Expanded substrate scope and improved reactivity of ether-forming cross-coupling reactions of organotrifluoroborates and acetals.
Expanded substrate scope and improved reactivity of ether-forming cross-coupling reactions of organotrifluoroborates and acetals.
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DOI：
10.1021/ja205174c
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发表时间：
2011-09-07
影响因子：
15
通讯作者：
Bode, Jeffrey W.
中科院分区：
文献类型：
--
作者：
Vo, Cam-Van T.;Mitchell, T. Andrew;Bode, Jeffrey W.
混合缩醛和有机三氟硼酸盐在简单、温和的条件下进行BF3•OEt2促进的交叉偶联得到二烷基醚。一项对反应伙伴的调查发现，在BF3•oet2促进的混合缩醛与炔基、烯基、芳基和杂芳基三氟硼酸钾偶联反应中，羟基甲酸酯离去基提高了区域选择性和产物收率，从而获得取代的二烷基醚。该离去基使反应在温和条件下（0°C， 5-60分钟）快速进行，并允许与缺乏电子的芳基三氟硼酸钾反应，而与其他缩醛底物反应性较低。通过核磁共振和x射线晶体学对反应机理和关键中间体的表征进行了研究，确定了羟甲酸酯部分作为可逆离去基的作用，用于稳定关键的氧羰基中间体，并且需要少量过量的有机二氟硼烷作为催化剂。作为硼亲核试剂的激活配体的次要作用也被考虑。这些研究为一类通过简单的、可预测的交叉偶联反应生成二烷基醚的试剂提供了基础。
Mixed acetals and organotrifluoroborates undergo BF3•OEt2 promoted cross-couplings to give dialkyl ethers under simple, mild conditions. A survey of reaction partners identified a hydroxamate leaving group that improves the regioselectivity and product yield in the BF3•OEt2-promoted coupling reaction of mixed acetals and potassium alkynyl-, alkenyl-, aryl- and heteroaryltrifluoroborates to access substituted dialkyl ethers. This leaving group enables reaction to proceed rapidly under mild conditions (0 °C, 5–60 min) and permits reactions with electron-deficient potassium aryltrifluoroborates that were less reactive with other acetal substrates. A study of the reaction mechanism and characterization of key intermediates by NMR and X-ray crystallography identified a role for the hydroxamate moiety as a reversible leaving group that serves to stabilize the key oxocarbenium intermediate and the need for a slight excess of organodifluoroborane to serve as a catalyst. A secondary role as an activating ligand for the boron nucleophile is also considered. These studies provide the basis for a general class of reagents that lead to dialkyl ethers by a simple, predictable cross-coupling reaction.
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