Luận án tiến sĩ Kỹ thuật hóa học: Application of Cu-MOF-74, Cu2(OBA)2(BPY), MOF-235 as catalysts for carbon-heteroatom bond forming reactions

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY

HO CHI MINH UNIVERSITY OF TECHNOLOGY

TRAN BOI CHAU

HO CHI MINH UNIVERSITY OF TECHNOLOGY

TRAN BOI CHAU

APPLICATION OF Cu-MOF-74, Cu2(OBA)2(BPY), MOF-235 AS CATALYSTS

FOR CARBON−HETEROATOM BOND FORMING REACTIONS

Major: Chemical Engineering Major code: 62520301

Independent examiner 1: Assoc Prof Dr Tran Ngoc Quyen Independent examiner 2: Assoc Prof Dr Ton That Quang

Examiner 1: Prof Dr Phan Dinh Tuan

Examiner 2: Assoc Prof Dr Nguyen Phuong Tung Examiner 3: Assoc Prof Dr Nguyen Quang Long

ADVISORS:

1 Prof Dr Phan Thanh Son Nam 2 Assoc Prof Dr Truong Vu Thanh

DECLARATION OF ORIGINALITY

I hereby declare that this is my own research study The research results and conclusions in this thesis are true, and are not copied from any other resources The literature references have been quoted with clear citation as requested

Thesis author

Tran Boi Chau

TÓM TẮT LUẬN ÁN

Luận án nhằm khảo sát hoạt tính xúc tác của Cu-MOF-74, Cu2(OBA)2(BPY) và 235 cho phản ứng hình thành liên kết carbondị tố Nội dung của luận án được trình bày thành bốn chương:

MOF-Chương 1: Tổng quan về Cu-MOF-74, Cu2(OBA)2(BPY), MOF-235 và phản ứng hình thành liên kết carbondị tố Chương 1 trình bày các ưu, nhược điểm của các vật liệu khung cơ kim (MOFs) khi được ứng dụng làm xúc tác; hoạt tính xúc tác của các vật liệu xốp, khung hữu cơ tâm đồng và tâm sắt cho phản ứng hình thành liên kết carbondị tố trong tổng hợp các aryl ethers và các hợp chất dị vòng 5 cạnh, 6 cạnh ngưng tụ với vòng benzene Đồng thời, trong chương 1 cũng nêu tóm tắt về phương pháp tổng hợp, đặc trưng hóa lý và một số các ứng dụng của ba loại vật liệu Cu-MOF-74, Cu2(OBA)2(BPY), MOF-235

Chương 2: Trình bày về thực nghiệm và kết quả của phân tích đặc trưng cấu trúc của Cu-MOF-74, Cu2(OBA)2(BPY), MOF-235 Các loại vật liệu được tổng hợp bằng phương pháp nhiệt dung môi và được phân tích các đặc trưng hóa lý bằng phương pháp nhiễu xạ tia X dạng bột (P-XRD), phổ hồng ngoại (FT-IR), kính hiển vi điện tử quét (SEM), kính hiển vi điện tử truyền qua (TEM), đo diện tích bề mặt bằng phương pháp hấp phụ đẳng nhiệt nitrogen, phương pháp nhiệt trọng lượng (TGA)

Chương 3: Trình bày kết quả khảo sát và bàn luận về hoạt tính xúc tác của

Cu-MOF-74 cho phản ứng ether hóa trực tiếp giữa N-(quinolin-8-yl)benzamides với

alcohols/phenols Hoạt tính xúc tác của MOF-235 cho phản ứng oxy hóa cộng vòng giữa benzyl alcohols và 2-aminophenols/2-aminothiophenols Hoạt tính xúc tác của Cu2(OBA)2(BPY) trong phản ứng tổng hợp quinazolines, 4H-3,1-benzoxazines Các

phản ứng được khảo sát tính dị thể và tái sử dụng xúc tác trong điều kiện phản ứng

Chương 4: Trình bày tóm tắt về các kết quả đạt được cũng như các đóng góp chính của luận án, đồng thời đề xuất một số hướng nghiên cứu tiếp theo

