高效镁催化的羧酸硼氢化反应

郑玉坤 曹旭 李佳 华海明 姚薇薇 赵斌林 马猛涛

引用本文: 郑玉坤, 曹旭, 李佳, 华海明, 姚薇薇, 赵斌林, 马猛涛. 高效镁催化的羧酸硼氢化反应[J]. 有机化学, 2020, 40(7): 2086-2093. doi: 10.6023/cjoc202003058 shu
Citation:  Zheng Yukun, Cao Xu, Li Jia, Hua Haiming, Yao Weiwei, Zhao Binlin, Ma Mengtao. Efficient Magnesium-Catalyzed Hydroboration of Carboxylic Acids[J]. Chinese Journal of Organic Chemistry, 2020, 40(7): 2086-2093. doi: 10.6023/cjoc202003058 shu

高效镁催化的羧酸硼氢化反应

    通讯作者: 姚薇薇yww0715@hotmail.com; 赵斌林zblchemistry@163.com; 马猛涛mengtao@njfu.edu.cn
  • 基金项目:

    国家自然科学基金(Nos. 21772093, 21372117)和江苏省自然科学基金(No. BK20181421)资助项目

摘要: 在温和的反应条件下, 较大空间位阻的氨基镁甲基化合物LMgCH3(THF)2 [L=N(Ar)(SiMe3), Ar=4, 2, 6-Me(CHPh2)2C6H2], 可作为高效催化剂前体应用于一系列芳香族和脂肪族羧酸与频哪醇硼烷的脱氧硼氢化反应中.在同样反应条件下观察到羧酸对酯的化学选择性硼氢化.基于密度函数理论(DFT)计算和化学计量反应, 提出了两种可能的羧酸硼氢化反应机理.

English

  • The reduction of carboxylic acids to alcohols is one of the very important chemical transformations in organic chemistry and a relatively difficult reaction compared to the reduction of aldehydes, ketones and esters. It is commonly performed using stoichiometric metal hydride reagents such as LiAlH4 or NaBH4 that posed an inherent safety risk and inorganic waste material due to their high reactivities. The catalytic hydrogenation of carboxylic acids required various precious transition metals that typically operated at the harsh reaction conditions (high temperatures and high pressures).[1-5] Another method is the transition metal (Ru, Rh, Ir, Fe, Mn, Cu etc.) catalyzed hydrosilylation of carboxylic acids to alcohols which is efficient, however, the limited substrate scope and the use of light-mediated protocols can be considered as principal drawbacks in the hydrosilylation.[6-13] In addition, most of hydrosilylation of carboxylic acids provided aldehyde products selectively depending on the molecular structure of the substrates and the metal catalysts.[6-7, 10-11] Sometimes the aliphatic acids were over-reduced via hydrosilylation to afford the corresponding alkane derivatives.[13]

    Recently the hydroboration has attracted considerable attention and showed high efficiency in the reduction of unsaturated organic compounds such as aldehyde, ketone, ester etc.[14-20] The more efficient and facile hydroboration was a plausible alternative protocol for the reduction of carboxylic acids to alcohol products. However, examples of metal-catalyzed hydroboration of carboxylic acids are very rare. Gunanathan et al.[21] recently reported the first example of Ru-catalyzed hydroboration of carboxylic acids. Soon afterwards, Leitner and Maji et al.[22] subsequently reported the Mn-catalyzed hydroboration of carboxylic acid, respectively. To date there is an increasing demand for green and sustainable alternative to replace transition metals catalyst. Compared to the transition metals, alkaline earth metal, especially magnesium offers earth-abundant, non-toxic, and environmentally benign properties in general. However, to the best of our knowledge, there is no example of main group metal catalyzed hydroboration of carboxylic acids in the literature. Taking into account the interest in developing new catalyst for the reduction of carboxylic acids, we herein describe an efficient and facile main group Mg-catalyzed hydroboration of carboxylic acids with HBpin under mild conditions. During this protocol, several groups reported independently the catalyst-free and solvent-free hydroboration of carboxylic acids quite recently, however, they required relatively excessive HBpin (4 equiv.) or longer reaction time (12 h).[23]

    Initially, the hydroboration reaction of benzoic acid with 3 equiv. of HBpin was performed at 60 ℃ under catalyst-free and solvent-free conditions. Only 30% yield was obtained after 1 h (Entry 1, Table 1). When 0.1 mol% magnesium complex 1 was added, the yield was increased to 82% in 2 h under the same reaction condition (Entry 2, Table 1). When a slightly excessive HBpin (3.1 equiv.) was employed, benzoic acid was completely converted into the corresponding benzyl boronate ester in quantitative yield within 2 h (Entries 4, 5, Table 1). If without catalyst, only 40% yield was obtained (Entry 3, Table 1). The low yield was observed when the similar reaction was performed at room temperature (Entry 6, Table 1). It is noteworthy that the new main group Mg-catalyzed protocol is more efficient than the corresponding transition metal Ru/ Mn-catalyzed hydroboration of carboxylic acid (Mg: 60 ℃, 2 h vs Ru/Mn: 60~115 ℃, 20~24 h).[21-22] Moreover, compared to catalyst-free approach, the Mg catalyst also offers some advantages such as less amount of HBpin or short reaction time.[23]

