School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Angew Chem Int Ed Engl. 2020 Dec 7;59(50):22403-22407. doi: 10.1002/anie.202007776. Epub 2020 Oct 7.
The synthetically versatile pinacol boronic ester group (Bpin) is generally thought of as a bulky moiety because of the two adjacent quaternary sp -hydribized carbon atoms in its diol backbone. However, recent diastereoselective reactions reported in the literature have cast doubt on this perception. Reported herein is a detailed experimental and computational analysis of Bpin and structurally related boronic esters which allows determination of three different steric parameters for the Bpin group: the A-value, ligand cone angle, and percent buried volume. All three parameters suggest that the Bpin moiety is remarkably small, with the planarity of the oxygen-boron-oxygen motif playing an important role in minimising steric interactions. Of the three steric parameters, percent buried volume provides the best correlation between steric size and diastereoselectivity in a Diels-Alder reaction.
通常认为，合成多功能频哪醇硼酸酯（Bpin）是一个庞大的部分，因为在其二醇骨架中有两个相邻的季碳原子的 sp 杂化。然而，文献中最近报道的立体选择性反应对此观点提出了质疑。本文报道了对 Bpin 和结构相关硼酸酯的详细实验和计算分析，这使得可以确定 Bpin 基团的三个不同的空间参数：A 值、配体锥角和百分比埋置体积。所有三个参数都表明 Bpin 部分非常小，氧-硼-氧基序的平面性在最小化空间相互作用方面起着重要作用。在 Diels-Alder 反应中，三个空间参数中，埋置体积百分比提供了与立体选择性之间最好的相关性。