ABSTRACT

Metal–organic frameworks (MOFs) are a well-known class of materials having the potential to become homo-hetero bridge Compared with homogeneous catalysts, MOF catalysts can be recycled and reutilized for several times; while compared with conventional heterogeneous catalysts, MOFs have structural and chemical tunability Metalorganic frameworks based on copper or iron metal sites have been used as catalysts for carbonheteroatom bond forming reactions These copper-based MOFs or iron-based MOFs are promising owing to the utilization of non-precious and less toxic metal salt species Herein; Cu-MOF-74, Cu2(OBA)2(BPY), MOF-235 were synthesized by solvothermal method, and characterized by P-XRD, SEM, TEM, TGA, FT-IR, nitrogen physisorption measurements Catalytic activities of these MOFs were investigated through carbonheteroatom bond forming reactions In fact, Cu-MOF-74

was used as a catalyst for the direct etherification of N-(quinolin-8-yl)benzamides with alcohols/phenols Besides, the synthesis of some N,N-, N,O-, N,S-heterocyclic

compounds was studied via two approaches One approach was based on the oxidative cyclization reaction between 2-aminophenols/2-aminothiophenols and alcohols catalyzed by MOF-235 The other was a one-pot, two-step process which involved the condensation of aldehydes with 2-aminobenzylamines/2-aminobenzyl alcohols/ 1,2-phenylenediamines in catalyst-free conditions, followed by oxidative dehydrogenation of CN bond catalyzed by Cu2(OBA)2(BPY) All the surveyed catalysts were examined for the heterogeneity and reutilization under reaction conditions To the best of our knowledge, these transformations under the studied reaction conditions have not been previously mentioned

ACKNOWLEDGMENT

This thesis has been carried out at Ho Chi Minh University of Technology since 2014 It may not be a very long time for someone else, but it will always be the suffering time as well as the best time in my life I have never ever thought that I could get through this tough time in life Fortunately, there always are my warmhearted and enthusiastic scientific advisors accompanying with me to my success The motivation of them always cheer me up at the right time and help me to go ahead to the final target I can never give enough my thanks to Prof Dr Phan Thanh Son Nam and Assoc Prof Dr Truong Vu Thanh It is impossible to recount all supports from my advisors, but these will be in my heart for the remaining of my life

It is an absolute misstep if I could not give my grateful thanks to Assoc Prof Dr Pham Thanh Quan, Assoc Prof Dr Le Thi Hong Nhan, Dr Phan Thi Hoang Anh All my lecturers are so kindness to boost me up whenever I got troubles to conduct the thesis or even some problems happened in personal life

Additionally, I would like to send my great thanks to all friends that I met in the laboratory for their excellent helps during the period of carrying experiments I give my sincerely thanks to Ha Quang Hiep, Doan Hoai Son, Duong Ngoc Tan Xuan, To Anh Tuong, Dang Van Hieu Among these friends, Ha Quang Hiep was the first person I met in the lab, who not only instructed me the lab instruments but also helped me close the gap between me with others

Finally, I would like to express my profound gratitude to my parents and also to my partner for providing me perfect support and encouragement during the entire course even in the hard time of writing this thesis

Tran Boi Chau

TABLE OF CONTENTS

1.1 Introduction to metalorganic frameworks 2

1.1.1 Possibility of catalytic application of MOFs 3

1.1.1.1 Limitations of MOFs as catalysts 3

1.1.1.2 The prospects of MOFs as catalysts 5

1.1.2 Factors affecting catalytic activities of MOFs 6

1.1.2.1 Influence of synthetic methods 6

1.1.2.2 Influence of ligands on catalytic performances 7

1.1.2.3 Influence of secondary building units 8

1.1.2.4 Influence of reaction solvents 8

1.2 MOF-235, Cu-MOF-74 and Cu2(OBA)2(BPY) 9

1.2.1 Synthesis, structure and physicochemical properties of MOF-235 9

1.2.2 Synthesis, structure and physicochemical properties of Cu2(OBA)2(BPY) 10

1.2.3 Synthesis, structure and physicochemical properties of Cu-MOF-74 11

1.3 Carbonheteroatom bond forming reactions for the synthesis of benzo-fused heterocycles 14

1.4 Carbonheteroatom bond forming reactions for the synthesis of aryl ethers 23

1.5 Aims and projects 29

2.1 Introduction 32

2.2 Experimental 32

2.2.1 Materials and instrumentation 32

2.2.2 Synthesis of Cu-MOF-74 33

2.2.3 Synthesis of Cu2(OBA)2(BPY) 33

2.2.4 Synthesis of MOF-235 34

2.3 Results and discussion 35

2.3.1 Characterization of Cu2(OBA)2(BPY) 35

3.2.1.2 Materials and instruments 48

3.2.2 Catalytic activity of Cu2(OBA)2(BPY) for the synthesis of N,N- and Heterocycles 49

N,O-3.2.2.1 Typical experiment 49

3.2.2.2 Materials and instruments 50

3.2.3 Catalytic activity of Cu-MOF-74 for the synthesis of aryl ethers 50

3.2.3.1 Typical experiment 50

3.2.3.2 Materials and instruments 51

3.3 Results and discussion 52

3.3.1 Catalytic activity of MOF-235 for the synthesis of N,O- and Heterocycles 52

N,S-3.3.2 Catalytic activity of Cu2(OBA)2(BPY) for the synthesis of N,N- and Heterocycles 64