    表 1

    Table 1.  Optimization of hydroboration of benzoic acid
    下载: 导出CSV
    Entry n Cat. 1/mol% t/℃ Time/h Yielda/%
    1 3.0 60 1 30
    2 3.0 0.1 60 2 82
    3 3.1 60 1 40
    4 3.1 0.1 60 1 61
    5 3.1 0.1 60 2 99
    6 3.1 0.1 25 2 15
    a The yield was determined by 1H NMR spectroscopy.

    With the optimized reaction conditions in hand, we decide to examine the scope of deoxygenative hydroboration of carboxylic acids. As shown in Table 2, the reduction of a wide range of aromatic and aliphatic carboxylic acids proceeded very well. For example, the deoxygenative hydroboration of a series of benzoic acid derivatives with electron-donating or electron-withdrawing groups such as Me, tBu, OMe, F, Cl, and Br were all efficiently converted into the corresponding alkyl boronate esters in high yield (3b~3h, Table 2). The reactivity of para-fluoro-substi- tuted of benzoic acid was slightly higher than that of ortho-fluoro-substituted benzoic acid (3e, 3f, Table 2). It was consistent with Ru-catalyzed hydroboration of para/ortho-bromo benzoic acid [21]. Even the acetoxy group was tolerated in the hydroboration of 4-acetoxybenzoic acid and afforded 97% yield (3i, Table 2). For the hydroboration of sterically bulky carboxylic acid such as 4-tert-butyl benzoic acid, the main group magnesium catalyst 1 only required 2 h to obtain 99% yield. However, when the same reaction was intervened within 8 h, the transition metal ruthenium catalyst only provided 70% yield. It needed to lengthen 24 h to afford 91% yield (Mg: 99% yield, 2 h vs. Ru: 70% yield, 8 h or 91% yield, 24 h).[21] In contrast to iron-catalyzed hydrosilylation of carboxylic acid, which involving the electron-rich carboxylic acids such as 4-tert-butyl benzoic acid and 4-methoxy benzoic acid led to over-reduction and resulted in alkane formation, [11] a especial reaction of benzoic acid with 4 equiv. of HBpin and 2 mol% catalyst loading of 1 was carried out at an elevated temperature (90 ℃) for 24 h. The selective formation of benzyl boronate ester was observed as an exclusive product. More sterically bulky benzoic acid derivatives such as 1-naphthoic acid and 2-naphthoic acid were also hydroborated in full conversion to the corresponding alkyl boronate esters (3j, 3k, Table 2).

    表 2

    Table 2.  Hydroboration of carboxylic acids catalyzed by magnesium complex 1a
    下载: 导出CSV
    a The yield was determined by 1H NMR spectroscopy. b HBpin (6.2 equiv.) was used.

    Encouraged by the excellent reactivity of 1 on the hydroboration of aromatic acids, we then extended the substrate scope to the aliphatic carboxylic acids. To our delight, the hydroboration of aliphatic carboxylic acids catalyzed by 1 was also successfully transformed to the corresponding alkyl boronate esters in excellent yields, irrespective of steric and electronic nature of the substrates. The initial hydroboration of acetic acid with HBpin proceeds rapidly to afford the ethyl boronate ester in quantitative yield (3l, Table 2). However, when increasing the carbon-chain length of acetic acid such as pentanoic acid and heptanoic acid, a gradually decreased conversion of product was obtained (94% and 93%, respectively, 3m, 3n, Table 2). Moreover, halide-substituted aliphatic carboxylic acid (3-chloropropionic acid) was subjected to hydroboration with HBpin, and 95% yield was observed. Similarly, hydroboration of many other aliphatic carboxylic acids, such as 2-cyclohexyl acetic acid, cyclohexanecarboxylic acid, pivalic acid, 3-phenylpropanoic acid and 4-phenyl- butyric acid, have provided the corresponding boronate esters in quantitative conversion under the standard conditions (3p~3t, Table 2). To our surprise, the hydroboration reaction of more sterically bulky 2-phenylpropanoic acid and diphenylacetic acid afforded high yields in the short reaction time (3u, 3v, Table 2). Furthermore, dicarboxylic acids (for instance, terephthalic acid and adipic acid) were treated with HBpin under the standard reaction conditions, the corresponding diboronate esters were also obtained in excellent conversions (3w, 3x, Table 2). Subsequently, these diboronate esters could be further hydrolyzed to diol products which are generally an important structural motif in many natural products and biological active substances. Compared with the traditional hydrogenation of diacid methods (generally involving either high temperature and pressure or low yield drawbacks) and the reported Ru/Mn- catalyzed method, [21-22] the Mg-catalyzed system displayed a much safer and higher efficient alternative protocol.

