Rhodium (Ⅲ)-catalyzed selective access to isoindolinones via formal[4+1]annulation of arylamides and propargyl alcohols


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本文作者相关文章
	Xu Youwei

	Wang Fen

	Yu Songjie

	Li Xingwei

Xu Youwei^a,b, Wang Fen^a, Yu Songjie^a, Li Xingwei^a

a. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China;
b. University of Chinese Academy of Sciences, Beijing 100049, China

Received 25 June 2017; Accepted 30 June 2017; Published 5 August 2017

^* Corresponding author. Li Xingwei, Tel: +86-411-84379089; E-mail:xwli@dicp.ac.cn

Foundation item: This work was supported by the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and the National Natural Science Foundation of China (21525208, 21472186)

Abstract: A mild and efficient oxidative synthesis of isoindolinones has been realized by Rh(Ⅲ)-catalyzed C-H activation of benzamides and[4 + 1] coupling with propargyl alcohols. This coupling system proceeds with broad substrate scope and mild conditions and provides a new approach to access the useful skeleton of γ-lactams with a stereogenic center.

Key words: Rhodium C-H activation [4+1]annulation Propargyl alcohol Isoindolinones

三价铑催化N-甲氧基苯甲酰胺与炔丙醇[4+1]环化合成异吲哚啉酮

许有伟^a,b, 王芬^a, 于松杰^a, 李兴伟^a

a. 中国科学院大连化学物理研究所, 辽宁大连 116023;
b. 中国科学院大学, 北京 100049

收稿日期：2017-06-25. 接受日期：2017-06-30. 出版日期：2017-08-05

基金项目：中国科学院大连化学物理研究所；国家自然科学基金（21525208，21472186）

通讯作者：李兴伟, 电话: (0411)84379089;电子信箱: xwli@dicp.ac.cn

摘要：含N杂环结构广泛存在于具有生物活性的药物或天然产物骨架中.本文开发了一个高效合成含N杂环骨架的方法，这类方法在近年来一直是研究热点.在最近几年，过渡金属催化C-H键活化并随后与不饱和键发生环化反应被认为是一种环境友好且原子经济性高的构建功能化杂环的方法.在这些金属催化的体系中，三价铑催化与炔烃的环化反应体系，被认为是一种高效且有实际意义的合成含N杂环的体系.在这类体系，特别是构建六元环的体系中，炔烃通常作为一个C₂合成子被广泛应用.为了克服这一局限性，Chang课题组和本课题组分别独立报道了通过三价铑催化，炔烃与芳烃硝酮偶联合成吲哚啉化合物，其中炔烃作为一个C₁合成子参与反应.另一方面，本课题组还报道了炔丙醇与吡咯烷苯甲酰胺通过C-H键活化合成1-异色满酮结构，其中由于电子效应，芳基-铑物种对于炔烃的插入是在炔烃的2位.基于上述工作，本文希望通过置换炔丙醇中芳基与烷基的位置，使芳基-铑物种对于炔烃插入的方向发生改变，进而生成联烯中间体，然后发生环化反应生成五元环内酰胺结构. 异吲哚啉酮骨架结构也是一类重要的含N杂环结构，广泛存在于多种天然产物及药物分子中，其合成方法受到广泛关注.尽管此前已有三价铑催化C-H官能团化的方法来构建异吲哚啉酮骨架结构，但通常需要活性极高或易爆的化合物作为反应底物.因此，本文报道一类以简单的炔丙醇与N-甲氧基苯甲酰胺作为起始原料，通过一步[4+1]环化合成异吲哚啉酮骨架结构.本文完成了32个不同官能团取代的异吲哚啉酮骨架结构的合成，反应均可以以中等到良好的收率得到目标产物.另外还进行了放大实验，结果表明可以以克级规模制备异吲哚啉酮化合物，反应剩余的Ag₂CO₃以及生成的单质银可以回收（收率78%）.总之，我们将N-甲氧基苯甲酰胺与炔丙醇在三价铑催化作用下通过C-H键活化的方法环化高效合成N-取代的异吲哚啉酮骨架结构，且该骨架结构含有一个手性中心.催化体系温和，官能团容忍度好.

