Catalysts BINOL

The above described reaction has been extended to the application of the AlMe-BINOL catalyst to reactions of acyclic nitrones. A series chiral AlMe-3,3 -diaryl-BINOL complexes llb-f was investigated as catalysts for the 1,3-dipolar cycloaddition reaction between the cyclic nitrone 14a and ethyl vinyl ether 8a [34], Surprisingly, these catalysts were not sufficiently selective for the reactions of cyclic nitrones with ethyl vinyl ether. Use of the tetramethoxy-substituted derivative llg as the catalyst for the reaction significantly improved the results (Scheme 6.14). In the presence of 10 mol% llg the reaction proceeded in a mixture of CH2CI2 and petroleum ether to give the product 15a in 79% isolated yield. The diastereoselectiv-ity was the same as in the acyclic case giving an excellent ratio of exo-15a and endo-15a of >95 <5, and exo-15a was obtained with up to 82% ee. 

Scheme 5-28 Asymmetric hydrophosphonylation of arylaldehy-des catalyzed by a heterobimetallic La/Li/BINOL catalyst (LLB)...

A related reaction using an In-BINOL catalyst that can be applied to aromatic aldehydes has been reported Takita, R. Yakura, K. Ohshima, T. ShibasaM, M. J. Am. Chem. Soc. 

The real promise of this catalytic reaction is the eventual development of an efficient enantioselective allylboration catalyzed by chiral Lewis acids. A stereoselective reaction using a substoichiometric amount of a chiral director has been reported, but only modest levels of stereo-induction were achieved with an aluminum-BINOL catalyst system (Eq. 19)P Recently, a chiral Brpnsted acid catalyzed system has been devised based on a diol-tin(IV) complex (Eq. 80). In this approach, aliphatic aldehydes provide enantioselectivities (up to 80% ee) higher than those of aromatic aldehydes when using the optimal complex 114. Although the levels of absolute stereoselectivity of this method remain too low for practical uses, promising applications are possible in double diastereoselection (see section on Double Diastereoselection ). 

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... 

Two other types of catalysts have been investigated for the enantioselective Strecker-type reactions. Chiral N-oxide catalyst 24 has been utilized in the trimethylsilyl cyanide promoted addition to aldimines to afford the corresponding aminonitriles with enantioselectivities up to 73% ee [14]. Electron-deficient aldimines were the best substrates, but unfortunately an equimolar amount of catalyst 24 was used in these reactions. The asymmetric Strecker addition of trimethylsilyl cyanide to a ketimine with titanium-based BINOL catalyst 25 gave fast conversions to quarternary aminonitriles with enantiomeric excesses to 59%... 

I. Catalytic, asymmetric hydrophosphonylation of imines promoted by the lanthanoid-potassium-BINOL catalyst (LnPB)... 

Empirical experimentation has revealed that the catalyst formed from a 1 1-1 1.4 ratio of Ln(0-i-Pr)3 (Ln = La or Yb) and 17 provides the maximum enantiomeric excesses for epoxidation (Figure 6). The 13C NMR spectrum of La-17 was not interpretable, suggesting that both the chiral Yb-17 catalyst and the La-17 catalyst exist as oligomers. Moreover, the catalytic epoxidation of 39 with Yb-17 displays a pronounced nonlinear effect (Figure 7). (For a treatment of nonlinear effects, see Chapter 5 in this volume.) Thus the oligomeric structure of these lanthanoid-BINOL catalysts may play a key role in the catalytic asymmetric... 

Mikami et al. studied the Diels-Alder reaction between a-methylstyrene and n-butyl glyoxylate catalyzed by a titanium binolate catalyst.76-78 Addition of 0.5 equivalents of (Zf)-BINOL to 1 equivalent of the racemic catalyst accelerated the reaction and gave the product with 89.8% ee (Scheme 20). Enantiopure catalyst derived solely from (/ )-BINOL gave the product with 94.5% ee. Here the amplification originates from the creation of a new chiral complex 9 of higher efficiency (rate and enantioselectivity) with respect to each enantiomer of the original racemic catalyst. 