    Based on the aforementioned excellent conversions of carboxylic acids to boronate esters, several representative boronate esters were selected to hydrolyze to afford the corresponding alcohols. The results were summarized in Table 3. Generally, the resultant boronate esters could be further hydrolyzed to the corresponding alcohols in high yields. For example, the benzoic acid (3a) was hydroborated to the benzyl boronate ester in 99% conversion, then directly hydrolyzed without separation. The corresponding benzyl alcohol (4a, Table 3) was obtained in 93% isolated yield after workup. Other carboxylic acid derivatives such as benzoic acid with electron-withdrawing group (3e) or aliphatic carboxylic acids (3m, 3q, 3s) also provided the corresponding alcohol products (4b~4e, Table 3) in very high isolated yield overall after successive hydroboration and hydrolysis. This protocol provides an efficient and mild method for the reduction of carboxylic acids to alcohols.

    表 3

    Table 3.  Hydrolysis of selected boronate esters to alcohols
    下载: 导出CSV
    Entry Carboxylic acid Conv.a/% Alcohol Yieldb/% (4)
    1 99 93 (4a)
    2 99 91 (4b)
    3 94 90 (4c)
    4 98 92 (4d)
    5 99 94 (4e)
    a The conversion was determined by 1H NMR spectroscopy. b Isolated yields.

    In addition, the challenging chemoselective hydroboration of carboxylic acid with esters using 1 was explored (Scheme 1). Under the same reaction conditions, equimolar amounts of benzoic acid, methyl benzoate and 3.1 equiv. of HBpin were treated with 0.1 mol% 1. It resulted in 99% conversion of benzoic acid to boronate ester in 2 h and the methyl benzoate remained intact as confirmed by 1H NMR analysis. Similarly, the competitive intermolecular catalytic hydroboration of benzoic acid with benzyl benzoate was also tested, and an exclusive formation of deoxygenated boronate ester from carboxylic acid was observed. 1H NMR analysis indicated the presence of more than 99% of unreacted benzyl benzoate in the reaction mixture.

    图式 1

    Scheme 1.  Chemoselective hydroboration of carboxylic acid with esters

    In order to gain a further understanding of the above Mg-catalyzed hydroboration of various carboxylic acids, quantum mechanical calculations were employed to probe the profile of the reaction between acetic acid and HBpin (Figure 1). Based on the theory calculations and stoichiometric reaction experimental results, two plausible reaction mechanisms were proposed. The overall catalytic cycle of reaction consists of two steps. The first step is the formation of boryl ester (RCOOBpin) (1st step, Scheme 2). There are two possible pathways for the formation of RCOOBpin. It could be achieved from a noncatalytic reaction of carboxylic acids and HBpin with liberation of dihydrogen which has been detected and confirmed by GC analysis reported by Gunanathan et al.[21] However, according to the density functional theory (DFT) calculation, the formation of boryl ester via the Mg-catalyzed pathway required fairly lower activation energy (9.0 vs. 51.0 kcal/mol, Figure 1). The acetic acid was firstly reacted with the magnesium hydride (LMgH) to generate the low-energy stable magnesium acetate complex int2b (-37.9 kcal/mol), and side product dihydrogen was confirmed as gas bubble that was observed in the beginning of the reaction. The intermediate int2b was further treated with HBpin to afford the boryl ester compound with concomitant elimination of LMgH.

    图 1

    Figure 1.  Calculated reaction pathway for hydroboration of carboxylic acid

    图式 2

    Scheme 2.  Proposed mechanism for the Mg-catalyzed hydroboration of carboxylic acid

    The second step of catalytic cycle also has two potential pathways (2nd step, Scheme 2). For the catalytic pathway A, the insertion of in situ generated LMgH species into the C=O bond of the previously formed boryl ester leads to the formation of the magnesium alkoxy boryl ester derivative (int2d). Next, the intermediate int2d was further reacted with HBpin which underwent a proton shift from boron atom to carbon atom to form a Mg—O—C—H four-membered ring (TS3d). The transition state TS3d decomposed spontaneously via C—O bond cleavage due to the process is exothermic (ΔG=-69.7 kcal/mol). Finally, the alkoxyboronate ester product was obtained with the elimination of magnesium boryloxide species (LMgOBpin) that was reacted with another equivalent of HBpin to regenerate the magnesium hydride with concomitant formation of diboryl ether by-product O(Bpin)2.