N,O-3.3.3 Catalytic activity of Cu-MOF-74 for the synthesis of aryl ethers 73

3.4 Summary 84

4.1 Thesis summary 864.2 Contribution of this thesis 874.3 Future works 87

LIST OF FIGURES

Figure 1.1 Schematic presentation for the construction of typical coordination polymers/MOFs from molecular building blocks [9] 2Figure 1.2 Typical curves observed in hot filtration test 4Figure 1.3 Different type of MOF active sites, including metal nodes, functionalized organic linkers, and guest species in the pores [31] 6Figure 1.4 The XRD patterns and the corresponding SEM images for the ZIF-8 samples synthesized by different methods (spray drying: ZIF-8-SP, microwave: ZIF-8-MW, room temperature: ZIF-8-RT, solvothermal: ZIF-8-SV) [32] 7Figure 1.5 (a) Inorganic building unit of MOF-235; (b) Single-crystal X-ray structure of MOF-235 (Fe, blue; O, red; Cl, teal; C, gray) [47] 9Figure 1.6 Coordination environment of copper in Cu2(OBA)2(BPY) [47] 10Figure 1.7.(a) 2D helical layers produced by Cu(II) and OBA ligands; (b) The 3D pillared-layer structure of Cu2(OBA)2(BPY) [47] 11Figure 1.8 (a) Coordination environment of Cu(II) centers in Cu-MOF-74 after thermal solvent removal (b) Inorganic SBUs crystalline framework (c) 3D honeycomb structure of Cu-MOF-74 (Cu, blue; O, red; C, gray) [52-54] 12Figure 1.9 Two possible paths for the conversion of amidine [86] 16Figure 1.10 A model of pillared-grid MOFs where circles indicate bimetal paddlewheels, red lines represent grid-forming ligands and blue lines represent pillar ligands [118] 27Figure 1.11 (a) Amino-functionalized tetracarboxylate ligand (b) Large spherical cages with diameter about 11 Å (c) Topology of [Cu6(L)3(H2O)6].(14DMF).9(H2O) (d) Unsaturated coordination space in MOF [121] 28Figure 2.1 P-XRD of the Cu2(OBA)2(PBY) 35Figure 2.2 FT-IR spectra of (a) the Cu2(OBA)2(BPY), (b) H2OBA, (c) 4,4’-bipyridine 36Figure 2.3 (a) SEM, (b) TEM micrographs of the Cu2(OBA)2(BPY) 36Figure 2.4 (a) Nitrogen adsorption/desorption isotherm, (b) Pore size distribution of the Cu2(OBA)2(BPY) 37Figure 2.5 TGA analysis of the Cu2(OBA)2(BPY) 37Figure 2.6 (a) P-XRD, (b) SEM micrograph, (c) TEM micrograph of the synthesized Cu-MOF-74 38

Figure 2.7 (a) Nitrogen adsorption/desorption isotherm, (b) Pore size distribution of

the Cu-MOF-74 39

Figure 2.8 TGA analysis of the Cu-MOF-74 39

Figure 2.9 FT-IR spectra of (a) the Cu-MOF-74, (b) 2,5-dihydroxyterephthalic acid 40Figure 2.10 P-XRD of the MOF-235 41

Figure 2.11 SEM micrograph of the MOF-235 41

Figure 2.12 TEM micrograph of the MOF-235 41

Figure 2.13 FT-IR spectra of (a) the MOF-235, (b) 1,4-benzenedicarboxylic acid 42

Figure 2.14 Nitrogen adsorption/desorption isotherm of the MOF-235 42

Figure 2.15 Pore size distribution of the MOF-235 42

Figure 2.16 TGA analysis of the MOF-235 43

Figure 3.1 Yield of 2-phenylbenzo[d]oxazole versus temperature 52

Figure 3.2 Yield of 2-phenylbenzo[d]oxazole versus oxidant 52

Figure 3.3 Yield of 2-phenylbenzo[d]oxazole versus oxidant quantity 54

Figure 3.4 Yield of 2-phenylbenzo[d]oxazole versus solvent 54

Figure 3.5 Yield of 2-phenylbenzo[d]oxazole versus catalyst amount 55

Figure 3.6 Yield of 2-phenylbenzo[d]oxazole versus reactant molar ratio 55

Figure 3.7 Leaching test of solid iron-based framework 57

Figure 3.8 Yield of 2-phenylbenzo[d]oxazole versus catalyst poison 57

Figure 3.9 Yield of 2-phenylbenzo[d]oxazole versus radical trapping reagent 57

Figure 3.10 Yield of 2-phenylbenzo[d]oxazole versus homogeneous iron catalyst 59

Figure 3.11 Yield of 2-phenylbenzo[d]oxazole versus heterogeneous catalyst 59

Figure 3.12 Catalyst reutilizing investigation 61

Figure 3.13 P-XRD results of the new (a) and reutilized (b) catalyst 61

Figure 3.14 FT-IR results of the new (a) and reutilized (b) catalyst 61

Figure 3.15 Yield of 2-(4-nitrophenyl)quinazoline versus heterogeneous catalyst 65

