Binaphthol Lewis acids

Binaphthol Lewis Acids. In search of Lewis acids which might provide better stereocontrol in these cycloadditions, we turned our attention to l,l -bi-2-naphthol as a potentially useful ligand for the titanium promoter complex. For synthetic reasons, we continued to use benzylic vinyl ethers and silyl vinyl ethers so that these protecting groups could be cleaved to provide the free alcohol later. 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

Lewis acids of chiral metal aryloxides prepared from metal reagents and optically active binaphthol derivatives have played a significant role in asymmetric synthesis and have been extensively studied.23 However, in Diels-Alder reactions, the asymmetric induction with chiral metal aryloxides is, in most cases, controlled by steric interaction between a dienophile and a chiral ligand. This kind of interaction is sometimes insufficient to provide a high level of enantioselectivity. 

Several chiral lanthanide(III) Lewis acid catalysts, derived from chiral binaphthols, have been used in the cycloaddition reactions of cyclopentadiene with substituted iV-acryloyl-1,3-oxazolidin-2-ones. A catalyst derived from ytterbium triflate, (R)-binaphthol... 

The chiral dialuminum Lewis acid 14, which is effective as an asymmetric Diels-Alder catalyst, has been prepared from DIBAH and BINOL derivatives (Scheme 12.12). " The catalytic activity of 14 is significantly greater than that of monoaluminum reagents. The catalyst achieves high reactivity and selectivity by an intramolecular interaction of two aluminum Lewis acids. Similarly, the chiral trialuminum Lewis acid 15 is quantitatively formed from optically pure 3-(2,4,6-triisopropylphenyl)binaphthol (2 equiv) and MeaAl (3 equiv) in CH2CI2 at room temperature (Scheme 12.12). " The novel structure of 15 has been ascertained by NMR spectroscopic analysis and measurement of the methane gas evolved. Trinuclear aluminum catalyst 15 is effective for the Diels-Alder reaction of methacrolein with cyclopentadiene. Diels-Alder adducts have been obtained in 99% yield with 92% exo selectivity. Under optimum reaction conditions, the... 

A chiral dinuclear Ti(IV) Lewis acid catalyst 18 can be prepared in situ from a 1 2 molar mixture of (7 )-3,3 -di(2-mesitylethynyl)binaphthol and Ti(Oi-Pr)4 at ambient temperature. The 3- and 3 -substituents on the chiral ligand are effective for preventing undesired aggregation between Ti(IV) complexes and increasing the enantioselectivity (up to 82% ee) in the Diels-Alder reaction of methacrolein with cyclopentadiene (Scheme 12.16). ... 

Asymmetric ene reactions can be achieved by using chiral Lewis acids as shown in Scheme 113 (275). Chiral binaphthol-based organoalu-... 

Yamamoto et al. have reported the asymmetric catalysis of a chiral Lewis acid in a carbonyl-ene reaction, which uses chloral as the enophile and an aluminum catalyst with enantiopure 3,3 -bissilylated binaphthol (BINOL) to give the corresponding homoallylic alcohol with 78% ee in 79% yield (Scheme 8C.2) [6]. It should be noted that 3,3 -diphenyl-BINOL-derived aluminum catalyst provides the racemic product in low yield. 

A mechanism has been proposed for the enantioselective Mikami ene reaction of a terminal alkene with a glyoxylic aldehyde using a chiral binaphthol as Lewis acid.80... 

Shibasaki and co-workers disclosed a general asymmetric Strecker-type reaction that was controlled by bifunctional Lewis acid-Lewis base catalyst 14 [10], N-Fluorenylimines 15 underwent catalytic asymmetric Strecker-type reactions with binaphthol catalyst 14 to give a-aminonitriles 16 in good to excellent enantioselectivities and yields (Scheme 6). a-Aminonitrile 16 (R = Ph) could then be converted to a-aminoamide 17 in several steps. Aromatic, aliphatic, heterocyclic and a,/f-unsaturated imines 15 were used as general substrates in these reactions. The origin of the highly enantioselective cataylsis by 14 is believed to be attributed to the simultaneous activation of imines and trimethylsilyl cyanide by the... 

The outstanding properties of binaphthol (BINOL) as a ligand in chiral Lewis acidic metal complexes were also demonstrated highly successfully by Mikami [108, 109] using a binol-titanium complex 2-69a. Even in the cycloaddition of methyl glyoxylate 2-66 to 1 -methoxy-1,3-butadiene 2-65 which usually shows only a low selectivity, a reasonable cis/trans-selectivity and an excellent enantioselectivity could be obtained in the presence of catalytic amounts of this complex. 

Chiral Lewis acids have been employed by Yamamoto et al. [197 -199] in order to carry out enantioselective aza Diels-Alder reactions starting from achiral substrates however, these transformations required stoichiometric amounts of the chiral mediator 3-16 which was generated in situ from (fl)-binaphthol and triphenylborate. The best results were obtained with the pyridine derivative 3-14 which afforded the desired cycloadduct 3-15 in high optical purity (Fig. 3-5). Using chiral imines, the authors found a high level of double asymmetric induction, and this methodology could be applied to the enantioselective total synthesis of two piperidine alkaloids. 