    However, an alternative reaction pathway B cannot be ruled out. The first step obtained boryl ester could be coordinated to in situ generated LMgH species to form the H—Mg—O—C four-membered ring (TS1c). The transition state TS1c then decomposed spontaneously into aldehyde and magnesium boryloxide species (LMgOBpin) via a C—O bond cleavage of boryl ester due to the favorable exothermic progress (ΔG=-44.1 kcal/mol). LMgOBpin was treated with HBpin to regenerate the magnesium hydride with elimination of diboryl ether by-product O(Bpin)2 as pathway A. The benzaldehyde can be observed in the crude 1H NMR spectrum of the stoichiometric reaction of benzoic acid (1 equiv.), HBpin (2 equiv.) and complex 1 (1 equiv.). The characterized C—H proton signal of aldehyde group at δ 10.04 (singlet) was observed in the corresponding 1H NMR spectrum which indicated the presence of free benzaldehyde (the corresponding value is δ 10.03 in the 1H NMR spectrum of pure benzaldehyde). Additionally, the in situ 1H NMR monitoring of the stoichiometric reaction between pentanoic acid (1 equiv.) and HBpin (2 equiv.) as well as complex 1 (1 equiv.) was also carried out. The aldehyde C—H signal of valeraldehyde resonated at δ 9.73 which was slightly different from that of the free valeraldehyde (δ 9.40) perhaps due to that the aldehyde was coordinated to the magnesium metal center and resulted in a small migration of chemical shift. However, the obtained peak type and coupling constant (triplet, 3JHH=1.8 Hz) are the same to the theoretical values of free valeraldehyde. All these information suggested the appearance of valeraldehyde. The subsequent reaction pathway underwent the catalytic hydroboration of aldehyde which proceeded smoothly and reported by many research groups. The generated aldehyde was immediately reacted with the magnesium hydride to afford the magnesium alkoxyl complex which further treated with HBpin to liberate the alkoxyboronate ester product and recover LMgH.

    In summary, we have demonstrated that monodentate amido magnesium methyl complex 1 as the first main group catalyst has been successfully employed for the highly efficient and selective hydroboration of various carboxylic acid. The results showed that both aromatic and aliphatic carboxylic acid as well as dicarboxylic acids could be hydroborated to the respective alkyl boronate esters, and selected boronate esters were further hydrolyzed to provide the corresponding alcohols in high isolated yields. In addition, the newly developed protocol also shows high chemoselectivity for carboxylic acid over esters. DFT calculations and stoichiometric reactions indicated that there were two possible different reaction pathways in the catalytic cycle. Overall, the new protocol not only provided another more efficient and mild method than the corresponding traditional hydrogenation and hydrosilylation for the reduction of carboxylic acid to alcohols, but also showed that the main group magnesium catalyst outperformed the transition metal ruthenium and manganese catalysts for deoxygenative hydroboration of carboxylic acid. This is quite different from the general trend that the catalytic efficiency of transition metals is much better than that of main group metals in many organic reactions. The main group elements may have “transition-metal- like” catalytic property or better as mentioned by Power in 2010.[24] We will continue to explore the catalytic application of magnesium complexes.

    All air-sensitive compounds were carried out using standard Schlenk-line or glovebox techniques under high- purity argon. Diethyl ether, toluene, tetrahydrofuran (THF) and hexane were dried and distilled from molten sodium. 1H NMR and 13C NMR spectra were recorded at 25 ℃ with a Bruker Avance III 600 MHz spectrometer and were referenced to the resonances of the solvent used. Complex 1 was prepared according to literature procedure.[25] Other reagents were used as received.

    To a Schlenk tube carboxylic acid (0.25 mmol), HBpin (0.775 mmol) and 1 (0.1 mol%) were added in the glove box. The reaction mixture was stirred for 2 h at 60 ℃. After that, the reaction mixture was evaporated under reduced pressure (to remove unreacted HBpin) and analyzed by 1H NMR and 13C NMR spectra after adding 0.5 mL of CDCl3.

    2-(Benzyloxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolane

    (3a):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.25 (s, 36H), 4.91 (s, 2H), 7.23~7.34 (m, 5H); 13C NMR (CDCl3, 151 MHz) δ: 24.6, 24.7, 66.7, 83.0, 83.2, 126.8, 127.4, 128.3, 139.3.

    4, 4, 5, 5-Tetramethyl-2-((4-methylbenzyl)oxy)-1, 3, 2-di-

    oxaborolane (3b):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.25 (s, 36H), 2.32 (s, 3H), 4.87 (s, 2H), 7.12 (d, J=7.8 Hz, 2H), 7.22 (d, J=7.8 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 21.2, 24.6, 24.7, 66.7, 83.0, 83.1, 83.2, 126.9, 129.0, 136.4, 137.0.

    2-((4-(tert-Butyl)benzyl)oxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-

    dioxaborolane (3c):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.26 (s, 36H), 1.31 (s, 9H), 4.89 (s, 2H), 7.27 (d, J=8.4 Hz, 2H), 7.35 (m, 2H); 13C NMR (CDCl3, 151 MHz) δ: 24.57, 24.61, 24.7, 31.5, 34.6, 66.6, 83.0, 83.2, 125.3, 126.7, 136.4, 150.4.