关键词：铑碳氢活化 [4+1]环化炔丙醇异吲哚啉酮

1 Introduction

N-Heterocyclic motifs are widely embedded in structurally complex biologically active natural products or pharmaceuticals. In this context, the development of efficient methods that deliver complex molecules containing N-heterocycle scaffolds has been of continuing interest. In recent years, transition metal-catalyzed direct C-H activation and sequential annulation with unsaturated bonds has been identified as an environment-friendly and atom-economic approach in the construction of an array of functionalized heterocyclic molecules [1-10].Among such transition metal-catalyzed systems, Cp*Rh(Ⅲ)-catalyzed annulation reactions using alkynes have emerged as useful and efficient strategies to form diverse N-heterocyclic compounds. In these systems, the alkynes mostly serve as a C₂ synthon, especially in the construction of six-membered rings, which may restrict the synthetic utility [11-29]. To grapple with this limitation [30-32], Chang's [33] and our group [34] had independently developed Rh(Ⅲ)-catalyzed coupling of arylnitrones with internal alkynes to produce indoline derivatives (Scheme 1, Eq. (1)). In 2013, our group also reported a Rh(Ⅲ)-catalyzed synthesis of isochroman-1-one via C-H activation and sequential insertion into the triple bond of propargyl alcohols, where the resulting aryl-Rh species is prone to insert into the 2-position of the propargyl alcohol as a result of the electronical effect of the aryl group (Scheme 1, Eq. (2)) [35]. In light of this work, We reasoned that the regioselective of the migratory insertion of the Rh-Ar group may be switched when using a propargyl alcohol with an alkyl terminus. Consequently, the resulting intermediate could deliver a ketenol intermediate, whose high reactivity may allow eventual formation of a five-membered lactam though subsequent annulation [36].

Scheme1. Rh(Ⅲ)-catalyzed annulation with alkynes.

N-substituted isoindolinones as an important class of heterocycles are widely present in many bioactive natural products and drug candidates [37-40]. The development of highly efficient approach to access isoindolinone derivatives has always been of ongoing interest in organic synthesis [41-44]. Although Rh(Ⅲ)-catalyzed C-H functionalization has been developed in the construction of isoindolinone cores, reactive and explosive diazo compounds have been typically used [45-50].Thus alternatives to access these isoindolinone skeletons from more simple and easy handling starting materials are still highly desirable. We now report a Rh(Ⅲ)-catalyzed regioselective [4 + 1] annulation of N-methoxybenzamides with easily available propargyl alcohols to construct various N-substituted isoindolinones [51].

2 Experimental

2.1 General

Unless otherwise noted, all reactions were carried out in flame-dried pressure tubes with a Teflon screw cap under air atmosphere. Anhydrous solvents were purified and dried by standard procedures. All commercially available reagents were used as received. ¹H and ¹³C NMR spectra were recorded using CDCl₃ as a solvent on a 400 MHz spectrometer at 25 ℃. The chemical shift is given in dimensionless δ values and is frequency referenced relative to SiMe₄ in ¹H and ¹³C NMR spectroscopy. High-resolution mass spectra were obtained on an Agilent Q-TOF 6540 spectrometer. All other solvents were obtained from commercial sources and were used as received. The N-methoxy amides 1 and propargyl alcohols 2 were prepared following a published procedure [50, 52-53].

2.2 General procedure for the synthesis of compounds 3 and 4

Amides (0.2 mmol), propargyl alcohols (0.3 mmol), [Cp*RhCl₂]₂ (2.5-4 mol%), AgOAc (10-16 mol%), Ag₂CO₃ (1.5 equiv.) and MeCN (2 mL) were charged into a pressure tube with a stir bar. The reaction mixture was stirred under air atmosphere at 30 ℃ for 24 h. After the solvent was removed under reduced pressure, the residue was purified by silica gel chromatography using PE/EA to afford the product.

2.3 Spectral data for products

3a. ¹H NMR (400 MHz, CDCl₃) δ 7.83-7.78 (m, 3H), 7.56-7.47 (m, 3H), 7.43-7.36 (m, 3H), 4.05 (s, 3H), 3.71 (d, J = 16.9 Hz, 1H), 3.37 (d, J = 16.9 Hz, 1H), 1.81 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.1, 164.8, 146.5, 137.0, 133.2, 132.1, 129.0, 128.5, 128.4, 127.9, 123.6, 122.5, 65.2, 64.6, 44.4, 23.9.

3b. ¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.4 Hz, 1H), 7.78 (d, J = 8.9 Hz, 2H), 7.54-7.52 (m, 1H), 7.48 (t, J = 7.4 Hz, 1H), 7.41 (t, J = 7.4 Hz, 1H), 6.85 (d, J = 8.9 Hz, 2H), 4.06 (s, 3H), 3.83 (s, 3H), 3.64 (d, J = 16.6 Hz, 1H), 3.31 (d, J = 16.6 Hz, 1H), 1.80 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 194.6, 164.7, 163.6, 146.6, 132.1, 130.3, 130.1, 129.0, 128.4, 123.6, 122.6, 113.7, 65.2, 64.8, 55.4, 44.1, 23.9.

3c. ¹H NMR (400 MHz, CDCl₃) δ 7.88 (d, J = 8.2 Hz, 2H), 7.82 (d, J = 7.4 Hz, 1H), 7.64 (d, J = 8.3 Hz, 2H), 7.50-7.49 (m, 2H), 7.46-7.41 (m, 1H), 4.05 (s, 3H), 3.68 (d, J = 16.8 Hz, 1H), 3.42 (d, J = 16.8 Hz, 1H), 1.81 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.2, 164.8, 146.1, 139.6, 134.4 (q, J_C-F = 32.7 Hz), 132.3, 129.1, 128.6, 128.2, 125.6 (q, J_C-F = 3.7 Hz), 123.7, 123.4 (q, J_C-F = 272.7 Hz), 122.2, 65.3, 64.5, 44.6, 24.0. HRMS calc. for C₁₉H₁₇F₃NO₃(M+H)⁺: 364.1155; Found: 364.1157.