Figure 3.16 Yield of 2-(4-nitrophenyl)quinazoline versus solvent 65

Figure 3.17 Yield of 2-(4-nitrophenyl)quinazoline in the case of omission of each reagent 66

Figure 3.18 Yield of 2-(4-nitrophenyl)quinazoline versus homogeneous catalyst 67

Figure 3.19 Leaching test of solid copper-based framework 71

Figure 3.20 Reusability of Cu2(OBA)2(BPY) 72

Figure 3.21 P-XRD of the fresh (a) and reused (b, after 5 runs) Cu2(OBA)2(BPY) 72

Figure 3.22 Yield of 2-ethoxy-N-(quinolin-8-yl)benzamide versus base 74

Figure 3.23 Yield of 2-ethoxy-N-(quinolin-8-yl)benzamide versus solvent 74

Figure 3.24 Yield of 2-ethoxy-N-(quinolin-8-yl)benzamide versus temperature 75

Figure 3.25 Yield of 2-ethoxy-N-(quinolin-8-yl)benzamide versus oxidant 75

Figure 3.26 Yield of 2-ethoxy-N-(quinolin-8-yl)benzamide versus catalyst amount 76

Figure 3.27 Yield of 2-ethoxy-N-(quinolin-8-yl)benzamide versus pyridine volume 76Figure 3.28 Yield of 2-ethoxy-N-(quinolin-8-yl)benzamide versus amount of base 77

Figure 3.29 Yield of 2-ethoxy-N-(quinolin-8-yl)benzamide versus heterogeneous catalyst 78

Figure 3.30 Reaction conversion versus heterogeneous catalyst 78

Figure 3.31 Yield of 2-ethoxy-N-(quinolin-8-yl)benzamide versus homogeneous catalyst 79

Figure 3.32 Reaction conversion versus homogeneous catalyst 79

Figure 3.33 Leaching test of solid copper-based framework 80

Figure 3.34 Catalyst reutilizing studies 80

Figure 3.35 P-XRD of fresh (a) and reused catalysts (b) 80

LIST OF SCHEMES

Scheme 1.1 Direct arylation of benzothiazole with aryl halide [50] 11

Scheme 1.2 Reaction of aryl iodides with N-H nucleophiles catalyzed by aCu-MOF-74 [59] 13

Scheme 1.3 Cross coupling approaches to form benzazoles [73-76] 14

Scheme 1.4 Synthesis of 4H-3,1-benzoxazines [78] 15

Scheme 1.5 Phillip’s method in the synthesis of benzazoles [79] 15

Scheme 1.6 Plausible reaction for Phillip benzazole synthesis [80] 15

Scheme 1.7 Conversion of amidine to 2-phenylbenzimidazole [86] 16

Scheme 1.8 Reaction between 2-aminobenzyl alcohol and propiophenone [87] 17

Scheme 1.9 Referred mechanism for reaction between 2-aminobenzyl alcohol and propiophenone [87] 18

Scheme 1.10 Reaction between 1,2-phenylenediamine and acetone to form dihydro-2,2,4-trimethyl-1H-1,5-benzodiazepine [88] 18

2,3-Scheme 1.11 Proposed mechanism for reaction between 1,2-phenylenediamine and acetone catalyzed by MIL-100 (Fe) to form benzodiazepine [89] 19

Scheme 1.12 Synthesis of 2-phenylquinazolin-4(3H)-one via a one-pot, two-step process [90] 19

Scheme 1.13 Probable reaction mechanism [90] 20

Scheme 1.14 Tandem process for the conversion of benzyl alcohol to phenylquinazolin-4(3H)-ones [91] 20

2-Scheme 1.15 Plausible reaction mechanism [91] 21

Scheme 1.16 Reaction of benzyl alcohol and 2-aminobenzamide [92] 22

Scheme 1.17 Plausible reaction mechanism [92] 22

Scheme 1.18 Palladium-catalyzed alkoxylation of N-methoxybenzamides [103] 24

Scheme 1.19 Copper-catalyzed phenoxylation of N-(quinolin-8-yl)benzamide derivatives [105] 24

Scheme 1.20 Ullmann coupling of phenol derivatives and aryl halides catalyzed by MOF-199 [110] 25

