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Volumn 130, Issue 16, 2008, Pages 5530-5541

Catalytic asymmetric alkylations of ketoimines. Enantioselective synthesis of N-substituted quaternary carbon stereogenic centers by Zr-catalyzed additions of dialkylzinc reagents to aryl-, alkyl-, and trifluoroalkyl-substituted ketoimines

Author keywords

[No Author keywords available]

Indexed keywords

ALKYLATION; CARBON; CATALYST ACTIVITY; SUBSTITUTION REACTIONS;

EID: 42949168465     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja8001343     Document Type: Article
Times cited : (154)

References (113)
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    • All structures were minimized according to the following sequence. The promising conformers from conformer distributions, obtained with the PM3 semi-empirical method, were optimized at the B3LYP level of theory. Conformer distributions were calculated with the Spartan '04 suite. Final stationary points on the potential energy surface were calculated with the Gaussian 03.D02 suite. Electronic configurations of the molecular systems were described by 6-31G(d,p) double-ζ basis set on H, C, N, and O; the basis sets for N and O were augmented with a single sp-type and a single d-type diffuse functions as supplied by Gaussian 03.D02. All basis sets are as supplied by the Gaussian 03.D02 suite. The valley nature of stationary points was confirmed by frequency calculation at the same level of theory: (a) Spartan 04: Pople, J. A, et al. J. Comput. Chem. 2000, 21, 1532-1548
    • All structures were minimized according to the following sequence. The promising conformers from conformer distributions, obtained with the PM3 semi-empirical method, were optimized at the B3LYP level of theory. Conformer distributions were calculated with the Spartan '04 suite. Final stationary points on the potential energy surface were calculated with the Gaussian 03.D02 suite. Electronic configurations of the molecular systems were described by 6-31G(d,p) double-ζ basis set on H, C, N, and O; the basis sets for N and O were augmented with a single sp-type and a single d-type diffuse functions as supplied by Gaussian 03.D02. All basis sets are as supplied by the Gaussian 03.D02 suite. The valley nature of stationary points was confirmed by frequency calculation at the same level of theory: (a) Spartan 04: Pople, J. A.; et al. J. Comput. Chem. 2000, 21, 1532-1548.
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    • With 0.5 mol, of the chiral ligand, use of lower amounts of the Zr salt (<7 mol , results in diminished reaction efficiency. Thus, with 1, 2, and 5 mol, Zr(Oi-Pr)4·HOi-Pr, under otherwise identical conditions, 27, 34, and 41% conversion to 6a is observed, respectively. Similarly, when catalytic alkylation is performed in the presence of increased amounts of the transition metal salt, lower conversions are obtained (e.g, 33% and 26% conversion with 10 and 15 mol, Zr(Oi-Pr) 4·HOi-Pr, It should be noted, however, that except for when 1-2 mol, of the Zr salt is used (87-88% ee, 6a is obtained in 94-96% ee. The mechanistic implications of the above observations, which do not uniformly apply to all ketoimines, are not clear but might suggest that the identity (stoichiometry) of the active chiral complex is not comprised of a 1:1 combination of the Zr salt and the chiral ligand see proposed predictive
    • 4·HOi-Pr). It should be noted, however, that except for when 1-2 mol % of the Zr salt is used (87-88% ee), 6a is obtained in 94-96% ee. The mechanistic implications of the above observations, which do not uniformly apply to all ketoimines, are not clear but might suggest that the identity (stoichiometry) of the active chiral complex is not comprised of a 1:1 combination of the Zr salt and the chiral ligand (see proposed predictive models in Figure 4).
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    • Similar side products are observed, even in Ti-catalyzed reactions of the more reactive α-aldimine esters with Et2Zn; see ref 61
    • 2Zn; see ref 61 .
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    • Extensive ligand screening studies did not lead to identification of a catalyst system that promotes alkylation with Et2Zn with improved product selectivity. Furthermore, for transformation of 2a to 21a, slow addition of the dialkylzinc reagent (over 22 h, under otherwise identical conditions (Tables 7 and 8, results in only ∼50% conversion and diminished enantioselectivity (82% ee vs 93% ee) without an improvement in product selectivity
    • 2Zn with improved product selectivity. Furthermore, for transformation of 2a to 21a, slow addition of the dialkylzinc reagent (over 22 h), under otherwise identical conditions (Tables 7 and 8), results in only ∼50% conversion and diminished enantioselectivity (82% ee vs 93% ee) without an improvement in product selectivity.
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    • 4·HOi-Pr (vs 20 mol %) are used, 21a-23a are generated in nearly equal amounts. Increasing the amount of the chiral ligand is detrimental as well: with 10 mol % 1 and 10 mol % of the Zr salt, the reaction outcome is the same as mentioned above.
    • 4·HOi-Pr (vs 20 mol %) are used, 21a-23a are generated in nearly equal amounts. Increasing the amount of the chiral ligand is detrimental as well: with 10 mol % 1 and 10 mol % of the Zr salt, the reaction outcome is the same as mentioned above.
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    • Attempts to prepare dialkylzinc reagents through reaction of the corresponding alkyllithium reagents with ZnCl2 for in situ use in Zr-catalyzed AA reactions resulted in variable conversions and complete lack of enantioslectivity <2% ee
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    • Experimental and theoretical studies suggest that Lewis base coordination to a dialkylzinc reduces Zn-C bond order, increasing alkylmetal nucleophilicity. See: (a) Hursthouse, M. B, Motevalli, M, O'Brien, P, Walsh, J. R, Jones. A. C. J. Mater. Chem. 1991, 1, 139-140
    • Experimental and theoretical studies suggest that Lewis base coordination to a dialkylzinc reduces Zn-C bond order, increasing alkylmetal nucleophilicity. See: (a) Hursthouse, M. B.; Motevalli, M.; O'Brien, P.; Walsh, J. R.; Jones. A. C. J. Mater. Chem. 1991, 1, 139-140.
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    • In one instance, catalytic asymmetric additions of HCN to aldimines have been shown to proceed through the thermodynamically less favored Z substrate isomer: Vachal, P.; Jacobsen, E. N J. Am. Chem. Soc. 2002, 124, 10012-10014.
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