3d. ¹H NMR (400 MHz, CDCl₃) δ 7.88-7.84 (m, 3H), 7.62-7.54 (m, 5H), 7.53-7.47 (m, 1H), 7.47-7.35 (m, 4H), 4.07 (s, 3H), 3.74 (d, J = 16.8 Hz, 1H), 3.41 (d, J = 16.8 Hz, 1H), 1.83 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.6, 164.8, 146.5, 145.8, 139.5, 135.6, 132.1, 129.0, 128.9, 128.5, 128.4, 128.2, 127.13, 127.08, 123.6, 122.4, 65.2, 64.6, 44.3, 23.9. HRMS calc. for C₂₄H₂₂NO₃(M+H)⁺: 372.1594; Found: 372.1596.

3e. ¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.3 Hz, 1H), 7.70 (d, J = 7.8 Hz, 2H), 7.56-7.44 (m, 2H), 7.41 (t, J = 7.2 Hz, 1H), 7.18 (d, J = 7.7 Hz, 2H), 4.05 (s, 3H), 3.67 (d, J = 16.8 Hz, 1H), 3.34 (d, J = 16.8 Hz, 1H), 2.36 (s, 3H), 1.80 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.7, 164.7, 146.6, 144.1, 134.5, 132.1, 129.2, 129.0, 128.4, 128.0, 123.6, 122.5, 65.2, 64.7, 44.2, 23.9, 21.6. HRMS calc. for C₁₉H₂₀NO₃(M+H)⁺: 310.1438; Found: 310.1440.

3f. ¹H NMR (400 MHz, CDCl₃) δ 7.83 (d, J = 7.4 Hz, 1H), 7.75 (d, J = 8.4 Hz, 2H), 7.54 (d, J = 7.5 Hz, 1H), 7.48 (t, J = 7.3 Hz, 1H), 7.43-7.39 (m, 3H), 4.05 (s, 3H), 3.70 (d, J = 16.9 Hz, 1H), 3.33 (d, J = 16.9 Hz, 1H), 1.81 (s, 3H), 1.30 (s, 9H). ¹³C NMR (100 MHz, CDCl₃) δ 195.7, 164.8, 157.0, 146.7, 134.5, 132.1, 129.0, 128.4, 127.9, 125.5, 123.6, 122.5, 65.2, 64.7, 44.3, 35.1, 31.0, 23.8. HRMS calc. for C₂₂H₂₆NO₃(M+H)⁺: 352.1907; Found: 352.1909.

3g. ¹H NMR (400 MHz, CDCl₃) δ 7.82-7.81 (m, 3H), 7.55-7.45 (m, 2H), 7.41 (t, J = 6.3 Hz, 1H), 7.06-7.02 (m, 2H), 4.05 (s, 3H), 3.64 (d, J = 16.7 Hz, 1H), 3.36 (d, J = 16.7 Hz, 1H), 1.80 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 194.5, 165.7 (d, J_C-F = 255.4 Hz), 164.7, 146.3, 133.4 (d, J_C-F = 2.9 Hz), 132.2, 130.6 (d, J_C-F = 9.4 Hz), 129.0, 128.5, 123.6, 122.4, 115.6 (d, J_C-F = 22.0 Hz), 65.2, 64.6, 44.3, 23.9. HRMS calc. for C₁₈H₁₇FNO₃(M+H)⁺: 314.1287; Found: 314.1290.

3h. ¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.4 Hz, 1H), 7.72 (d, J = 8.4 Hz, 2H), 7.50-7.47 (m, 2H), 7.43-7.40 (m, 1H), 7.34 (d, J = 8.4 Hz, 2H), 4.04 (s, 3H), 3.63 (d, J = 16.8 Hz, 1H), 3.35 (d, J = 16.8 Hz, 1H), 1.79 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 194.9, 164.8, 146.2, 139.7, 135.3, 132.2, 129.3, 129.0, 128.8, 128.5, 123.6, 122.3, 65.2, 64.5, 44.3, 23.9. HRMS calc. for C₁₈H₁₇ClNO₃(M+H)⁺: 330.0891; Found: 330.0894.

3i. ¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.4 Hz, 1H), 7.64 (d, J = 8.4 Hz, 2H), 7.52-7.49 (m, 4H), 7.45-7.40 (m, 1H), 4.04 (s, 3H), 3.63 (d, J = 16.8 Hz, 1H), 3.34 (d, J = 16.8 Hz, 1H), 1.79 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.1, 164.8, 146.2, 135.7, 132.2, 131.8, 129.4, 129.0, 128.5, 128.5, 123.7, 122.3, 65.3, 64.5, 44.3, 23.9. HRMS calc. for C₁₈H₁₇BrNO₃(M+H)⁺: 374.0386; Found: 374.0389.