An asymmetric Michael addition reaction to a,(i-unsaturatcd /V-acylpyrrolcs was developed <07TL2815>. The reaction was mediated by a La-based BINOL catalyst system. 

If the metal-binaphthyl complex is not fitted directly into the cyclic transition state, it becomes difficult to explain the asymmetric inductions observed. The following rule seems to be generally valid for both BINOL and BINAP complexes The complexation of carbonyl or imine moieties by (R)-binaphthyl-metal complexes results in a shielding of the si face, the reaction proceeds from the re face. Correspondingly, the opposite principle applies when (STbinaphthyl complexes are used. All aldol reactions and carbonyl-ene reactions which are catalyzed by binaphthyl complexes abide by this rule [18], and the scheme can also be applied to the addition of ketene-silyl-acetals to imines with boron-BINOL catalysts [19]. 

Very recently, a limited survey of the CAB and Keck BINOL methodology with crotyltributyltin was conducted by Marshall and Palovich (Table 3) [50b]. A modified CAB, prepared from the 2,6-dimethoxybenzoic ester of (l ,jR)-tartaric acid, and 1.5 equiv. BH3-THF was used in the addition of crotyltributyltin and allyltributyltin to representative achiral aldehydes in the presence of 2 equiv. (Cp3C0)20. Addition to crotyltin proceeded with good to excellent diastereoselectivity and enantioselectivity to give syn adducts in 70-93 % ee as major products (78 22-92 8). The addition of allylstannane to cyclohexanecarboxaldehyde afforded the (R) adduct in 55 % ee. In contrast, the use of Keck s BINOL catalyst gave an allyl adduct in 87 % ee. Addition of crotylstannane to cyclohexanecarboxaldehyde with this catalyst led, however, to a 65 35 mixture of syn and anti adducts 43 (R = Me) and 44 (R = Me) in 95 % and 49 % ee. 

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. 

The same reaction was investigated with the substituted BINOL catalyst 98 and initially it was found to be inferior to catalysts prepared from the bis-sulfonamides. Surprisingly, it was found that in the presence of 1 equiv. diethyl ether high asymmetric induction could be achieved as summarized in Table 17 [72]. The reactions are also greatly accelerated by the presence of ether. It was suggested that a pentacoordi-nate aluminum species is involved in this reaction. The effect of ether was observed for all reactions whether or not an ether linkage was present in the substrate. The effect falls off with more hindered ethers and with amines. Another remarkable aspect of this reaction is that the catalyst to substrate ratio can be reduced to 10 mol % although the induction does fall off to some extent. 

A crossed-pinacol coupling was reported using Et2Zn and with a BINOL catalyst gave good enantioselectivity. A combination of Mg and MeaSiCl was also used to a crossed-pinacol. ... 

Faller, J. W., Sams, D. W. I., Liu, X. Catalytic Asymmetric Synthesis of Homoallylic Alcohols Chiral Amplification and Chiral Poisoning in a Titanium/BINOL Catalyst System. J. Am. Chem. Soc. 1996,118, 1217-1218. 

In a direct comparison, the CAB catalyst proved to be superior to the titanium binol catalyst in the reaction of branched aldehydes in both reaction times and selectivity. Finally, the CAB-promoted reaction of a chiral aldehyde with ( )-85 was examined (Scheme 10-48). When the aldehyde (7 )-106 reacts with the 2-bute-nylstannane in the presence of the CAB catalyst, a 98/2 mixture of diastereomers is obtained. The CAB-promoted reaction of the aldehyde (S)-106 with the (E)-85 affords a 90/10 mixture of diastereomers. This adduct is the minor isomer from the BF3-promoted addition of ( )-85 to aldehyde (S)-106. Thus, the CAB-promoted additions are strongly reagent controlled, essentially overriding the intrinsic facial preference of the aldehyde substrate. 

Scheme 5.2.83 Allylation reactions with (S)- and (R)-BINOL catalysts...

Scheme 5.2.84 Examples of enantioselective allylations utilizing nonracemic BINOL catalysts...

Scheme 3.5 Enantioselective carbon-carbon bond formation in a fluorous biphasic system (top), and synthesis of the fluorous BINOL catalyst 18 (bottom) (Cso = camphorsulfonyl) [20],...

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