Scheme 1.21 Coupling reaction of nitroarenes and substituted phenols catalyzed by Cu2(BDC)2(DABCO) [111] 25

Scheme 1.22 Reaction of 2-hydroxybenzaldehydes and dioxane catalyzed by

Cu2(BPDC)2(BPY) [120] 27

Scheme 1.23 Plausible reaction mechanism of dioxane and 2-hydroxybenzaldehyde [121] 29

Scheme 2.1 Synthetic procedure of the Cu-MOF-74 33

Scheme 2.2 Synthetic procedure of the Cu2(OBA)2(BPY) 34

Scheme 2.3 Synthetic procedure of the MOF-235 34

Scheme 3.1 Synthesis of 2-phenylbenzoxazole 48

Scheme 3.2 Synthesis of 2-(4-nitrophenyl)quinazoline 50

Scheme 3.3 Etherification of N-(quinolin-8-yl)benzamide with ethanol 51

Scheme 3.4 Plausible reaction pathway 58

Scheme 3.5 Synthesis of 2-(4-nitrophenyl)quinazoline 64

Scheme 3.6 Plausible reaction mechanism for the oxidative dehydrogenation reaction of 2-substituted 1,2-dihydro- 4H-3,1-benzoxazine 72

Scheme 3.7 Reactions with large scale and the synthesis of targeted bioactive compounds 83

Scheme 3.8 Plausible mechanism for alkoxylation of N-(quinolin-8-yl)benzamide 84

LIST OF TABLES Table 1.1 Structure and characteristics of MOF-235, Cu2(OBA)2(BPY), Cu-MOF-74 13

Table 2.1 Characteristics of synthesized Cu2(OBA)2(BPY), Cu-74 and 235 44

MOF-Table 3.1 The synthesis of 2-arylbenzoxazoles and 2-arylbenzothiazoles via the pot oxidative cyclization reaction 62

one-Table 3.2 Scope of reactions 68

Table 3.3 Reaction scope for alkoxylation and phenoxylation 81

LIST OF ABBREVIATIONS

Acac Acetylacetonate AQ Aminoquinoline CHP Cumyl hydroperoxide DEC Diethyl carbonate DTBP Di-tert-butyl peroxide

BCMIM 1,3-Bis(carboxymethyl)imidazole BDC 1,4-Benzenedicarboxylate

BET Brunauer–Emmett–Teller BPDC 4,4-Biphenyldicarboxylate BPY 4,4’-Bipyridine

BTC 1,3,5-Benzenetricarboxylate CSD Cambridge Structural Database DABCO 1,4-Diazabicyclo [2.2.2] octane DCM Dichloromethane

DMA N,N-Dimethylacetamide

DMAP 4-Dimethylaminopyridine DMF N,N-Dimethylformamide

DMSO Dimethyl sulfoxide GC Gas chromatography

FT-IR Fourier Transform Infrared Spectroscopy FE-SEM Field emission scanning electron microscopy H2OBA 4,4’-Oxybis(benzoic)acid

H2DHTP 2,5-Dihydroxyterephthalic acid H2BDC 1,4-Benzenedicarboxylic acid H2BPDC 4,4’-Biphenylcarboxylic acid

HKUST Hong Kong University of Science and Technology

DMC Dimethyl carbonate

MIL Matériaux de I’Institut Lavoisier MS Mass Spectrometry

NMR Nuclear Magnetic Resonance NMP N-Methyl-2-pyrrolidone

NMO N-Methylpyrrolidine

OBA 4,4’-Oxybis(benzoate)

PEG Polyethylene glycol

PhI(OAc)2 Phenyliodine(III) diacetate P-XRD Powder X-ray Diffraction SBUs Secondary Building Units SEM Scanning Electron Microscopy

VNU-18 Vietnam National University-Ho Chi Minh City TBHP tert-Butyl hydroperoxide

TEA Triethanolamine

TEM Transmission Electron Microscopy TGA Thermogravimetric Analysis TPA Terephthalic acid

ZIF Zeolitic Imidazole Framework

INTRODUCTION

Metal–organic frameworks (MOFs) have recently emerged as versatile materials in the field of heterogeneous catalysis due to their high surface area, modular nature and high crystallinity MOFs have been used as catalysts for many carboncarbon, carbonheteroatom bond forming reactions

Among molecules containing carbonheteroatom linkages, aryl ether compounds as well as compounds consisting of a five-membered or six-membered heterocyclic ring fused to benzene nucleus exhibit diverse biological properties For example, benzimidazole derivatives are used as drugs for psychiatric, hypertension, cancer treatment [1, 2] Substituted quinazolines display antibacterial, antitubercular and antiviral abilities [3, 4] Diphenyl ethers and their halo derivatives are used as herbicide and fungicides