3j. ¹H NMR (400 MHz, CDCl₃) δ 8.30 (s, 1H), 7.92-7.78 (m, 5H), 7.60-7.38 (m, 5H), 4.08 (s, 3H), 3.84 (d, J = 16.8 Hz, 1H), 3.51 (d, J = 16.8 Hz, 1H), 1.86 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.0, 164.8, 146.6, 135.5, 134.3, 132.3, 132.1, 129.7, 129.5, 129.0, 128.6, 128.4 (two signals overlapped), 127.7, 126.8, 123.6, 123.4, 122.5, 65.3, 64.7, 44.5, 23.9. HRMS calc. for C₂₂H₂₀NO₃(M+H)⁺: 346.1438; Found: 346.1439.

3k. ¹H NMR (400 MHz, CDCl₃) δ 7.84 (d, J = 6.3 Hz, 1H), 7.67-7.38 (m, 5H), 7.37-7.24 (m, 2H), 4.07 (s, 3H), 3.71 (d, J = 16.9 Hz, 1H), 3.36 (d, J = 16.9 Hz, 1H), 2.35 (s, 3H), 1.82 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.4, 164.9, 146.6, 138.4, 137.1, 134.1, 132.2, 129.1, 128.49, 128.46, 128.45, 125.2, 123.7, 122.6, 65.3, 64.7, 44.6, 23.9, 21.3. HRMS calc. for C₁₉H₂₀NO₃(M+H)⁺: 310.1438; Found: 310.1439.

3l. ¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.4 Hz, 1H), 7.76 (s, 1H), 7.65 (d, J = 7.7 Hz, 1H), 7.50-7.40 (m, 4H), 7.32 (t, J = 7.9 Hz, 1H), 4.05 (s, 3H), 3.65 (d, J = 16.9 Hz, 1H), 3.36 (d, J = 16.9 Hz, 1H), 1.80 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 194.8, 164.8, 146.2, 138.4, 134.9, 133.1, 132.2, 129.9, 129.0, 128.5, 128.0, 126.0, 123.7, 122.3, 65.3, 64.5, 44.4, 24.0. HRMS calc. for C₁₈H₁₇ClNO₃(M+H)⁺: 330.0891; Found: 330.0894.

3m. ¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.5 Hz, 1H), 7.54-7.51 (m, 1H), 7.46-7.41 (m, 2H), 7.31-7.21 (m, 3H), 7.04 (d, J = 6.7 Hz, 2H), 4.03 (s, 3H), 3.59 (d, J = 15.6 Hz, 1H), 3.52 (d, J = 15.6 Hz, 1H), 3.11 (d, J = 16.3 Hz, 1H), 2.88 (d, J = 16.3 Hz, 1H), 1.68 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 204.1, 164.6, 146.3, 133.3, 132.3, 129.4, 128.8, 128.7, 128.5, 127.1, 123.7, 122.0, 65.2, 64.4, 51.1, 47.6, 23.7. HRMS calc. for C₁₉H₂₀NO₃(M+H)⁺: 310.1438; Found: 310.1440.

3n. ¹H NMR (400 MHz, CDCl₃) δ 7.81-7.79 (m, 3H), 7.52 (t, J = 7.1 Hz, 1H), 7.47-7.38 (m, 4H), 7.32 (d, J = 7.1 Hz, 1H), 4.06 (s, 3H), 3.75-3.61 (m, 2H), 1.62-1.55 (m, 1H), 0.67-0.62 (m, 2H), 0.52-0.41 (m, 1H), 0.07-0.02 (m, 1H). ¹³C NMR (100 MHz, CDCl₃) δ 195.6, 165.1, 142.9, 137.2, 133.2, 131.5, 130.3, 128.51, 128.49, 127.9, 123.6, 122.5, 67.1, 64.6, 41.8, 18.4, 2.9, 1.3. HRMS calc. for C₂₀H₂₀NO₃(M+H)⁺: 322.1438; Found: 322.1439.

3o. ¹H NMR (400 MHz, CDCl₃) δ 7.87 (d, J = 7.3 Hz, 1H), 7.78 (d, J = 7.6 Hz, 2H), 7.50-7.44 (m, 4H), 7.38 (t, J = 7.5 Hz, 2H), 7.22 (t, J = 7.2 Hz, 2H), 7.18-7.11 (m, 1H), 7.05 (d, J = 7.2 Hz, 2H), 4.10 (s, 3H), 3.72 (d, J = 16.9 Hz, 1H), 3.50 (d, J = 16.9 Hz, 1H), 2.68-2.60 (m, 1H), 2.54-2.39 (m, 2H), 2.06-1.99 (td, J = 12.8, 4.5 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃) δ 195.8, 166.0, 144.4, 140.9, 137.0, 133.2, 132.3, 130.0, 128.5 (two signals overlapped), 128.4, 128. 1, 127.8, 125.9, 123.7, 122.3, 67.6, 64.7, 43.9, 37.7, 29.3. HRMS calc. for C₂₅H₂₄NO₃(M+H)⁺: 386.1751; Found: 386.1751.