    2-((4-Methoxybenzyl)oxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-di-

    oxaborolane (3d):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.27 (s, 36H), 3.79 (s, 3H), 4.85 (s, 2H), 6.86 (d, J=8.4 Hz, 2H), 7.27 (d, J=8.4 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 24.55, 24.60, 24.7, 55.3, 66.5, 83.0, 83.16, 83.18, 113.8, 128.6, 131.6, 159.1.

    2-((4-Fluorobenzyl)oxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-diox-aborolane (3e):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.23 (s, 36H), 4.85 (s, 2H), 6.98 (t, J=8.4 Hz, 2H), 7.29 (m, 2H); 13C NMR (CDCl3, 151 MHz) δ: 24.56, 24.60, 24.7, 66.1, 83.1, 83.2, 115.1, 115.2, 128.68, 128.73, 135.1, 161.5, 163.1.

    2-((2-Fluorobenzyl)oxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-diox-aborolane (3f):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.24 (s, 36H), 4.98 (s, 2H), 6.98~7.43 (m, 4H); 13C NMR (CDCl3, 151 MHz) δ: 24.55, 24.59, 24.64, 60.86, 60.89, 83.18, 83.19, 115.0, 115.1, 124.02, 124.04, 126.4, 126.5, 128.87, 128.90, 129.0, 129.1, 159.4, 161.1.

    2-((4-Chlorobenzyl)oxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-diox-aborolane (3g):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.27 (s, 36H), 4.88 (s, 2H), 7.29 (m, 4H); 13C NMR (CDCl3, 151 MHz) δ: 24.55, 24.60, 24.7, 66.0, 83.2, 128.2, 128.5, 133.2, 137.8.

    2-((2-Bromobenzyl)oxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-diox-aborolane (3h):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.25 (s, 36H), 4.96 (s, 2H), 7.11~7.49 (m, 4H, ArH); 13C NMR (CDCl3, 151 MHz) δ: 24.57, 24.61, 24.7, 66.4, 83.16, 83.18, 83.23, 121.6, 127.4, 127.9, 128.7, 132.3, 138.4.

    4-(((4, 4, 5, 5-Tetramethyl-1, 3, 2-dioxaborolan-2-yl)oxy)-methyl)phenyl acetate (3i):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.24 (s, 36H), 2.26 (s, 3H), 4.88 (s, 2H), 7.02 (m, 2H), 7.33 (d, J=8.4 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 21.1, 24.6, 24.7, 66.1, 83.05, 83.12, 121.4, 127.8, 136.9, 150.0, 169.5.

    4, 4, 5, 5-Tetramethyl-2-(naphthalen-1-ylmethoxy)-1, 3, 2-

    dioxaborolane (3j):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.27 (s, 36H), 5.41 (s, 2H), 7.43~8.05 (m, 7H); 13C NMR (CDCl3, 151 MHz) δ: 24.6, 24.7, 65.1, 83.1, 83.2, 123.6, 125.0, 125.4, 125.7, 126.2, 128.2, 128.6, 131.1, 133.7, 134.8.

    4, 4, 5, 5-Tetramethyl-2-(naphthalen-2-ylmethoxy)-1, 3, 2-

    dioxaborolane (3k):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.27 (s, 36H), 5.10 (s, 2H), 7.45 (m, 3H), 7.81 (m, 4H); 13C NMR (CDCl3, 151 MHz) δ: 24.60, 24.63, 24.7, 66.9, 83.1, 83.2, 125.0, 125.3, 125.8, 126.1, 127.7, 128.0, 128.1, 133.0, 133.4, 136.8.

    2-Ethoxy-4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolane

    (3l):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.19 (t, J=7.2 Hz, 3H), 1.23 (s, 36H), 3.86 (q, J=7.2 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 17.3, 24.6, 24.7, 60.7, 82.7, 83.1.

    2-(Hexyloxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolane (3m):[23b] 1H NMR (CDCl3, 600 MHz) δ: 0.84 (t, J=7.2 Hz, 3H), 1.21 (s, 36H), 1.27~1.28 (m, 4H), 1.49~1.54 (m, 2H), 3.78 (t, J=6.6 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 14.1, 22.4, 24.58, 24.62, 27.8, 31.2, 65.0, 82.6, 83.2.

    4, 4, 5, 5-Tetramethyl-2-(octyloxy)-1, 3, 2-dioxaborolane

    (3n):[23b] 1H NMR (CDCl3, 600 MHz) δ: 0.84 (t, J=7.2 Hz, 3H), 1.22 (s, 36H), 1.28~1.30 (m, 6H), 1.50~1.54 (m, 2H), 3.79 (t, J=6.6 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 14.1, 22.7, 24.55, 24.58, 24.62, 25.6, 29.0, 31.5, 31.9, 65.0, 82.7, 83.2.

