Cationic Co(I)-Intermediates for Hydrofunctionalization Reactions: Regio- and Enantioselective Cobalt-Catalyzed 1,2-Hydroboration of 1,3-Dienes
Cationic Co(I)-Intermediates for Hydrofunctionalization Reactions: Regio- and Enantioselective Cobalt-Catalyzed 1,2-Hydroboration of 1,3-Dienes
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DOI：
10.1021/jacs.8b13812
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发表时间：
2019-05-08
影响因子：
15
通讯作者：
RajanBabu, T. V.
中科院分区：
文献类型：
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作者：
Duvvuri, Krishnaja;Dewese, Kendra R.;RajanBabu, T. V.
近年来，烯烃的催化硼氢化反应主要集中在简单烯烃和苯乙烯衍生物上，而1，3-二烯的反应很少，据报道主要是通过1，4-加成反应得到烯丙基硼酸酯。发现1，n-双二苯基膦烷配合物[Ph_2P-（CH_2）（n）-PPh_2] CoX_2; n = 1-5）或来自（2-恶唑啉基）-苯基二芳基膦配合物[（G-PHOX）CoX 2]（G =恶唑啉环上的4-取代基）实现频哪醇硼烷（HBPin）对多种1，3-二烯取决于所选配体。已发现优化1，2-加成的条件。反应性催化剂可以在四[（3，5-三氟甲基）苯基]硼酸钠（NaBARF）存在下使用三甲基铝、甲基铝氧烷或活化锌由钴（II）络合物产生。配合物（dppp）CoCl 2对各种线性末端1，3-二烯和2-取代1，3-二烯的加成反应具有最好的结果（1，2-与1，4-加成比>95：5）。[（PHOX）CoX_2]（X = Cl，Br）配合物主要与直链未取代的1，3-二烯进行1，4-加成，但令人惊讶的是，与2-取代或2，3-二取代的1，3-二烯进行选择性的1，2-加成。分离并充分表征（X射线晶体学）Co（I）-络合物（dppp）（3）Co2 Cl 2和[（S，S）-BDPP]（3）Co2 Cl 2不催化反应，除非被刘易斯酸或NaBARF活化，表明阳离子Co（I）物质在催化循环中的关键作用。2-取代的1，3-二烯的区域选择性和对映选择性1，2-硼氢化反应最好使用通过用锌和NaBARF活化手性膦基恶唑啉-钴（II）络合物制备的催化剂来完成。容许许多常见的官能团，其中包括-OBn、-OTBS、-OTs、N-邻苯二甲酰亚胺基，并且对于包括1-链烯基环烷-1-烯的几种二烯，获得> 95：5的er。这种操作简单的反应扩展了不对称硼氢化反应的领域，以提供直接获得许多几乎对映体纯的高烯丙基硼酸酯，这是不容易通过目前的方法。通过C-BPin到C-芳基的转化，将所得硼酸酯转化为相应的醇、三氟硼酸钾盐、N-BOC胺和芳基衍生物。
Much of the recent work on catalytic hydroboration of alkenes has focused on simple alkenes and styrene derivatives with few examples of reactions of 1,3-dienes, which have been reported to undergo mostly 1,4-additions to give allylic boronates. We find that reduced cobalt catalysts generated from 1,n-bis-diphenylphosphinoalkane complexes [Ph2P-(CH2)(n)-PPh2]CoX2; n = 1-5) or from (2-oxazolinyl)-phenyldiarylphosphine complexes [(G-PHOX)CoX2] (G = 4-substituent on oxazoline ring) effect selective 1,2-, 1,4-, or 4,3-additions of pinacolborane (HBPin) to a variety of 1,3-dienes depending on the ligands chosen. Conditions have been found to optimize the 1,2-additions. The reactive catalysts can be generated from the cobalt(II)-complexes using trimethylaluminum, methyl aluminoxane, or activated zinc in the presence of sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate (NaBARF). The complex, (dppp)CoCl2, gives the best results (ratio of 1,2- to 1,4-addition >95:5) for a variety of linear terminal 1,3-dienes and 2-substituted 1,3-dienes. The [(PHOX)CoX2] (X = Cl, Br) complexes give mostly 1,4-addition with linear unsubstituted 1,3-dienes, but, surprisingly, selective 1,2-additions with 2-substituted or 2,3-disubstituted 1,3-dienes. Isolated and fully characterized (X-ray crystallography) Co(I)-complexes, (dppp)(3)Co2Cl2 and [(S,S)-BDPP](3)Co2Cl2, do not catalyze the reaction unless activated by a Lewis acid or NaBARF, suggesting a key role for a cationic Co(I) species in the catalytic cycle. Regio- and enantioselective 1,2-hydroborations of 2-substituted 1,3-dienes are best accomplished using a catalyst prepared via activation of a chiral phosphinooxazoline-cobalt(II) complex with zinc and NaBARF. A number of common functional groups, among them, -OBn, -OTBS, -OTs, N-phthalimido-groups, are tolerated, and er's > 95:5 are obtained for several dienes including 1-alkenylcycloalk-1-enes. This operationally simple reaction expands the realm of asymmetric hydroboration to provide direct access to a number of nearly enantiopure homoallylic boronates, which are not readily accessible by current methods. The resulting boronates have been converted into the corresponding alcohols, potassium trifluororoborate salts, N-BOC amines, and aryl derivatives by C-BPin to C-aryl transformation.