3p. ¹H NMR (400 MHz, CDCl₃) δ 7.83 (d, J = 7.1 Hz, 1H), 7.61 (t, J = 6.9 Hz, 1H), 7.54-7.38 (m, 4H), 7.13 (t, J = 7.2 Hz, 1H), 7.10-7.01 (m, 1H), 4.05 (s, 3H), 3.73 (d, J = 17.1 Hz, 1H), 3.45 (d, J = 17.3 Hz, 1H), 1.76 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 194.4 (d, J_C-F = 3.9 Hz), 164.9, 161.4 (d, J_C-F = 253.6 Hz), 146.4, 134.6 (d, J_C-F = 9.1 Hz), 132.1, 130.4 (d, J_C-F = 2.4 Hz), 129.2, 128.4, 125.9 (d, J_C-F = 13.0 Hz), 124.5 (d, J_C-F = 3.4 Hz), 123.7, 121.9, 116.5 (d, J_C-F = 23.9 Hz), 65.2, 64.5, 48.8 (d, J_C-F = 7.8 Hz), 24.5. HRMS calc. for C₁₈H₁₇FNO₃(M+H)⁺: 314.1187; Found: 314.1189.

3q. ¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.4 Hz, 1H), 7.59 (d, J = 4.8 Hz, 1H), 7.56 (d, J = 3.6 Hz, 1H), 7.53-7.47 (m, 2H), 7.45-7.39 (m, 1H), 7.05 (t, J = 4.4 Hz, 1H), 4.07 (s, 3H), 3.60 (d, J = 16.1 Hz, 1H), 3.29 (d, J = 16.1 Hz, 1H), 1.81 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 189.0, 164.7, 146.3, 144.4, 134.3, 132.3, 132.2, 128.8, 128.5, 128.2, 123.7, 122.6, 65.3, 64.7, 45.4, 23.7. HRMS calc. for C₁₆H₁₆NO₃S(M+H)⁺: 302.0845; Found: 302.0846.

4a. ¹H NMR (400 MHz, CDCl₃) δ 7.89-7.76 (m, 3H), 7.53 (t, J = 7.4 Hz, 1H), 7.41 (t, J = 7.7 Hz, 2H), 7.31 (dd, J = 8.5, 2.2 Hz, 1H), 7.11 (td, J = 8.9, 2.3 Hz, 1H), 4.04 (s, 3H), 3.74 (d, J = 17.3 Hz, 1H), 3.37 (d, J = 17.3 Hz, 1H), 1.80 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.8, 165.4 (d, J_C-F= 250.0 Hz), 164.0, 149.0 (d, J_C-F = 9.6 Hz), 136.8, 133.4, 128. 6, 127.9, 125.9 (d, J_C-F = 9.6 Hz), 125.0 (d, J_C-F = 2.4 Hz), 116.1 (d, J_C-F = 23.3 Hz), 110.5 (d, J_C-F = 24.7 Hz), 65.5, 64.4 (d, J_C-F = 2.5 Hz), 44.3, 23.8.

4b. ¹H NMR (400 MHz, CDCl₃) δ 7.83 (d, J = 7.5 Hz, 2H), 7.76 (d, J = 8.1 Hz, 1H), 7.60-7.50 (m, 2H), 7.43-7.39 (m, 3H), 4.04 (s, 3H), 3.73 (d, J = 17.3 Hz, 1H), 3.39 (d, J = 17.3 Hz, 1H), 1.79 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.7, 163.9, 148.1, 138.5, 136.7, 133.4, 129.0, 128.6, 127.9, 127.6, 124.9, 123.2, 65.3, 64.4, 44.1, 23.9.

4c. ¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.5 Hz, 2H), 7.73 (d, J = 1.0 Hz, 1H), 7.69 (d, J = 8.1 Hz, 1H), 7.58-7.50 (m, 2H), 7.41 (t, J = 7.7 Hz, 2H), 4.04 (s, 3H), 3.72 (d, J = 17.3 Hz, 1H), 3.39 (d, J = 17.3 Hz, 1H), 1.78 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.6, 164.0, 148.2, 136.7, 133.4, 131.9, 128.6, 128.1, 127.8, 126.8, 126.0, 125.1, 65.3, 64.3, 44.0, 23.9.

4d. ¹H NMR (400 MHz, CDCl₃) δ 7.79 (d, J = 7.5 Hz, 2H), 7.70 (d, J = 7.7 Hz, 1H), 7.51 (t, J = 7.3 Hz, 1H), 7.38 (t, J = 7.6 Hz, 2H), 7.30 (s, 1H), 7.21 (d, J = 7.7 Hz, 1H), 4.04 (s, 3H), 3.68 (d, J = 16.8 Hz, 1H), 3.36 (d, J = 16.8 Hz, 1H), 2.36 (s, 3H), 1.78 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.2, 165.1, 146.8, 142.9, 137.1, 133.2, 129.3, 128.5, 127.9, 126.2, 123.5, 123.0, 65.2, 64.5, 44.5, 23.9, 22.0.