    2-(4-Chlorobutoxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-dioxabo-

    rolane (3o):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.22 (s, 36H), 1.98 (m, 2H), 3.60 (t, J=6.0 Hz, 2H), 3.96 (t, J=5.4 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 24.6, 24.7, 34.3, 41.4, 61.6, 82.95, 83.04, 83.1.

    2-(2-Cyclohexylethoxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-diox-aborolane (3p):[23b] 1H NMR (CDCl3, 600 MHz) δ: 0.87~0.90 (m, 2H), 1.10~1.16 (m, 3H), 1.24 (s, 36H), 1.40~1.43 (m, 3H), 1.59~1.70 (m, 5H), 3.85 (t, J=6.6 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 24.6, 24.7, 26.4, 26.7, 33.3, 34.0, 39.0, 62.9, 82.7, 83.2.

    2-(Cyclohexylmethoxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-diox-

    aborolane (3q):[23b] 1H NMR (CDCl3, 600 MHz) δ: 0.89~0.94 (m, 2H), 1.10~1.13 (m, 3H), 1.23 (s, 36H), 1.46~1.50 (m, 1H), 1.60~1.70 (m, 5H), 3.61 (d, J=6.6 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 24.55, 24.59, 24.62, 25.9, 26.6, 29.4, 39.4, 70.4, 82.7, 83.16, 83.18.

    4, 4, 5, 5-Tetramethyl-2-(neopentyloxy)-1, 3, 2-dioxaboro-lane (3r):[23b] 1H NMR (CDCl3, 600 MHz) δ: 0.86 (s, 9H), 1.24 (s, 36H), 3.48 (s, 2H); 13C NMR (CDCl3, 151 MHz) δ: 24.58, 24.62, 26.1, 32.4, 75.0, 82.7, 83.2.

    4, 4, 5, 5-Tetramethyl-2-(3-phenylpropoxy)-1, 3, 2-dioxa-

    borolane (3s):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.28 (s, 36H), 1.87~1.92 (m, 2H), 2.70 (t, J=7.8 Hz, 2H), 3.88 (t, J=6.0 Hz, 2H), 7.17~7.29 (m, 5H); 13C NMR (CDCl3, 151 MHz) δ: 24.55, 24.60, 24.7, 32.0, 33.2, 64.2, 82.8, 83.2, 125.8, 128.4, 128.5, 141.9.

    4, 4, 5, 5-Tetramethyl-2-(4-phenylbutoxy)-1, 3, 2-dioxabo-

    rolane (3t):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.28 (s, 36H), 1.60~1.73 (m, 4H), 2.64 (t, J=7.8 Hz, 2H), 3.88 (t, J=6.0 Hz, 2H), 7.16~7.28 (m, 5H); 13C NMR (CDCl3, 151 MHz) δ: 24.57, 24.60, 24.64, 27.5, 31.1, 35.6, 64.8, 82.7, 83.13, 83.17, 125.7, 128.3, 128.5, 142.5.

    4, 4, 5, 5-Tetramethyl-2-(2-phenylpropoxy)-1, 3, 2-dioxa-

    borolane (3u):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.17 (s, 12H), 1.25 (s, 24H), 1.27 (d, J=7.2 Hz, 3H), 2.93~2.99 (m, 1H), 3.84~3.97 (m, 2H), 7.16~7.31 (m, 5H); 13C NMR (CDCl3, 151 MHz) δ: 17.6, 24.53, 24.56, 41.4, 70.4, 82.7, 83.09, 83.14, 126.4, 127.6, 128.3, 143.8.

    2-(2, 2-Diphenylethoxy)-4, 4, 5, 5-tetramethyl-1, 3, 2-diox-

    aborolane (3v):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.15 (s, 12H), 1.27 (s, 24H), 4.24 (t, J=7.2 Hz, 1H), 4.41 (d, J=6.6 Hz, 2H), 7.17~7.28 (m, 10H); 13C NMR (CDCl3, 151 MHz) δ: 24.53, 24.55, 24.58, 52.6, 67.9, 82.7, 83.05, 83.13, 126.5, 128.4, 128.5, 141.8.

    1, 4-Bis(((4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)-

    oxy) methyl)benzene (3w):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.24 (s, 72H), 4.88 (s, 4H), 7.28 (s, 4H); 13C NMR (CDCl3, 151 MHz) δ: 24.61, 24.67, 66.6, 83.0, 83.1, 126.8, 138.5.

    1, 6-Bis((4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)-

    oxy) hexane (3x):[23b] 1H NMR (CDCl3, 600 MHz) δ: 1.19 (s, 72H), 1.29~1.30 (m, 4H), 1.48~1.51 (m, 4H), 3.76 (t, J=6.6 Hz, 4H). 13C NMR (CDCl3, 151 MHz) δ: 24.5, 24.6, 25.3, 31.4, 64.8, 82.6, 83.1.

