Carboxylation of organoboronic esters catalyzed by N-heterocyclic carbene copper(I) complexes
Carboxylation of organoboronic esters catalyzed by N-heterocyclic carbene copper(I) complexes
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DOI：
10.1002/anie.200801857
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Hou, Zhaomin
中科院分区：
文献类型：
--
作者：
Ohishi, Takeshi;Nishiura, Masayoshi;Hou, Zhaomin
二氧化碳（CO2）是一种有吸引力的、廉价的、无毒的C1源。[1]然而，由于其高的热力学稳定性和低的反应性，使用CO2作为C1源以形成C1-C4键通常需要高度亲核的有机金属试剂，例如烷基锂化合物和格氏试剂。亲核性较低的有机硼化合物虽然容易获得，但通常不与CO2反应。近年来，过渡金属催化的碳亲核试剂与CO2的加成反应引起了人们的广泛关注。[2，3]在此背景下，Iwasawa和同事报道了在铑（I）化合物和添加剂存在下用CO2催化羧基化芳基-和烯基硼酸酯。[3b]由于各种官能化有机硼酸酯的容易获得，该反应对于官能化羧酸衍生物的合成是潜在有用的。[4]然而，不幸的是，Rh催化剂体系仅显示出对官能团的有限耐受性。尽管羰基和氰基在反应条件下存活，但更反应性的官能部分，如溴、碘和乙烯基，似乎不耐受。此外，由于催化剂体系的复杂性，关于活性催化剂物种和反应机理的信息很少。这些困难限制了Rh催化剂体系的应用范围。因此，寻找新的催化剂用于更有效的、选择性的CO2转化以及催化过程的澄清是有意义和重要的。本文报道了一种用于芳基硼酸酯和烯基硼酸酯与CO2的羧化反应的优良的N-杂环卡宾铜（I）催化剂体系。该Cu催化剂体系不仅表现出更高的官能团耐受性，而且还可以提供结构上可表征的活性催化剂物种，从而为催化过程的机理方面提供前所未有的见解。[5]据报道，带有N-杂环卡宾（NHC）配体的铜配合物可作为各种羰基化合物转化的有效催化剂，[6]如α，β-不饱和羰基化合物的共轭还原，[6 b]酮的氢化硅烷化，[6c]以及CO2的还原。[7]此外，还报道了许多铜化合物促进有机硼化合物与亲电试剂的亲核加成，例如α，β-不饱和羰基化合物和烯丙基碳酸酯。[8]这些结果鼓励我们研究有机硼酸酯与CO2通过使用N-杂环卡宾铜配合物作为催化剂的羧化反应。首先，我们研究了4-甲氧基苯基硼酸2，2-二甲基-1，3-丙二醇酯（1a）与CO2的反应，使用由CuCl、IPr· HCl [9]和tBuOK原位生成的N-杂环卡宾铜物种。在THF 708 C中，在5 mol % CuCl、5 mol% IPr· HCl和2 mmol tBuOK的存在下，1a（1 mmol）与CO2（1 atm）的反应顺利进行，在酸水解后几乎定量地得到羧化产物4-甲氧基苯甲酸2a（表1，条目1）。在相同条件下，在不存在CuCl或NHC配体或tBuOK的情况下不发生反应。使用体积较小的NHC配体（如IMes· HCl）代替IPr· HCl导致2a的产率较低（表1，条目4）。CuBr、CuI和CuII化合物CuCl 2和Cu（OAc）2的效果不如CuCl（表1，条目5-9）。进一步筛选显示，分离的卡宾氯化铜（I）络合物[（IPr）-CuCl][6 b，c]也可以显示出对于该反应的高活性，并且催化剂负载可以用1.05 mmol tBuOK降低至1 mol %（表1，条目9和10）。我们...
Carbon dioxide (CO2) is an attractive, cheap, and nontoxic C1 source.[1] However, because of its high thermodynamic stability and low reactivity, the use of CO2 as a C1 source for CÀC bond formation usually requires highly nucleophilic organometallic reagents, such as alkyllithium compounds and Grignard reagents. Less nucleophilic organoboron compounds, though easily available, usually do not react with CO2. Recently, transition-metal-catalyzed addition of carbon nucleophiles to CO2 has attracted much attention.[2, 3] In this context, Iwasawa and co-workers have reported the catalytic carboxylation of aryl-and alkenylboronic esters with CO2 in the presence of a rhodium (I) compound and additives.[3b] This reaction is potentially useful for the synthesis of functionalized carboxylic acid derivatives because of the easy availability of various functionalized organoboronic esters.[4] Unfortunately, however, the Rh catalyst systems showed only limited tolerance toward functional groups. Although carbonyl and cyano groups survived the reaction conditions, more reactive functional moieties, such as bromo, iodo, and vinyl groups, seemed intolerant. Moreover, little information about the active catalyst species and the reaction mechanism was available because of the complexity of the catalyst systems. These difficulties have limited the application scope of the Rh catalyst systems. The search for new catalysts for more efficient, selective CO2 transformation as well as the clarification of the catalytic process is therefore of interest and importance. We report herein an excellent N-heterocyclic carbene copper (I) catalyst system for the carboxylation of aryl-and alkenylboronic esters with CO2. This Cu catalyst system not only showed higher functional-group tolerance, but could also afford structurally characterizable active catalyst species, thus offering unprecedented insight into the mechanistic aspects of the catalytic process.[5] Copper complexes bearing N-heterocyclic carbene (NHC) ligands have been reported to act as efficient catalysts for the transformation of various carbonyl compounds,[6] such as conjugate reduction of α, β-unsaturated carbonyl compounds,[6b] hydrosilylation of ketones,[6c] and also for the reduction of CO2.[7] In addition, many copper compounds have also been reported to promote nucleophilic addition of organoboron compounds to electrophiles, such as α, β-unsaturated carbonyls and allylic carbonates.[8] These results encouraged us to examine the carboxylation of organoboronic esters with CO2 by use of N-heterocyclic carbene copper complexes as catalysts. At first we examined the reaction of 4-methoxyphenylboronic acid 2, 2-dimethyl-1, 3-propanediol ester (1a) with CO2 using N-heterocyclic carbene copper species generated in situ from CuCl, IPr· HCl [9] and tBuOK. In the presence of 5 mol% of CuCl with 5 mol% of IPr· HCl and 2 mmol of tBuOK in THF 708C, the reaction of 1a (1 mmol) with CO2 (1 atm) took place smoothly to afford the carboxylation product 4-methoxybenzoic acid 2a almost quantitatively after acidic hydrolysis (Table 1, entry 1). The reaction did not occur in the absence of CuCl or an NHC ligand or tBuOK under the same conditions. The use of a less bulky NHC ligand, such as IMes· HCl, instead of IPr· HCl led to a lower yield of 2a (Table 1, entry 4). CuBr, CuI, and the CuII compounds CuCl2 and Cu (OAc) 2 were less effective than CuCl (Table 1, entries 5–9). Further screening revealed that the isolated carbene copper (I) chloride complex [(IPr)-CuCl][6b, c] could also show high activity for this reaction, and the catalyst loading could be reduced to 1mol% with 1.05 mmol of tBuOK (Table 1, entries 9 and 10). We …