Despite the indispensability of these biologically active compounds, MOF-catalyzed reactions for the synthesis of these skeletons have still been limited Therefore, this thesis investigated the catalytic performance of Cu-MOF-74, Cu2(OBA)2(BPY) and

MOF-235 for the synthesis of benzazole, quinazoline, 4H-3,1-benzoxazine, aryl ether

derivatives The reason for choosing MOF targets was based on the utilization of analogous metal salts/complexes as catalysts in the synthetic procedures of these surveyed compounds [5-8] Besides, other factors such as less toxic and non-noble metallic precursors should be taken into account In addition to the reutilization of the materials in consecutive cycles, the outstanding catalytic activities of these copper, iron-based MOFs compared to that of analogous homogeneous catalysts in the studied reaction conditions were ascribed to the structure of these metalorganic frameworks Hence, the investigations of using MOFs as catalysts in organic transformations have been attracting attraction in recent years

CHAPTER 1 Cu-MOF-74, Cu2(OBA)2BPY, MOF-235 AND CARBON HETEROATOM BOND FORMING REACTIONS

1.1 Introduction to metalorganic frameworks

The discovery of ordered porous materials has created great opportunities for new applications in heterogeneous catalysis Metalorganic frameworks (MOFs) are well-ordered porous structures designed by the coordinative bonds between metal ions (or metal clusters) and organic linkers The metal ions, including monovalent (Cu+, Ag+, etc.), divalent (Cu2+, Zn2+, Mg2+, etc.), trivalent (Al3+, Cr3+, Ga3+, Fe3+, In3+, etc.) or tetravalent (V4+, Zr4+, Ti4+, Hf4+, etc.) metal cations, have been used as inorganic nodes to construct MOFs The common organic linkers used in the MOF synthesis are carboxylates, imidazolates, phosphonates, etc Based on a wide choice of metal ions and bridging ligands, variety of MOF structures with one dimensional, two to three dimensional networks have been synthesized to meet changing demands

Figure 1.1 Schematic presentation for the construction of typical coordination polymers/MOFs from molecular building blocks [9]

1.1.1 Possibility of catalytic application of MOFs

Metalorganic frameworks (MOFs), as a new class of porous crystalline compounds, consist of second building units and organic ligands Owing to the tunability of both organic and inorganic components, various MOF structures have been synthesized and applied in broad range of fields from gas storage and gas separation [10-13] to chemical sensors [14], nanofluids [15] In addition to these extraordinary applications, MOFs are proved to be promising catalysts for organic transformations However, the difficulties and prospects of using MOFs as catalysts need to be further analyzed

1.1.1.1 Limitations of MOFs as catalysts

One of the obstacles that can limit catalytic application of MOFs is their mechanical resistance MOFs must be mechanically stable enough to retain their crystalline structure, porosity and surface area in a certain compression or in a certain pressure [16] Due to variable interactions, including strong coordination bonds to weaker dispersion forces and hydrogen bonds, MOFs showed complicated mechanical resistance properties Many MOFs were found to suffer from reversible or irreversible amorphization under mechanical loading, while others exhibited drastic pressure-induced phase transitions, associated with striking bond rearrangements [17]

Another issue needs to be analyzed is the thermal and chemical stability of MOFs In most cases, thermal instability is a result of bond-breaking between the nodes and organic linkers, followed by linker combustion The thermal stability of MOFs is frequently screened by thermal-gravimetric analysis (TGA) and differential thermal analysis (DTA) In a typical TGA experiment, during progressively raising the temperature under a flowing inert N2 or He gas, guests were first released, followed by the loss of coordinated solvent molecules and then by the decomposition of frameworks Under TGA condition, some zirconium-based MOFs UiO-66 retained their crystalline structure at the temperature above 500oC [18] However, the data coming from TGA have to be taken with caution when MOFs are used under catalytic conditions It should be noted that TGA was carried out under a flowing inert gas (N2 or He) for a limited time, while reactions proceeded in certain solvents, in acidic or basic media, or in the presence of certain functional groups for a relatively long reaction time In these certain

situations, MOFs might leach at a rather lower temperature than the data obtained from TGA Leaching was an undesirable process in heterogeneous catalysts because it ruined solid catalysts in the long term and destroyed the shape selectivity of these catalysts And most importantly, it led to inaccurate determination of catalytic activity Leaching process was detected by chemical analysis of the filtrate of the reaction medium or by comparison of the P-XRD patterns between the fresh and the used catalysts It should be noted that metal ions were just an integral part of the whole metalorganic framework Hence, if a relatively small amount of catalyst was leached, it was difficult to screen by chemical analysis or by P-XRD P-XRD was used when catalyst was leached in a mass causing the frameworks to collapse In these cases, the most common alternative procedure was a hot filtration test The reaction was carried out under normal condition, as the conversion of reaction was partially performed (approximately 30% in conversion), the catalyst would be removed and the filtrate was continuously conducted under the same reaction conditions If no trace of leached metal passed to the liquid medium, no further conversion should be observed in the filtrate