    Upon completion of the hydroboration reaction, the resultant boronate ester was purified by column chromatography over silica-gel using ethyl acetate/hexane (V:V= 1:10) mixture as eluents to obtain the corresponding pure primary alcohols.

    Phenylmethanol (4a):[21] 1H NMR (CDCl3, 600 MHz) δ: 2.75 (s, 1H), 4.62 (s, 2H), 7.30~7.38 (m, 5H); 13C NMR (151 MHz, CDCl3) δ: 65.1, 127.1, 127.6, 128.6, 140.9.

    (4-Fluorophenyl)methanol (4b):[21] 1H NMR (CDCl3, 600 MHz) δ: 2.55 (s, 1H), 4.58 (s, 2H), 7.00~7.03 (m, 2H), 7.27~7.29 (m, 2H); 13C NMR (CDCl3, 151 MHz) δ: 64.5, 115.3, 115.5, 128.8, 128.9, 136.61, 136.63, 161.5, 163.2.

    Pentan-1-ol (4c):[21] 1H NMR (CDCl3, 600 MHz) δ: 0.90 (t, J=7.2 Hz, 3H), 1.31~1.33 (m, 4H), 1.55 (s, 2H), 2.01 (s, 1H), 3.60~3.63 (m, 2H); 13C NMR (CDCl3, 151 MHz) δ: 14.1, 22.6, 28.0, 32.5, 63.0.

    Cyclohexylmethanol (4d):[21] 1H NMR (CDCl3, 600 MHz) δ: 0.85~0.92 (m, 2H), 1.10~1.25 (m, 3H), 1.43~1.44 (m, 1H), 1.63~1.73 (m, 5H), 2.15 (s, 1H), 3.39 (dd, J=1.62 Hz, J=6.48 Hz, 2H); 13C NMR (CDCl3, 151 MHz) δ: 25.9, 26.7, 29.7, 40.5, 68.7.

    3-Phenylpropan-1-ol (4e):[21] 1H NMR (CDCl3, 600 MHz) δ: 1.91~1.96 (m, 2H), 2.75 (t, J=7.8 Hz, 2H), 2.83 (s, 1H), 3.69 (t, J=6.6 Hz, 2H), 7.24~7.35 (m, 5H); 13C NMR (CDCl3, 151 MHz) δ: 32.1, 34.2, 62.0, 125.8, 128.38, 128.43, 141.9.

    Supporting Information NMR spectroscopic spectra, chemoselective hydroboration, stoichiometric reaction, and computational studies. The Supporting Information is avai- lable free of charge via the Internet at http://sioc-journal. cn/.


    1. [1]

      Ullrich, J.; Breit, B. ACS Catal. 2018, 8, 785.

    2. [2]

      Korstanje, T. J.; Vlugt, J. I.; Elsevier, C. J.; de Bruin, B. Science 2015, 350, 298.

    3. [3]

      Cui, X.; Li, Y.; Topf, C.; Junge, K.; Beller, M.Angew. Chem., Int. Ed. 2015, 54, 10596.

    4. [4]

      Stein, T.; Meuresch, M.; Limper, D.; Schmitz, M.; Holscher, M.; Coetzee, J.; Cole-Hamilton, D. J.; Klankermayer, J.; Leitner, W. J.Am. Chem. Soc. 2014, 136, 13217.

    5. [5]

      Brewster, T. P.; Miller, A. J. M.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2013, 135, 16022.

    6. [6]

      Zhang, M.; Li, N.; Tao, X.; Ruzi, R.; Yu, S.; Zhu, C. Chem. Commun. 2017, 53, 10228.

    7. [7]

      Corre, Y.; Rysak, V.; Trivelli, X.; Agbossou-Niedercorn, F.; Michon, C. Eur. J. Org. Chem. 2017, 4820.

    8. [8]

      Nguyen, T. V. Q.; Yoo, W. J.; Kobayashi, S. Adv.Synth. Catal. 2016, 358, 452.

    9. [9]

      Fernandez-Salas, J. A.; Manzini, S.; Nolan, S. P.Adv. Synth. Catal. 2014, 356, 308.

    10. [10]

      Zheng, J.; Chevance, S.; Darcel, C.; Sortais, J. B. Chem. Commun.2013, 49, 10010.

    11. [11]

      Misal Castro, L. C.; Li, H.; Sortais, J. B.; Darcel, C. Chem.Commun. 2012, 48, 10514.

    12. [12]

      Matsubara, K.; Iura, T.; Maki, T.; Nagashima, H. J. Org.Chem. 2002, 67, 4985.

    13. [13]

      Gevorgyan, V.; Rubin, M.; Liu, J. X.; Yamamoto, Y. J. Org.Chem. 2001, 66, 1672.

    14. [14]

      Chong, C. C.; Kinjo, R. ACS Catal. 2015, 5, 3238.

    15. [15]

      Bismuto, A.; Cowley, M. J.; Thomas, S. P. ACS Catal. 2018, 8, 2001.

