R-BINOL

The 2-pyrones can behave as dienes or dienophiles depending on the nature of their reaction partners. 3-Carbomethoxy-2-pyrone (84) underwent inverse Diels-Alder reaction with several vinylethers under lanthanide shift reagent-catalysis [84] (Equation 3.28). The use of strong traditional Lewis acids was precluded because of the sensitivity of the cycloadducts toward decarboxylation. It is noteworthy that whereas Yb(OTf)j does not catalyze the cycloaddition of 84 with enolethers, the addition of (R)-BINOL generates a new active ytterbium catalyst which promotes the reactions with a moderate to good level of enantio selection [85]. 

Possible racemisation of imines, derivatives of amino acids and R(—)-myrtenal, has been examined by Dufrasne et al.1 After 72 h, no significant effect on chiral purity was observed. For imines being derivatives of chiral primary amines and the a-substituted 8-keto-aldehydes, no evidence of epimerisation has been indicated by the NMR measurements.3 For a series of imines, being derivatives of amino acids or amino acid esters and (R)-BINOL reagents, Chin et al.5 have tested the possibility of epimerization under experiment conditions. It was shown that R S ratio has changed only slightly, and after 24 h, the difference was lower than 10%. 

On the basis of the splitting of several JH signals in the 20- and 40-membered U02 complexes, [11], [12] being derivatives of (R)-binol, Amato et al.27 have suggested the presence of a conformational equilibrium. For the 20-membered complex, the rate constant at 300 K was... 

Enantioselective Diels-Alder reactions proceed smoothly in the presence of a chiral Sc catalyst, prepared in situ from Sc(OTf)3, R)- I )-l,l -bi-2-napluhol [(R)-BINOL], and a tertiary amine in dichloromethane.58 The catalyst is also effective in Diels-Alder reactions of an acrylic acid derivative with dienes (Scheme 14). 

BINOL and related compounds have proved to be effective catalysts for a variety of reactions. Zhang et al.106a and Mori and Nakai106b used an (R)-BINOL-Ti(OPr )4 catalyst system in the enantioselective diethylzinc alkylation of aldehydes, and the corresponding secondary alcohols were obtained with high enantioselectivity. This catalytic system works well even for aliphatic aldehydes. Dialkylzinc addition promoted by TifOPr1 in the presence of (R)- or (A)-BINOL can give excellent results under very mild conditions. Both conversion of the aldehyde and the ee of the product can be over 90% in most cases. The results are summarized in Table 2-14. 

Asymmetric ring opening of 3,4-epoxy cyclopentanone (desymmetrization) catalyzed by 2 mol% of an (R)-BINOL modified aluminum complex affords the (4/ )-hydroxy enone in 95% ee at 98% yield (Scheme 7-29).2... 

The self-assembly of a chiral Ti catalyst can be achieved by using the achiral precursor Ti(OPr )4 and two different chiral diol components, (R)-BINOL and (R,R)-TADDOL, in a molar ratio of 1 1 1. The components of less basic (R)-BINOL and the relatively more basic (R,R)-TADDOL assemble with Ti(OPr )4 in a molar ratio of 1 1 1, yielding chiral titanium catalyst 118 in the reaction system. In the asymmetric catalysis of the carbonyl-ene reaction, 118 is not only the most enantioselective catalyst but also the most stable and the exclusively formed species in the reaction system. 

The idea of enantioselective activation was first reported by Mikami and Matsukawa111 for carbonyl-ene reactions. Using an additional catalytic amount of (R)-BINOL or (/ )-5.5 -dichloro-4,4, 6,fi -tctramcthyl biphenyl as the chiral activator, (R)-ene products were obtained in high ee when a catalyst system consisting of rac-BINOL and Ti(OPri)4 was employed for the enantioselective carbonyl ene reaction of glyoxylate (Scheme 8-54). Amazingly, racemic BINOL can also be used in this system as an activator for the (R)-BINOL-Ti catalyst, affording an enhanced level of enantioselectivity (96% ee). 

This strategy has recently been extended to optically active stereosequences, either by using a chiral protective group (carbamate) as an inductor, or by using (S)- or (R)-BINOL-TiCl2 as the catalyst for the Mukayiama reaction [29]. 

Asymmetric epoxidation of ( -benzylideneacetophenone using THE La-(R)-BINOL-Ph3PO/CUMENE hydroperoxide system... 

In a 10 mL round-bottomed flask equipped with a magnetic stirrer bar were placed activated molecular sieves 4A (44.9 mg), (R)-BINOL (6.4 mg), triphenylphosphine oxide (18.7 mg), and anhydrous tetrahydrofuran (0.9 mL) and the mixture was stirred for 5 minutes under argon. To this suspension was added a suspension of La(0 z-Pr)3 (7.1 mg) in tetrahydrofuran (1.4 mL) by a syringe. After stirring for 1 hour at room temperature, cumene hydro-... 

The method is applicable to a wide range of substrates. Table 4.4 gives various a, (3-enones that can be epoxidized with the La-(R)-BINOL-Ph3PO/ROOH system. The substituents (R1 and R2) can be either aryl or alkyl. Cumene hydroperoxide can be a superior oxidant for the substrates with R2 = aryl group whereas t-butyl hydroperoxide (TBHP) gives a better result for the substrates with R1 = R2 = alkyl group. 

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