Figure 1.2 Typical curves observed in hot filtration test

Other alternative for the hot filtrate test was the three phrases test In the three phrases test, one of the reaction partners was grafted on the solid which was different from MOF The solid was then screened for product formation If MOF was leached, the substrate anchored on the solid would be converted into products

1.1.1.2 The prospects of MOFs as catalysts

From another aspect, MOFs have been known as remarkable heterogeneous catalysts because of their extremely high porosity, high surface area and crystalline structure Among the porous materials, MOFs are crystalline porous solids with ultrahigh porosity MOF-210 exhibits the highest Langmuir specific surface area of 10400 m2/gam and pore volume of 3.60 cm3/gam [19] The sizes of channels and cavities in MOFs can be varied from a few angstroms to several nanometers depending on the length of ligands and the connectivity of the networks This is an advantage of using MOFs as heterogeneous catalysts because diffusion of reactants and products to and from the active sites is often a rate limiting step Note that this is in contrast to zeolites, for which the diversity of framework types is limited due to the use of a few number of building units such as [SiO4] and [AlO4] tetrahedral units Owing to modular nature, size and shape as well as the chemical environments of the pores can be tuned to make MOFs more selective toward guest incorporation Pore spaces in MOFs not only affect the chemical nature of molecules and reactions due to the confinement effect, but also impact on the transition states formed during the reaction occurring within the pores [20, 21] Due to host–guest interactions, the substrates were activated and oriented around a catalytic site, leading to modification of reactivity of the adsorbates and the selectivity of the reaction towards specific products

Another advantage of using MOFs as catalysts is the highly crystalline structures In MOF topologies, the position of each atom, the coordination and oxidation state of metal ion will be clarified This crystalline nature of MOFs will facilitate establishing structure–activity relationships of the catalytic centers of the frameworks The diversity of MOF synthetic methodology is also one of the benefits for catalytic application The active sites can be introduced at the metallic sites, at the organic linker or inside the pore system by as-synthesis or post-synthesis methodology [22] For example, some MOFs such as MIL-101 (Fe, Cr, Al) [23], HKUST-1 [24] contained unsaturated metal centers at the nodes of the frameworks that could directly coordinate to the substrate and catalyze chemical transformations These MOFs were referred as as-synthesized active MOFs Unfortunately, only a few proportions of the whole family

of MOFs could be directly used as catalysts in the as-synthesized form Other MOFs were required to be modified to achieve certain catalytic properties by a large variety of techniques known as post-synthetic modification, including grafting [25], encapsulation [26], metal [27] or ligand exchange [28], anchoring [29], covalent modification [30], etc

Figure 1.3 Different type of MOF active sites, including metal nodes, functionalized organic linkers, and guest species in the pores [31]

1.1.2 Factors affecting catalytic activities of MOFs

Up to now, the catalytic activities of MOFs in many organic transformations have been studied The data reported indicated that the catalytic performance was affected by many factors, including MOF synthetic approaches, the reaction temperature, reaction solvents and MOF construction, etc

1.1.2.1 Influence of synthetic methods

To demonstrate the effect of synthetic methods on catalytic performance, ZIF-8 was prepared in different ways The results showed that shape and size of ZIF-8 particles were changed depending on the synthetic methods Smaller crystal size was obtained when crystallization rate increased

Figure 1.4 The XRD patterns and the corresponding SEM images for the ZIF-8 samples synthesized by different methods (spray drying: ZIF-8-SP, microwave: ZIF-8-

MW, room temperature: ZIF-8-RT, solvothermal: ZIF-8-SV) [32]

The rate of crystallization was also found to affect the formation of defect sites leading to different catalytic performance of MOFs To study the defect concentration in ZIF-8 series synthesized by various methods, including ZIF-8-spray, ZIF-8-microwave, ZIF-8-room temperature, ZIF-8-solvothermal; the temperature programmed desorption of NH3 (NH3-TPD); CO2 (CO2-TPD) or CO (CO-TPD) were carried out All data obtained from different samples exhibited that ZIF-8-spray possessed the highest amounts of defects, followed by ZIF-8-microwave, ZIF-8-room temperature, ZIF-8-solvothermal, respectively This study confirmed that the amounts of defects depended on the synthetic approaches In addition, there existed a correlation between the number of defects and catalytic performances owing to the presence of large number of acid and basic sites originating from the defect structure As a result, the more defect sites were created, the better catalytic activities performed [32]