4e. ¹H NMR (400 MHz, CDCl₃) δ 8.42 (d, J = 1.1 Hz, 1H), 8.32 (d, J = 8.3 Hz, 1H), 8.01 (d, J = 8.3 Hz, 1H), 7.83 (d, J = 7.8 Hz, 2H), 7.55 (t, J = 7.3 Hz, 1H), 7.42 (t, J = 7.7 Hz, 2H), 4.08 (s, 3H), 3.78 (d, J = 17.5 Hz, 1H), 3.55 (d, J = 17.6 Hz, 1H), 1.83 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.2, 162.4, 150.4, 147.5, 136.4, 135.2, 133.7, 128.8, 127.9, 124.7, 124.1, 118.0, 65.3, 64.6, 43.5, 24.2.

4f. ¹H NMR (400 MHz, CDCl₃) δ 7.89 (d, J = 7.9 Hz, 1H), 7.81 (d, J = 7.5 Hz, 2H), 7.71 (s, 1H), 7.63 (d, J = 7.9 Hz, 1H), 7.54-7.47 (m, 3H), 7.45-7.35 (m, 5H), 4.09 (s, 3H), 3.76 (d, J = 16.7 Hz, 1H), 3.41 (d, J = 16.7 Hz, 1H), 1.86 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.2, 164.7, 147.1, 145.4, 140.3, 137.1, 133.2, 128.8, 128.5, 128.0, 127.9, 127.8, 127.6, 127.4, 124.0, 121.4, 65.3, 64.8, 44.5, 23.9.

4g. ¹H NMR (400 MHz, CDCl₃) δ 7.80 (d, J = 7.6 Hz, 2H), 7.74 (d, J = 8.4 Hz, 1H), 7.51 (t, J = 7.2 Hz, 1H), 7.39 (t, J = 7.5 Hz, 2H), 7.02 (s, 1H), 6.92 (d, J = 8.3 Hz, 1H), 4.03 (s, 3H), 3.79 (s, 3H), 3.69 (d, J = 16.8 Hz, 1H), 3.34 (d, J = 16.8 Hz, 1H), 1.79 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.3, 165.3, 163.1, 148.9 137.1, 133.2, 128.5, 127.9, 125.2, 121.1, 114.8, 107.8, 65.3, 64.6, 55.5, 44.6, 23.8.

4h. ¹H NMR (400 MHz, CDCl₃) δ 7.79 (d, J = 7.6 Hz, 2H), 7.51 (t, J = 7.2 Hz, 1H), 7.43-7.28 (m, 4H), 7.15 (d, J = 7.1 Hz, 1H), 4.03 (s, 3H), 3.65 (d, J = 16.7 Hz, 1H), 3.38 (d, J = 16.7 Hz, 1H), 2.71 (s, 3H), 1.78 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.3, 166.2, 147.2, 137.8, 137.2, 133.1, 131.6, 130.3, 128.5, 127.9, 126.0, 119.6, 65.1, 63.8, 44.4, 24.2, 17.3.

4i. ¹H NMR (400 MHz, CDCl₃) δ 9.16 (d, J = 8.3 Hz, 1H), 7.96 (d, J = 8.4 Hz, 1H), 7.86-7.78 (m, 3H), 7.66-7.59 (m, 2H), 7.57-7.45 (m, 2H), 7.36 (t, J = 7.6 Hz, 2H), 4.09 (s, 3H), 3.75 (d, J = 16.7 Hz, 1H), 3.43 (d, J = 16.7 Hz, 1H), 1.85 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.3, 166.7, 147.1, 137.1, 133.2, 133.0, 133.0, 129.1, 128.5, 128.1, 128.1, 127.9, 126.7, 124.1, 122.8, 119.4, 65.3, 64.3, 44.3, 23.7.

4j. ¹H NMR (400 MHz, CDCl₃) δ 7.80 (d, J = 7.5 Hz, 2H), 7.52 (t, J = 7.1 Hz, 1H), 7.47-7.31 (m, 5H), 4.03 (s, 3H), 3.68 (d, J = 17.1 Hz, 1H), 3.40 (d, J = 17.1 Hz, 1H), 1.77 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.9, 163.1, 149.1, 136.9, 133.4, 132.9, 131.5, 130.1, 128.7, 127.9, 125.5, 120.9, 65.2, 63.6, 44.2, 24.2. HRMS calc. for C₁₈H₁₇ClNO₃(M+H)⁺: 330.0891; Found: 330.0892.

4k. ¹H NMR (400 MHz, CDCl₃) δ 7.80 (d, J = 7.7 Hz, 2H), 7.55-7.36 (m, 4H), 7.30 (d, J = 7.6 Hz, 1H), 7.04 (t, J = 8.7 Hz, 1H), 4.02 (s, 3H), 3.68 (d, J = 17.1 Hz, 1H), 3.43 (d, J = 17.1 Hz, 1H), 1.78 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 195.7, 162.2 (d, J_C-F = 2.0 Hz), 158.6 (d, J_C-F = 260.8 Hz), 149.1 (d, J_C-F = 2.6 Hz), 136.8, 134.0 (d, J_C-F = 7.7 Hz), 133.3, 128.6, 127.8, 118.3 (d, J_C-F = 4.1 Hz), 116.5 (d, J_C-F = 13.3 Hz), 115.8 (d, J_C-F = 19.3 Hz), 65.2, 64.2, 44.0, 24.3. HRMS calc. for C₁₈H₁₇FNO₃(M+H)⁺: 314.1187; Found: 314.1187.