Transformations involving chiral catalysts most efficiently lead to optically active products. The degree of enantioselectivity rather than the efficiency of the catalytic cycle has up to now been in the center of interest. Compared to hydrogenations, catalytic oxidations or C-C bond formations are much more complex processes and still under development. In the case of catalytic additions of dialkyl zinc compounds[l], allylstan-nanes [2], allyl silanes [3], and silyl enolethers [4] to aldehydes, the degree of asymmetric induction is less of a problem than the turnover number and substrate tolerance. Chiral Lewis acids for the enantioselective Mukaiyama reaction have been known for some time [4a - 4c], and recently the binaphthol-titanium complexes 1 [2c - 2e, 2jl and 2 [2b, 2i] have been found to catalyze the addition of allyl stannanes to aldehydes quite efficiently. It has been reported recently that a more active catalyst results upon addition of Me SiSfi-Pr) [2k] or Et2BS( -Pr) [21, 2m] to bi-naphthol-Ti(IV) preparations. 

An alternative reaction, the addition of an olefinic C—H bond to C=0, is catalyzed by Lewis acidic titanium aryloxide complexes chiral binaphthol ligands give high optical yields.96... 

In 1985 we reported a new cyclization process employing a chiral zinc reagent derived from dimethylzinc and optically active binaphthol [13]. The reaction proceeds smoothly at low temperature to generate the cyclization product with reasonable asymmetric induction. Since then many chiral Lewis-acid catalysts have been reported in the literature and the resulting process is now an essential tool for many asymmetric syntheses (Eq. 7). 

The Brpnsted acid-assisted chiral Lewis acid (BLA) 28, prepared from a 1 2 molar ratio mixture of a trialkylborate and optically pure binaphthol, is also an excellent chiral promoter for the aza Diels-Alder reaction of imines with Danishefsky dienes (Eqs 44 and 45). Enantioselectivity and double diastereodifferentiation in reactions employing chiral 28 are slightly better than those using chiral 27 [41]. 

Vederas and co-workers have reported the ene reaction of an allylglycinate 262 with methyl glyoxylate using copper Lewis acids (Sch. 57) [104], Chiral binaphthol-titanium complexes did not catalyze the ene reaction and only the starting material was recovered. 

Another example of ring-opening of the oxirane ring is the kinetic resolution of racemic oxiranes by use of chiral Lewis acid catalysts, including Zr(0-t-Bu)4, with (f )-(-F)-binaphthol [22],... 

Optically active l,l -binaphthols are among the most important chiral ligands of a variety of metal species. Binaphthol-aluminum complexes have been used as chiral Lewis acid catalysts. The l,T-binaphthyl-based chiral ligands owe their success in a variety of asymmetric reactions to the chiral cavity they create around the metal center [107,108]. In contrast with the wide use of these binaphthyls, the polymer-supported variety has been less popular. The optically active and sterically regular poly(l,l -bi-naphthyls) 96 have been prepared by nickel-catalyzed dehalogenating polycondensation of dibromide monomer 95 (Sch. 7) [109] and used to prepare the polybinaphthyl aluminum(III) catalyst 97 this had much greater catalytic activity than the corresponding monomeric catalyst when used in the Mukaiyama aldol reaction (Eq. 29). Unfortunately no enantioselectivity was observed in the aldol reaction. 

A mechanism has been proposed for the enantioselective Mikami ene reaction of a terminal alkene with a glyoxylic aldehyde using a chiral binaphthol as Lewis acid. Stereoselective synthesis of 5-amino esters via asymmetric aldol-type and aza-Diels-Alder reactions has been reviewed. Siliranes react cleanly with benzaldehyde to produce oxasilacyclopentanes—with inversion— under conditions of Bu OK catalysis enolizable aldehydes yield sdyl enol ethers. ... 

Chiral (helical) Lewis acids for asymmetric Diels-Alder catalysis are prepared from titanium tetraisopropoxide 5 and a chiral binaphthol ligand 4 [13]. The titanium reagent 6 plays an important role as chiral template for the fixation of a,fi-unsaturated aldehydes and thereby for the enantioface recognition of substrates. The asymmetric Diels-Alder reaction, e. g., of cyclopentadiene 7 and acrolein 8, is effected in the presence of catalytic (P)-6 (10 mol%), producing the endo adduct 9 (R =R2=H) in 88% ee (Scheme 1). 

Asymmetric ene Reaction In 1988 Yamamoto and coworkers provided the first indication that asymmetry in ene-reactions could be induced by catalytic amounts of chiral Lewis acids in the presence of 4-A molecular sieves (Scheme 6.64) [88]. They described the first example of asymmetric ene-reaction between prochiral, halogenated aldehydes and alkenes catalyzed by chiral binaphthol-derived aluminum complexes. The hindered 3,3-silyl substituents in the chiral catalyst are essential to achieve good enantioselectivity and high yield. In fact, the use of a catalyst derived from MesAl and 3,3 -biphenylbinaphthol led to the racemic product in a low yield. 

Enantiomerically pure binaphthol is used as a chiral auxiliary. For example, it has been used to prepare chiral aluminum hydride reducing agents, chiral Lewis acids catalysts,8 and chiral crown ethers.9... 

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