Highly Selective Methods for Synthesis of Internal (α-) Vinylboronates through Efficient NHC-Cu-Catalyzed Hydroboration of Terminal Alkynes. Utility in Chemical Synthesis and Mechanistic Basis for Selectivity
Highly Selective Methods for Synthesis of Internal (α-) Vinylboronates through Efficient NHC-Cu-Catalyzed Hydroboration of Terminal Alkynes. Utility in Chemical Synthesis and Mechanistic Basis for Selectivity
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DOI：
10.1021/ja2007643
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发表时间：
2011-05-25
影响因子：
15
通讯作者：
Hoveyda, Amir H.
中科院分区：
文献类型：
--
作者：
Jang, Hwanjong;Zhugralin, Adil R.;Hoveyda, Amir H.
提出了用于末端炔的位点选择性硼氢化的Cu催化方法，其中主要产生内或α-乙烯基硼酸酯（高达>98%）。反应由1-5摩尔%的铜的N-杂环卡宾（NHC）络合物催化，该络合物容易由N-芳基取代的市售咪唑啉鎓盐制备，并且在市售双-（频哪醇合）二硼[B-2（pin）2]和1.1当量的MeOH存在下在-50至-15 ℃下进行3-24小时。炔丙醇和胺以及衍生的苄基醚、叔丁基醚或甲硅烷基醚以及各种酰胺是特别有效的底物;各种芳基取代的末端炔也是合适的，其中用带有吸电子取代基的底物实现更高的α-选择性。α-选择性Cu-催化的硼氢化适合于克级程序（1摩尔%催化剂负载）。机制的研究，表明α选择性产生的NHC配体和炔烃基板的结构和电子属性。与所提出的假设一致，与衍生自N-金刚烷基取代的咪唑啉盐的Cu络合物的催化反应在相同种类的底物和类似条件下提供高的β选择性。
Cu-catalyzed methods for site-selective hydroboration of terminal alkynes, where the internal or alpha-vinylboronate is generated predominantly (up to >98%) are presented. Reactions are catalyzed by 1-5 mol % of N-heterocyclic carbene (NHC) complexes of copper, easily prepared from N-aryl-substituted commercially available imidazolinium salts, and proceed in the presence of commercially available bis-(pinacolato)diboron[B-2(pin)2] and 1.1 equiv of MeOH at -50 to -15 degrees C in 3-24 h. Propargyl alcohol and amine and the derived benzyl, tert-butyl, or silyl ethers as well as various amides are particularly effective substrates; also suitable are a wide range of aryl-substituted terminal alkynes, where higher alpha-selectivity is achieved with substrates that bear an electron-withdrawing substituent. alpha-Selective Cu-catalyzed hydroborations are amenable to gram-scale procedures (1 mol % catalyst loading). Mechanistic studies are presented, indicating that alpha selectivity arises from the structural and electronic attributes of the NHC ligands and the alkyne substrates. Consistent with suggested hypotheses, catalytic reactions with a Cu complex, derived from an N-adamantyl-substituted imidazolinium salt, afford high beta selectivity with the same class of substrates and under similar conditions.