4l. ¹H NMR (400 MHz, CDCl₃) δ 7.80 (d, J = 7.7 Hz, 2H), 7.51 (t, J = 7.3 Hz, 1H), 7.45-7.36 (m, 3H), 7.05 (d, J = 7.6 Hz, 1H), 6.86 (d, J = 8.4 Hz, 1H), 4.00 (s, 3H), 3.95 (s, 3H), 3.62 (d, J = 16.6 Hz, 1H), 3.36 (d, J = 16.6 Hz, 1H), 1.77 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.2, 164.5, 157.2, 149.2, 137.1, 133.8, 133.1, 128.5, 127.9, 115.9, 114.3, 110.6, 65.1, 63.8, 55.8, 44.3, 24.1. HRMS calc. for C₁₉H₂₀NO₄(M+H)⁺: 326.1387; Found: 326.1389.

4m. ¹H NMR (400 MHz, CDCl₃) δ 7.80 (d, J = 7.5 Hz, 2H), 7.63 (s, 1H), 7.51 (t, J = 7.4 Hz, 1H), 7.42-7.36 (m, 3H), 7.29 (d, J = 7.8 Hz, 1H), 4.04 (s, 3H), 3.68 (d, J = 16.9 Hz, 1H), 3.35 (d, J = 16.8 Hz, 1H), 2.38 (s, 3H), 1.78 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.2, 165.0, 143.8, 138.5, 137.0, 133.2, 133.0, 129.0, 128.5, 127.9, 123.8, 122.3, 65.2, 64.5, 44.5, 24.0, 21.3.

4n. ¹H NMR (400 MHz, CDCl₃) δ 7.87 (d, J = 7.4 Hz, 2H), 7.62 (d, J = 4.8 Hz, 1H), 7.56 (t, J = 7.4 Hz, 1H), 7.44 (t, J = 7.7 Hz, 2H), 7.27-7.26 (m, 1H), 4.08 (s, 3H), 3.87 (d, J = 17.4 Hz, 1H), 3.21 (d, J = 17.4 Hz, 1H), 1.81 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.4, 162.7, 156.8, 136.8, 134.6, 133.5, 130.8, 128.7, 127.9, 123.3, 65.7, 65.7, 45.7, 22.7. HRMS calc. for C₁₆H₁₆NO₃S (M+H)⁺: 302.0845; Found: 302.0846.

4o. ¹H NMR (400 MHz, CDCl₃) δ 7.84-7.80 (m, 3H), 7.56-7.47 (m, 3H), 7.44-7.38 (m, 3H), 4.42-4.34 (m, 1H), 4.25-4.17 (m, 1H), 3.72 (d, J = 16.9 Hz, 1H), 3.34 (d, J = 16.9 Hz, 1H), 1.81 (s, 3H), 1.37 (t, J = 7.1 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 196.3, 165.0, 146.6, 137.0, 133.2, 132.0, 129.1, 128.5, 128.4, 127.9, 123.6, 122.5, 73.2, 64.6, 44.6, 23.8, 14.0. HRMS calc. for C₁₉H₂₀NO₃(M+H)⁺: 310.1438; Found: 310.1438.

3 Results and discussion

The coupling of N-methoxybenzamide (1a) with propargyl alcohol (2a) was selected as a model reaction for screening of the reaction parameters (Table 1). The initial reaction of 1a with 2a was carried out in the presence of 4 mol% of [RhCp*Cl₂]₂ as the catalyst at 60 ℃ in MeCN. Traces of desired product 3a were detected using AgOAc as an oxidant, while the yield of 3a was dramatically improved to 64% when Ag₂CO₃was used (Table 1, entries 1 and 2). Lowering the reaction temperature to 30 ℃ afforded 3a in 73% yield (Table 1, entry 3), but lowering the amount of Ag₂CO₃ to 1 equiv. resulted in a slightly lower yield (Table 1, entry 4). Screening of the solvent gave MeCN as an optimal medium (Table 1, entries 5−9). With K₂CO₃ or HOAc being an additive, the yields of the desired product were decreased to 63% or 54%, respectively (Table 1, entries 10 and 11). To our delight, introduction of AgOAc (16 mol%) as an additive gave 3a in 76% yield (Table 1, entry 12). Switching the air atmosphere to O₂ atmosphere resulted in an inferior result (Table 1, entry 13). A similar yield was still obtainable when the catalyst loading was lowered to 2.5 mol% (Table 1, entry 14).

Table 1
Optimization of reaction conditions.