    16. [16]

      Das, U. K.; Higman, C. S.; Gabidullin, B.; Hein, J. E.; Baker, R. T.ACS Catal. 2018, 8, 1076.

    17. [17]

      Weidner, V. L.; Barger, C. J.; Delferro, M.; Lohr, T. L.; Marks, T. J.ACS Catal. 2017, 7, 1244.

    18. [18]

      Barman, M. K.; Baishya, A.; Nembenna, S. Dalton Trans. 2017, 46, 4152.

    19. [19]

      Lortie, J. L.; Dudding, T.; Gabidullin, B. M.; Nikonov, G. I. ACS Catal. 2017, 7, 8454.

    20. [20]

      (a) Li, Y.; Cheng, Y.; Shan, C.; Zhang, J.; Xu, D.; Bai, R.; Qu, L.; Lan, Y. Chin. J. Org.Chem. 2018, 38, 1885 (in Chinese).
      (李园园, 程玉华, 单春晖, 张敬, 徐冬冬, 白若鹏, 屈凌波, 蓝宇, 有机化学, 2018, 38, 1885.)
      (b) Wang, L.; Sun, W.; Liu, C. Chin. J. Catal.2018, 39, 1725.
      (c) He, Z.; Zhu, Q.; Hu, X.; Wang, L.; Xia, C.; Liu, C. Org. Chem.Front. 2019, 6, 900.
      (d) Xuan, Q.; Song, Q. Org. Lett. 2016, 18, 4250.
      (e) Luo, M.; Zang, S.; Yao, W.; Zheng, J.; Ma, M. Sci.Sin. Chim. 2020, 50, 639.

    21. [21]

      Kishan, S.; KrishnaKumar, V.; Gunanathan, C. ACS Catal. 2018, 8, 4772.

    22. [22]

      (a) Erken, C.; Kaithal, A.; Sen, S.; Weyhermüller, T.; Hö lscher, M.; Werlé, C.; Leitner, W. Nat. Commun. 2018, 9, 4521.
      (b) Barman, M. K.; Das, K.; Maji, B. J. Org. Chem. 2019, 84, 1570.

    23. [23]

      (a) Harinath, A.; Bhattacharjee, J.; Panda, T. K. Chem. Commun.2019, 55, 1386.
      (b) Wang, W.; Luo, M.; Zhu, D.; Yao, W.; Xu, L.; Ma, M. Org. Biomol.Chem. 2019, 17, 3604.
      (c) Xu, X.; Yan, D.; Zhu, Z.; Kang, Z.; Yao, Y.; Shen, Q.; Xue, M. ACS Omega.2019, 4, 6775.

    24. [24]

      Power, P. P. Nature 2010, 463, 171.

    25. [25]

      Ma, M.; Li, J.; Shen, X.; Yu, Z.; Yao, W.; Pullarkat, S. A. RSC Adv.2017, 7, 45401.

  • Scheme 1  Chemoselective hydroboration of carboxylic acid with esters

    Figure 1  Calculated reaction pathway for hydroboration of carboxylic acid

    Scheme 2  Proposed mechanism for the Mg-catalyzed hydroboration of carboxylic acid

    Table 1.  Optimization of hydroboration of benzoic acid

    Entry n Cat. 1/mol% t/℃ Time/h Yielda/%
    1 3.0 60 1 30
    2 3.0 0.1 60 2 82
    3 3.1 60 1 40
    4 3.1 0.1 60 1 61
    5 3.1 0.1 60 2 99
    6 3.1 0.1 25 2 15
    a The yield was determined by 1H NMR spectroscopy.
    下载: 导出CSV

    Table 2.  Hydroboration of carboxylic acids catalyzed by magnesium complex 1a

    a The yield was determined by 1H NMR spectroscopy. b HBpin (6.2 equiv.) was used.
    下载: 导出CSV

    Table 3.  Hydrolysis of selected boronate esters to alcohols

    Entry Carboxylic acid Conv.a/% Alcohol Yieldb/% (4)
    1 99 93 (4a)
    2 99 91 (4b)
    3 94 90 (4c)
    4 98 92 (4d)
    5 99 94 (4e)
    a The conversion was determined by 1H NMR spectroscopy. b Isolated yields.
    下载: 导出CSV
  • 加载中
计量
  • PDF下载量:  22
  • 文章访问数:  2080
  • HTML全文浏览量:  209
文章相关
  • 发布日期:  2020-07-01
  • 收稿日期:  2020-03-25
  • 修回日期:  2020-04-26
  • 网络出版日期:  2020-05-11
通讯作者: 陈斌, bchen63@163.com
  • 1. 

    沈阳化工大学材料科学与工程学院 沈阳 110142

  1. 本站搜索
  2. 百度学术搜索
  3. 万方数据库搜索
  4. CNKI搜索

/

返回文章