Titanium catalysts binaphthol catalyst

Yamamoto et al. have reported a chiral helical titanium catalyst, 10, prepared from a binaphthol-derived chiral tetraol and titanium tetraisopropoxide with azeotropic removal of 2-propanol [16] (Scheme 1.22, 1.23, Table 1.9). This is one of the few catalysts which promote the Diels-Alder reaction of a-unsubstituted aldehydes such as acrolein with high enantioselectivity. Acrolein reacts not only with cyclo-pentadiene but also 1,3-cyclohexadiene and l-methoxy-l,3-cyclohexadiene to afford cycloadducts in 96, 81, and 98% ee, respectively. Another noteworthy feature of the titanium catalyst 10 is that the enantioselectivity is not greatly influenced by reaction temperature (96% ee at... 

Yamamoto and colleagues prepared chiral titanium catalyst 420 from titanium tetraisopropoxide and chiral binaphthol 419 (equation 126). This catalyst gave high asymmetric inductions in various Diels-Alder reactions of a,/J-unsaturated aldehydes with cyclopen-tadiene and 1,3-cyclohexadiene260. 

Chiral titanium complexes 4 and 5, which were developed as chiral catalysts for asymmetric carbonyl-ene reactions with prochiral glyoxylate esters [50], were first apphed to the catalytic asymmetric allylation of carbonyl compounds by Mikami and Nakai (Scheme 5) [9]. The titanium catalysts are prepared from (S)-binaphthol and diisopropoxytitanium dihahde (X=C1 and Br) in the presence of 4 A molecular sieves. Using these catalysts, glyoxylates are enantio- and diastereoselectively allylated with allylic trimethylsilanes or allylic tributylstan-nanes. High levels of enantioselectivity and syn selectivity are observed for (E)-crotylsilane and -stannane. The syn selective allylation reaction is believed to proceed mainly through an antiperiplanar transition state. 

Keck almost simultaneously reported two procedures using chiral titanium catalysts 6A and 6B for the enantioselective addition of allyltributyltin to aldehydes [11]. In the first procedure, the catalyst 6A is prepared from a 1 1 mixture of (R)-binaphthol and titanium tetraisopropoxide. The second procedure for the preparation of 6B, in contrast, requires a 2 1 mixture of BINOL, Ti(0 Pr)4, and a catalytic amount of CF3SO3H or CF3CO2H. Using 10 mol % of the catalyst 6A or 6B, a variety of aromatic, aliphatic, and a,P-unsaturated aldehydes are efficiently transformed into the corresponding optically active homoallylic alcohols with high enantioselectivity. An improved procedure was later published for the catalytic asymmetric allylation reactions using the 2 1 BINOL/Ti catalytic system [12]. 

Titanium complexes that are similar to Duthaler s ( 2.5.2) can be generated from TiCl4, Ti(Or-Pr)4 and diacetoneglucose 1.48. These complexes catalyze asymmetric hetero-Diels-Alder reactions, and give high enantiomeric excesses [827], Corey and coworkers [828] also prepared a chiral titanium catalyst derived from cis-/V-sulfonyl-2-amino-1 -indanol and used this to catalyze asymmetric Diels-Alder reactions. Buchwald and coworkers [829, 830] have proposed the use of titanocene-binaphthol catalysts for asymmetric hydrogenation of imines or trisubsti-tuted olefins. 

The use of titanium catalysts formed from (S)- or (/ )-binaphthol 1.44 and H(Oi-Pr)4 or Ti(0/-Pr)2Cl2 has been proposed by Keck, Umani-Ronchi and their cowoikers [1218-1221] for the asymmetric allylation of aldehydes with CH2=C(R)CH2SnBu3 (R = H,Me). These reactions occur near room temperature in the presence of molecular sieves, and excellent yields and enantiomeric excesses are obtained (Figure 6.53). 

Nakajima also developed an enantioselective addition of lithium acet-ylides to ketones in the presence of chiral lithium binaphtholate catalyst 8 (Scheme 2.7). This is the first example of the catalytic enantioselective addition of lithium acelylides to carhonyl compounds without the aid of other metal sources, such as titanium(iv) and zinc(n) species. 

Asymmetric Friedel-Crafts Reactions with Fluoral Catalyzed by Chiral Binaphthol-Derived Titanium Catalysts... 

Asymmetric catalysis of Friedel-Crafts reaction with fluoral is established using chiral binaphthol-derived titanium catalysts with or without asymmetric activation to provide a practical synthetic route not only for chiral a-trifluorobenzylalcohols but so for highly enantiopure functionalized jS-trifluoroaldols through the sequential diastereoselective reactions of the resultant vinyl ethers or silyl enol ethers with electrophiles. 

