL complexes (M = Li, Na, K, and Ba) are commonly useful for the catalytic asymmetric Michael reaction. 

Third heterobimetallic asymmetric catalyst reported by Shibasaki et al., gallium-sodium-BINOL complex (GaSB) 26 and indium-potassium-BINOL complex (InPB), are also rather effective catalysts for asymmetric Michael reactions, and GaSB was better than InPB in terms of enantioselectivity. The GaSB catalyst was prepared from GaCl3, NaO Bu (4 mol equiv. to... 

In 2002, Sasai et al. reported the synthesis of dendrimer heterobimetallic multi-funchonal chiral catalysts, containing up to 12 chiral BINOL units at the periphery (Figure 4.36) [107]. The insoluble dendrihc heterobimetallic mulhfunchonal chiral AlLibis(binaphthoxide) (ALB) complexes were obtained by treahng these dendrimer ligands with AlMcj and n-BuIi. The resulhng dendrimer-supported ALB... 

The LLB type catalysts were also successfully applied in the asymmetric nitroaldol reaction of the quite um-eactive a,a-difluoro aldehydes. In general, catalytic asymmetric syntheses of fluorine-containing compounds are rather difficult [32]. However, catalytic asymmetric nitroaldol reaction of a broad variety of a,a-difluoro aldehydes 20,22,24,26,28, and 30 proceeded satisfactorily when using the heterobimetallic asymmetric catalysts with modified, 6,6 -disubstitut-ed BINOL ligands [33] (Scheme 7). The best results were obtained with the sa-marium(III) complex (5mol%) generated from 6, 6 -bis (triethylsilyl)ethy-nyl BINOL with enantioselectivities up to 95% ee. 

In 1992, Shibasaki et al. reported for the time an application of chiral heterobimetallic lanthanoid complexes (LnLB) as chiral catalysts in asymmetric catalysis, namely the catalytic asymmetric nitroaldol reaction (Henry reaction), which is one of the most classical C-C bond forming processes [11]. Additionally, this work represents the first enantioselective synthesis of (3-nitroalcohol compounds by the way of enantioselective addition of nitroalkanes to aldehydes in the presence of a chiral catalyst. The chiral BINOL based catalyst was prepared starting from anhydrous LaCl3 and an equimolar amount of the dialkali metal salt of BINOL in the presence of a small amount of water [9]. 

In the presence of the sodium-containing heterobimetallic catalyst (R)-LSB (10 mol%), the reaction of enone 52 with TBHP (2 equiv) was found to give the desired epoxide with 83% ee and in 92% yield [56]. Unfortunately LSB as well as other bimetallic catalysts were not useful for many other enones. Interestingly, in marked contrast to LSB an alkali metal free lanthanoid BINOL complex, which was prepared from Ln(0- -Pr)3 and (R)-BINOL or a derivative thereof (1 or 1.25 molar equiv) in the presence of MS 4A (Scheme 17), was found to be applicable to a range of enone substrates. Regarding enones with an aryl-substitu-ent in the a-keto position, the most effective catalytic system was revealed when using a lanthanum-(.R/)-3-hydroxymethyl-BINOL complex La-51 (l-5mol%) and cumene hydroperoxide (CMHP) as oxidant. The asymmetric epoxidation proceeded with excellent enantioselectivities (ees between 85 and 94%) and yields up to 95%. 

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. ... 

One of the more reactive and selective catalysts of this type involves a bifunctional catalyst containing an alkali metal cation and an anionic lanthanide complex resulting from addition of excess binolate with lanthanide halides. Such catalysts have been used in asymmetric nitroaidol (Henry) reactions of ketones. Heterobimetallic Li-La alkoxo complexes (Figure 4.15) catalyzed these reactions with particularly high enantioselectivity. ... 

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