Asymmetric induction in hydrogen-mediated reductive aldol additions to α-amino aldehydes catalyzed by rhodium: Selective formation of syn-stereotriads directed by intramolecular hydrogen-bonding

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作者：
Jung, Cheol-Kyu [1 ]
Krische, Michael J. [1 ]
机构：
[1] Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX 78712, United States
来源：
Journal of the American Chemical Society | 2006年 / 128卷 / 51期
关键词：
Rhodium-catalyzed hydrogenation of methyl vinyl ketone and ethyl vinyl ketone in the presence of N-Boc-α-aminoaldehydes 3a-8a at ambient temperature and pressure results in reductive C-C coupling to furnish aldol adducts 3b-8b and 3c-8c, respectively, which incorporate stereotriads that embody high levels of syn-aldol selectivity accompanied by high levels of anti-Felkin-Anh control. The collective data are consistent with a catalytic mechanism involving addition of the Z(O)-rhodium enolate to the sterically less-encumbered aldehyde π-face of an intramolecularly hydrogen-bonded chelate through a Zimmerman-Traxler type transition structure. Stereochemical assignments are supported by single-crystal X-ray diffraction analysis of 5b-O-3,5-dinitrobenzoate, iso-5b, N-Me-iso-5b-O-3,5-dinitrobenzoate, and 7b. As revealed by HPLC analysis, optical purity of the stereochemically labile α-aminoaldehydes is completely preserved under the conditions of hydrogen-mediated aldol coupling. Deletion of the intramolecular hydrogen bond, as in the case of N-methyl-N-Boc-L-leucinal N-Me-5a, inverts stereoselectivity to furnish the Felkin-Anh product N-Me-iso-5b in 17% yield. Additionally, reactions performed in the presence of tert-amyl alcohol (10 equiv) exhibit markedly lower levels of anti-Felkin-Anh control (7:1 versus ≥ 20:1). The collective studies suggest that intramolecular hydrogen bonding plays a key role in both activating the α-aminoaldehyde toward addition and directing facial selectivity. © 2006 American Chemical Society;
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页码：17051 / 17056
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