Binaphthol-derived titanium complexes [64], prepared from chiral ligands 65 (Figure 3.13), also performed very well in the cycloadditions of conjugated aldehydes with cyclic and acyclic dienes. Judging from the absolute configurations of endo and exo adducts, this catalyst should cover the re-face of carbonyl on its u tz-coordination to s-trans a,/l-unsaturated aldehydes, and hence dienes should approach selectively from the si-face. 

A similar nonlinearity is seen in the ene reaction of methyl glyoxylate and a-methylstyrene (Scheme 43) (69). Thus, the reaction catalyzed by a complex in situ formed from dibromo(diisopropoxy)titanium(IV) and (/ )-binaphthol in 33% ee affords the chiral adduct in 91% ee with the same enantioselectivity as would have been obtained had enantiomerically pure binaphthol been used. Molecular weight measurements suggest the catalyst is a dinuclear titanium compound, although the structure has not been elucidated. This nonlinear effect is interpreted by the difference in the dissociation constant of the diastereomeric dimers as... 

A full account5 describes the enantioselective carbonyl-ene reaction of glyoxylate esters catalyzed by a binaphthol-derived chiral titanium complex that is potentially useful for the asymmetric synthesis of a-hydroxy esters of biological and synthetic importance.6 The present procedure is applicable to a variety of 1,1-disubstituted olefins to provide ene products in extremely high enantiomeric purity by the judicious choice of the dichloro or dibromo chiral catalyst (see Table). In certain glyoxylate-ene reactions involving removal of a methyl hydrogen, the dichloro catalyst... 

Uemura et al. [49] found that (R)-1,1 -binaphthol could replace (7 ,7 )-diethyl tartrate in the water-modified catalyst, giving good results (up to 73% ee) in the oxidation of methyl p-tolyl sulfoxide with f-BuOOH (at -20°C in toluene). The chemical yield was close to 90% with the use of a catalytic amount (10 mol %) of the titanium complex (Ti(0-i-Pr)4/(/ )-binaphthol/H20 = 1 2 20). They studied the effect of added water and found that high enantioselectivity was obtained when using 0.5-3.0 equivalents of water with respect to the sulfide. In the absence of water, enantioselectivity was very low. The beneficial effect of water is clearly established here, but the amount of water needed is much higher than that in the case of the catalyst with diethyl tartrate. They assumed that a mononuclear titanium complex with two binaphthol ligands was involved, in which water affects the structure of the titanium complex and its rate of formation. 

Mikami reported a highly enantioselective carbonyl-ene reaction where a chiral titanium complex 11 prepared from enantiomerically pure binaphthol (BINOL) and Ti(0-i-Pr)2Br2 catalyzed a glyoxylate-ene reaction with a-methylstyrene to give chiral homoallyl alcohol 12 with 94.6% ee [22]. In this reaction, a remarkable asymmetric amplification was observed and almost the same enantioselectivity (94.4% ee) was achieved by using chiral catalyst prepared... 

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. 

Oguni has reported asymmetric amplification [12] ((-i-)-NLE) in an asymmetric carbonyl addition reaction of dialkylzinc reagents catalyzed by chiral ami-noalcohols such as l-piperidino-3,3-dimethyl-2-butanol (PDB) (Eq. (7.1)) [13]. Noyori et al. have reported a highly efficient aminoalcohol catalyst, 2S)-3-exo-(dimethylamino)isobomeol (DAIB) [14] and a beautiful investigation of asymmetric amplification in view of the stability and lower catalytic activity of the het-ero-chiral dimer of the zinc aminoalcohol catalyst than the homo-chiral dimer (Fig. 7-5). We have reported a positive non-linear effect in a carbonyl-ene reaction [15] with glyoxylate catalyzed by binaphthol (binol)-derived chiral titanium complex (Eq. (7.2)) [10]. Bolm has also reported (-i-)-NLE in the 1,4-addition reaction of dialkylzinc by the catalysis of nickel complex with pyridyl alcohols [16]. 

A bimetallic titanium complex of BINOL derivative can be used to catalyze the asymmetric carbonyl-ene reaction [46]. Insoluble polymeric catalyst 74 was prepared from a self-assembly of Ti(OiPr)4 and non-crosshnked copolymers with (R)-binaphthol pendant groups (Scheme 3.22) [47]. The self-assembled polymeric Ti complex is insoluble in organic solvent and catalyzed the carbonyl-ene reaction of glyoxylate 75 and a-methylstyrene 76. When the reaction of 75 and 76 was carried out with 20mol% of 74 in Gl pCf at room temperature, an 85% yield of the product with 88% ee was obtained. Following its recovery by filtration, this catalyst was reused five times with full retenhon of its activity and enantioselectivity, without further treatment... 

Diels-Alder reactions. Many excellent Lewis acid catalysts for the Diels-Alder reaction are formed from enantiomeric binaphthols and substituted binaph-thols. Ytterbium-, aluminum-, and titanium-based species have been developed. A chiral iron catalyst" is also quite effective, but a zirconocene-based catalyst is disappointing in terms of asymmetric induction. 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

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