Rhodium- or iridium-catalyzed trans-hydroboration of terminal alkynes, giving (Z)-1-alkenylboron compounds
Rhodium- or iridium-catalyzed trans-hydroboration of terminal alkynes, giving (Z)-1-alkenylboron compounds
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DOI：
10.1021/ja0002823
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发表时间：
2000-05-24
影响因子：
15
通讯作者：
Miyaura, N
中科院分区：
文献类型：
--
作者：
Ohmura, T;Yamamoto, Y;Miyaura, N
1-烯基硼化合物是一种多用途的有机合成试剂，而炔的硼氢化反应是合成1-烯基硼化合物的一条实用途径。1，2尽管最近的研究主要集中在金属催化的邻苯二酚硼烷（1a）或频哪醇硼烷（1b）的硼氢化反应上，但非催化的1，3和催化的4，5反应都通过反马氏和硼烷与末端炔的顺式加成得到（E）-1-烯基硼酸酯。因此，（Z）-1-烯基硼化合物已经通过替代的两步法合成。6，7我们希望报道在Rh（I）-或Ir（I）-PiPr 3络合物和Et 3 N存在下，末端炔与1a或1b的正式反式硼氢化反应，以产生顺式-1-烯基硼酸酯（方案1）。为了便于分析，在分离2-4之前，将得自1a的所有产物转化为相应的频哪醇酯。1-辛炔的催化硼氢化的选择结果示于表1中。由[Rh（cod）Cl] 2和PiPr 3（4当量）原位产生的催化剂在室温下在1小时内完成1-辛炔的硼氢化（条目1）。8存在大于1当量的Et 3 N对于实现高产率和高顺式选择性是关键的，因为类似的反应在不存在Et 3 N的情况下产生所有异构体2-4的混合物（条目2）。将常规顺式硼氢化逆转为反式硼氢化的另一个主要因素是使用超过硼烷试剂的炔，因为当使用稍微过量的1a时（E）-异构体3占优势（条目3）。该反应最初产生（Z）-烯基硼酸酯2，但Rh-H物质的加成/消除顺序将2异构化为更稳定的（E）-异构体3。图9a，B PiPr 3的空间和电子效应也在影响
Hydroboration of alkynes is a practical route for the syntheses of 1-alkenylboron compounds which are a versatile reagent for organic synthesis. 1, 2 Although much attention has been recently focused on the metal-catalyzed hydroboration with catecholborane (1a) or pinacolborane (1b), both the uncatalyzed1, 3 and the catalyzed reactions4, 5 yield (E)-1-alkenylboronates through the anti-Markovnikov and syn-addition of borane to terminal alkynes. Thus,(Z)-1-alkenylboron compounds have been synthesized by an alternative, two-step method. 6, 7 We wish to report a formal trans-hydroboration of terminal alkynes with 1a or 1b to yield cis-1-alkenylboronates in the presence of a Rh (I)-or Ir (I)-PiPr3 complex and Et3N (Scheme 1). For convenience of the analyses, all products derived from 1a were converted into the corresponding pinacol esters prior to isolation of 2-4. The selected results for the catalytic hydroboration of 1-octyne are shown in Table 1.The catalyst in situ generated from [Rh (cod) Cl] 2 and PiPr3 (4 equiv) completed the hydroboration of 1-octyne within 1 h at room temperature (entry 1). 8 The presence of more than 1 equiv of Et3N was critical to achieve high yield and high cis-selectivity because a similar reaction resulted in a mixture of all isomers 2-4 in the absence of Et3N (entry 2). Another dominant factor reversing the conventional cis-hydroboration to the trans-hydroboration was the use of alkyne in excess of the borane reagent because (E)-isomer 3 was predominated when using a slightly excess of 1a (entry 3). The reaction initially yields (Z)-alkenylboronate 2, but an addition/elimination sequence of Rh-H species isomerizes 2 to a more stable (E)-isomer 3. 9a, b The steric and electronic effects of PiPr3 also play a major role in influencing