Lanthanum-BINOL catalysts

A bimetallic catalyst prepared from BINOL and lithium aluminum hydride has been found to result in useful asymmetric induction in the Pudovik reaction [17]. The (f )-ALB catalyst 64 (10 mol %) facilitates the addition of dimethyl phosphite to a variety of electron-rich and electron-poor aryl aldehydes in high yield with induction in the range 71-90 % ee. The nature of the solvent is important in this reaction—the induction for addition to benzaldehyde dropped from 85 % ee to 65 % ee when the solvent was changed from toluene to dichloromethane. Aluminum seems to be a key to the success of this reaction, because reaction with benzaldehyde was not as successful with other bimetallic catalysts. BINOL catalysts with lanthanum and potassium gave only 2 % ee, a catalyst with lanthanum and sodium gave a low 32 % ee, and a catalyst with lanthanum and lithium gave only a 28 % ee [18]. Aliphatic aldehydes were not successfully hydrophosphonylated with dimethyl phosphite by catalyst 64 (Sch. 9). Induction was low (3-24 % ee) for unbranched and branched substrates. a,/3-Unsaturated aldehydes were, however, reported to work nearly as well as aryl aldehydes with four examples in the range 55-89 % ee. The failure of aliphatic aldehydes with this catalyst can be overcome by reduction of the product obtained from reactions with a,)3-unsaturated aldehydes. As illustrated by the reduction of 67 with palladium on carbon, this can be done without epimerization of the a-hydroxy phos-phonate. 

Interestingly, almost the first results in asymmetric hydrophosphonylation with acyclic imines revealed that the lanthanum-potassium-BINOL catalyst LPB was more effective than the analog sodium and lithium complexes LSB and LLB, both of which have been shown to be highly efficient in asymmetric C-C bond formations (see Sect. 3). As a representative example, in the presence of LPB (20 mol%) and 5 equiv of dimethyl phosphite the hydrophosphonylation of im-... 

The catalytic activity of a lanthanum (R)-BINOL complex tethered either on silica (62a) or MCM-41 (62b) was evaluated for the enantioselective nitroaldol reaction of cyclohexanecarboxaldehyde (Se), hexanal (Sf), iso-butyraldehyde (Sg) and hydro-cinnamaldehyde (Sh) with nitromethane inTHF (Scheme 12.22) [166]. The silica-anchored lanthanum catalyst 62a gave 55-76% e.e. and yields up to 87%, while the PMS-immobilized catalyst 62b revealed slightly higher e.e.s (57-84%) for the same aldehydes. The homogeneous counterparts showed similar catalytic performance, albeit within a shorter reaction time. The increased enantioselectivity observed for the MCM-41 hybrid catalyst 62b was explained by transformations inside the channels, which is also reflected by lower yields due to hindered diffusion. The recyclability of the immobilized catalysts 62b was checked with hydrocin-namaldehyde (Ph). It was found that the reused catalyst gave nearly the same enantioselectivities after the fourth catalytic run, although the time period for achieving similar conversion increased from initially 30 to 42 h. 

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. 

Asymmetric amplification has also been observed in lanthanum-catalyzed nitro-aldol reaction, Shibasaki used a chiral lanthanum complex 15 prepared from LaCl3 and dilithium alkoxide of chiral BINOL for the enantioselective aldol reaction between naphthoxyacetaldehyde 14 and nitromethane (Scheme 9.11) [26]. When chiral catalyst 15 was prepared from BINOL with 56% ee, the corresponding aldol adduct 16 with 68% ee was obtained. This result indicates that the lanthanum 15 complex should exist as oligomer(s). 

This type of chiral lanthanum catalyst was found to be applicable for epoxidation of a range of enone substrates. Thus 35 was converted to 36 with 86% ee and in 93% yield, and 37 was transformed to 38 with 85% ee in 85% yield (Table 2, entries 1,4, and 6). The enantioselectivity of the asymmetric epoxidations could be substantially improved by the use of (/ )-3-hydroxymethyl-BINOL (32) instead of 17 (Table 2, entries 2, 3, 5, and 7). Namely, 34, 36, and 38 were obtained in excellent yields with 91, 94, and 83% ee, respectively. 

The hypothesis that the structure of these catalysts is not always in accordance with a monomeric structure has been expressed for both the titanium-BINOL complexes [30] and the corresponding lanthanum species. In the latter case, this hypothesis has been corroborated impres-... 

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. 

It was proposed that a Lewis acid lanthanum center controls the direction of the carbonyl function and activates the enone while the sodium alkoxide forms enolate intermediates and regenerates the catalyst by hydrogen abstraction (Scheme 6). Other Ln/alkali metal combinations, including La/Li, show negligible asymmetric induction, yet give almost racemic products in excellent yield. In contrast, alkali-metal free BINOL ester enolate complexes catalyze Michael reactions with high enantioselectivities, albeit at lower temperatures. 

ASYMMETRIC NITROALDOL REACTION USING LANTHANUM-LITHIUM-BINOL DERIVATIVE COMPLEX AS A CATALYST... 

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 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%. 

Nef reaction. Optically active /3-hydroxy nitroalkanes are obtained in the condensation of nitromethane with aldehydes using a BINOL-lanthanum complex as catalyst. 

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. ... 

Shibasaki developed chiral lanthanum(iii) sodium(i) tris(binaphtholate) 67, which was prepared from La(Oz-Pr)3, (J )-BINOL, and NaOf-Bu, as the first example of a chiral sodium-containing heterobimetallic catalyst (Scheme 2.39). X-ray analysis of 67 showed that it consists of LaNas-CeoHaeOe 6THF H2O, which contains a central La(iii) atom, three (1 )-BINOL molecules, and three sodium atoms in the core structure. Catalyst 67 efficiently promoted the highly enantioselective Michael reaction... 

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. 

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