With the optimized reaction conditions established, we next investigated the scope and generality of the propargyl alcohols in this catalytic system (Table 2, entries 1-17). Fortunately, various propargyl alcohols bearing both electron-donating (3b, 3e, 3f, 3k) and -withdrawing (3c, 3d) as well as halogen (3g, 3h, 3i, 3l) substituents at the meta or para positions of the benzene ring of the propargyl alcohol are all compatible. However, introduction a fluorine atom into the ortho position of benzene ring caused a lower reactivity (3p). In addition to the benzene ring of the propargyl alcohol, a naphthalene ring (3j), a thiophene ring (3q) and a benzyl group (3m) on the 1-position all resulted in good yields of the desired products. Besides methyl group, cyclopropyl (3n) and phenylethyl (3o) substituted propargyl alcohols could also undergo this annulation smoothly in moderate to high yields.

Table 2
Scope of substrates.

The substrate scope of the N-methoxybenzamides was next explored under modified conditions (Table 2, entries 18-32). In general, N-methoxybenzamides bearing different electron-donating and -withdrawing groups at the ortho and para positions all coupled smoothly with propargyl alcohol 2a, and the desired cyclization products were isolated in moderate to good yields (4a-4d, 4f-4l). However, 4-nitro-substituted benzamide exhibited lower reactivity (4e). The C-H activation of meta-methyl substituted substrate occurred selectively at the less hindered position to give product 4m in 66% yield. Furthermore, the substrate is not limited to N-methoxybenzamide; both the thiophene ring (4n) and N-ethoxybenzamides (4o) also coupled in good yields.

To demonstrate the synthetic utility of the reaction, a gram scale reaction between 1i and 2a has been performed (Scheme 2), and the desired product 4i was isolated in good yield (75%) even with a reduced catalyst loading. It should be noted that Ag₂CO₃ as the oxidant could be recovered effectively (78% recovery), and oxidation using the recycled Ag₂CO₃ resulted in isolation of the product 3a in 56% yield.

Scheme2. Gram-scale preparation of 4i.

Several deuterated experiments have been carried out to probe the reaction mechanism. Treatment of 1a with 2a-d₁under the standard conditions provided 3a-d₁ with deuterium essentially at the C-1 position (Scheme 3, Eq. (1)). Moreover, H/D exchange was observed at the C-1 position of 3a when D₂O was introduced into the catalytic system, indicative of the irreversibility of a β-H elimination process under the catalytic conditions (Scheme 3, Eqs. (2) and (3)). A significant kinetic isotope effect (k_H/k_D = 5.3) was observed from an intermolecular competitive coupling using an equivalent molar mixture of 1a and 1a-d₅ (Scheme 3, Eq. (4)). In addition, two parallel reactions using 1a and 1a-d₅, a KIE value of 1.8 was obtained on the basis of ¹H NMR analysis (Scheme 3, Eq. (5)). These results suggest that C-H activation is probably involved in the turnover-limiting step.

Scheme3. Mechanism study experiments.

On the base of previous work and our preliminary mechanistic experiments, three plausible pathways are proposed in Scheme 4. Initially, an active catalyst Cp*Rh^Ⅲ is likely generated by anion exchange with AgOAc, followed by directed C-H activation of N-methoxybenzamides 1 to give intermediate Ⅰ. Subsequent regioselective insertion of the propargyl alcohol gives a seven-membered rhodacycle Ⅱ, which then undergoes β-H elimination and tautomerization to give an intermediate Ⅳ.

Scheme4. Proposed mechanism for the synthesis of isoindolinone.

Following the formation of Ⅳ, several pathways may be possible. In path a, the C=C bond in Ⅳ inserts into the Rh-H bond to give an alkyl intermediate Ⅴ, which undergoes reductive elimination to yield product 3 together with formation of a Rh^Ⅰ species. Alternatively, intermediate Ⅳ might also undergo reductive elimination to form intermediate Ⅵ (path b), followed by aza-Michael addition to give the final product [12]. The intermediate Ⅳ might also directly undergo Rh-N bond insertion into the C=C bond to give the intermediate Ⅶ, which releases the final product 3 by reductive elimination. Finally, the Rh^Ⅰ species could be reoxidized by Ag₂CO₃ to complete the catalytic cycle. Based on our H/D exchange studies, path b seems less likely. This is because if the path b is a primary pathway, we would expect loss of level of deuteration in the product when 2a-d₁ was used as a result of exchange of the labile N-H proton in intermediate Ⅵ with adventitious water in the solvent or the added (deuterated) water (Scheme 3, Eq. (1)). Furthermore, with introduction of 10 equiv. D₂O, the degree of deuteration at the methylene position is expected to be markedly increased (Scheme 3, Eqs. (2) and (3)). Although paths a and c cannot be distinguished at this stage, on the basis of generally higher tendency of migratory insertion of a hydride group then other groups, we tentatively prefer pathway a.

4 Conclusions

A highly efficient Rh-catalyzed annulation of N-methoxybenzamide and propargyl alcohol via C-H activation has been disclosed, leading to the efficient synthesis of a series of N-substituted isoindolinones bearing a stereogenic center. The catalytic reaction features mild reaction conditions, good function group toleration, and high reaction efficiency. Moreover, this protocol endows alkyne with an unusual role of C₁ synthon, and this method may find applications in the formation of related complex molecules.

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