Stereoselectivity in (Acyloxy)borane-Catalyzed Mukaiyama Aldol Reactions

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• 作者：Joshua M. Lee ; Xin Zhang ; Per-Ola Norrby ; Paul Helquist ; Olaf Wiest
• 刊名：Journal of Organic Chemistry
• 出版年：2016
• 出版时间：July 1, 2016
• 年：2016
• 卷：81
• 期：13
• 页码：5314-5321
• 全文大小：421K
• 年卷期：0
• ISSN：1520-6904

文摘

The origin of diastereo- and enantioselectivity in a Lewis acid-catalyzed Mukaiyama aldol reaction is investigated using a combination of dispersion corrected DFT calculations and transition state force fields (TSFF) developed using the quantum guided molecular mechanics (Q2MM) method. The reaction proceeds via a closed transition structure involving a nontraditional hydrogen bond that is 3.3 kJ/mol lower in energy than the corresponding open transition structure. The correct prediction of the diastereoselectivity of a Mukaiyama aldol reaction catalyzed by the conformationally flexible Yamamoto chiral (acyloxy) borane (CAB) requires extensive conformational sampling at the transition structure, which is achieved using a Q2MM-derived TSFF, followed by DFT calculations of the low energy conformational clusters. Finally, a conceptual model for the rationalization of the observed diastereo- and enantioselectivity of the reaction using a closed transition state model is proposed.