Asymmetric Activation of Racemic Catalysts

An alternative but conceptually opposite strategy has been reported for asymmetric catalysis by racemic catalysts. A chiral activator selectively activates one enantiomer of a racemic chiral catalyst. A higher level of catalytic efficiency by more than two orders of magnitude kact kx]0 ), in addition to a higher enantio-selectivity, might be attained than that achieved by an enantio-pure catalyst (Xaei% ee x% ee) (Fig. 7-2). 

The ene reaction is one of the simplest ways for C-C bond formation, which converts readily available olefins with C-H bond activation at an allylic site by 

The activation of the enantio-pure (f )-binolato-Ti(OiPr)2 catalyst (2) is also synthetically useful by further addition of (7 )-binol (Eq. (7.19) and Table 7-3). The reaction proceeded quite smoothly to provide the carbonyl-ene product in higher chemical yield (82.1%) and enantioselectivity (96.8% ee) than those without additional binol (94.5% ee, 19.8%) (Run 2 vs. 1). Comparing the results of enantiomer-selective activation of the racemic catalyst (89.8% ee, R) (Table 7-2, Run 4) with those of the enantio-pure catalyst [with (96.8% ee, R) or without activator (94.5% ee, / )], the reaction catalyzed by the (/ )-binolato-Ti(0/Pr)2/(/ )-bi-nol complex (2 ) is calculated to be 26.3 times as fast as that catalyzed by the (S)-binolato-Ti(OiPr)2 (2) in the racemic case (Fig. 7-9 a). Indeed, kinetic studies show that the reaction catalyzed by the (/ )-binolato-Ti(0/Pr)2/(R)-binol complex (2 ) is 25.6 (=fcac/ ) times as fast as that catalyzed by the (f )-binoIato-Ti(OiPr)2 (2). These results imply that the racemic ( )-binolato-Ti(0/Pr)2 (2) and half-molar amount of (/ )-binol assemble preferentially into the (7 )-binolato-Ti(OiPr)2/(/ )-bi- 

The great advantage of asymmetric activation of the racemic binolato-Ti(OiPr)2 complex (2) is highlighted in a catalytic version (Table 7-2, Run 5). High enantioselectivity (80.0% ee) is obtained by adding less than the stoichiometric amount (0.25 molar amount) of additional (R)-binol. A similar phenomenon on enantiomer-selective activation has been observed in aldol (Eq. (7.20)) [53J and 

Activation of the (f )-binolato-Ti(OiPr)2 (2) by highly acidic and sterically demanding alcohols as achiral rather than chiral activators is also effective to provide higher levels of enantioselectivity than those attained by the parent enantio-pure binolato-Ti(OiPr) catalyst (2) in the Mukaiyama aldol reaction of silyl enol ethers (Eq. (7.22)) [55]. 

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Table 7-4. Asymmetric activation of racemic binaps-RuCiT catalyst (3) by enantio-pure dpen ) ...

These new generation catalysts have been applied to the asymmetric activation of an inactive racemic metal compound by a non-racemic enantiopure ligand, called a vitamer (Scheme 20a). In contrast, a racemic catalyst can interact with an enantiopure chiral poison (asymmetric... 

ASYMMETRIC ACTIVATION AND DEACTIVATION OF RACEMIC CATALYSTS (a) Aldol reaction... 

Combination of the asymmetric activation and asymmetric deactivation protocols as asymmetric activation/deactivation can be achieve the difference in catalytic activity between the two enantiomers of racemic catalysts can be maximized through selective activation and deactivation of enantiomeric catalyst, respectively (Scheme 8.15). 

The asymmetric activation can be done by a chiral activator through in situ diastereomer interconversion of the tropos ligand of racemic catalysts (Scheme 8.22). One possible case is that selective complexation of a chiral activator with one enantiomer of a racemic catalyst occurs. The remaining enantiomeric catalyst may then isomerize and complex with the chiral activator leading to a single diastereomer (Scheme 8.22a). The other case is that nonselective complexation of a tropos catalyst with a chiral activator initially provides a 1 1 ratio of activated diastereomers, which would isomerize to the single diastereomeric activated complex (Scheme 8.22b). 

Asymmetric Polymerization. The chiral organoaluminum catalyst is utilized for asymmetric polymerization of racemic a-methyl and p-methyl p-lactones. Optically active polymers pos-... 

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