A Theoretical Investigation on the Strecker Reaction Catalyzed by a TiIV‐Complex Catalyst Generated from a Cinchona Alkaloid, Achiral Substituted 2, 2′‐Biphenol, and Tetraisopropyl Titanate. Issue 5 (12th December 2012)

• Record Type: Journal Article
• Authors: Su, Zhishan; Li, Weiyi; Wang, Jun; Hu, Changwei; Feng, Xiaoming
• Is Part Of: Chemistry. Volume 19:Issue 5(2013)
• Page Start: 1637
• Page End: 1646
• Publication Date: 2012-12-12
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201202237
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The mechanism and the origin of selectivity of the asymmetric Strecker reaction catalyzed by a Ti^IV‐complex catalyst generated from a cinchona alkaloid, achiral substituted 2, 2′‐biphenol, and tetraisopropyl titanate have been investigated by DFT and ONIOM methods. The calculations indicate that the reaction proceeds through a dual activation mechanism, in which Ti^IV acts as Lewis acid to activate the electrophile aldimine substrate, whereas the tertiary amine in cinchona alkaloid works as Lewis base to promote the activation and isomerization of HCN. The CC bond formation step is predicted to be the selectivity‐controlling step in the reaction with an energy barrier of 9.3 kcal mol⁻¹. The "asymmetric activation" is achieved by the transfer of asymmetry from the chiral cinchonine ligand to the axially flexible achiral biphenol ligand through coordination interaction with the central metal Ti^IV. The large steric hindrance from the 3, 3′‐position substitute of biphenol, combined with the quinoline fragment of cinchona alkaloid and the orientation of hydrogen bonding of <italic>i</italic>PrOH, play a key role in controlling the stereoselectivity, which is in good agreement with the experimental observations.</p> </abstract>