1.1.2.2 Influence of ligands on catalytic performances

Ligands used in MOFs can be classified into several categories such as N-containing

heterocyclic ligands, carboxylate ligands, cyano ligands, sulfonyl ligands, phosphoryl ligands, etc Not only coordinating groups on ligands took part in MOF construction, but also the length and substituents on ligands were found to play important roles in catalysis of organic transformations The effect of spacer between two coordinating groups on the catalytic activities of MOFs has been studied With the long spacer, MOFs had large pores for higher accessibility of reactants to active sites Substituents on ligand also impacted on either the steric or electronic effect of the binding properties, resulting in the different catalytic activities [33-35] In addition, these substituents themselves contained the functionalized groups that could act as catalysts Up to now, many MOFs with functionalized organic bridging ligands have been reported as catalysts for organic transformations such as esterification [36], Knoevenagel condensation [37], aldol condensation [38], etc

1.1.2.3 Influence of secondary building units

SBUs directly affect the construction of MOFs Through the chosen configuration of SBU, the desired construction as well as some special properties, including the robustness, the optical or electric conduction, catalytic properties can be predicted [33] These coordinatively saturated or unsaturated metal centers have been used as catalysts in Friedländer reactions [39], isomerization reactions [40], cyanosilylation reactions [41], oxidation reactions [42], Friedel-Crafts alkylation [43], etc

1.1.2.4 Influence of reaction solvents

Although solvents used in most reactions were optimized to get the best results, but the effects of solvents are still unclear Solvents not only stabilized the transition state but also affected the catalytic performance of MOFs [44] Solvents used in reactions either might block the active metal sites of MOFs or competed with reactants in adsorption on these active sites [40] as well as affected the stability of MOFs [35]

In summary, metalorganic frameworks have been found to fill the gap between heterogeneous catalysts and homogeneous catalysts because of inheriting all the benefits from these two types of catalysts The high tunability along with the possibility of grafting the catalytic sites into most components of structure, the well-defined

structure are the outstanding characteristics of homogeneous catalysts However, the homogeneous catalysts were unable to be recovered from the reaction medium, and lack of size and shape selectivity as compared to that of heterogeneous catalysts As a result, MOFs have become good candidates in catalytic application, especially in the field of asymmetric catalysis for chiral environments or multi-functionalized MOFs for coupling multi-components in one-pot reactions

1.2 MOF-235, Cu-MOF-74 and Cu2(OBA)2(BPY)

1.2.1 Synthesis, structure and physicochemical properties of MOF-235

[Fe3O(1,4-BDC)3(DMF)3][FeCl4-](DMF)3, also known as MOF-235, was first synthesized in 2005 by solvothermal method It was constructed from ferric chloride hexahydrate and terephthalic acid in the presence of ethanol and DMF The particles of MOF-235 have octahedron morphology in which each iron atom is trivalent yielding an overall cationic (+1 per formula unit) framework This charge is balanced by FeCl4-

counterion which is located in the hexagonal pore of the structure The diameter of the primary hexagonal channel in MOF-235 is 6.7 Å [45, 46]

MOF-235 was first used as adsorbent in gas separation and environmental treatment It was found as a potential adsorbent for the separation of CH4 from a mixture of CH4, H2, CO2 The absolute adsorption capacity was observed highest toward CH4.These results were attributed to the high pore volume and large number of open metal sites in MOF-235 [48] Because of the special structure in which the framework had the positive charge balanced by FeCl4 counterions, MOF-235 could adsorb very large amount of both anionic dye methyl orange and cationic dye methylene blue via an electrostatic interaction between the dyes and the adsorbent [49]

1.2.2 Synthesis, structure and physicochemical properties of Cu2(OBA)2(BPY)

Cu2(OBA)2(BPY) was synthesized by adding aqueous solution of Cu(NO3)2.3H2O into the mixture of 4,4’-oxybis(benzoic) acid (H2OBA) and 4,4’-bipyridine (BPY) In the structure of Cu2(OBA)2(BPY), Cu(II) ions have a trigonal bipyramid geometry formed by four carboxylate oxygen atoms and a nitrogen atom of the 4,4’-bipyridine The Cu–O bond length is assumed in the range 1.952–2.172 Å while the Cu–N bond length is 1.999 Å [47] Hence, the coordination geometry around the copper(II) atoms can be

regarded as a Jahn–Teller-distorted trigonal bipyramid

Figure 1.6 Coordination environment of copper in Cu2(OBA)2(BPY) [47].

In the structure of Cu2(OBA)2(BPY), OBA ligands take responsibility of producing 2D layers in which the carboxylate groups of OBA ligands are connected with the Cu(II) cations forming an eight-membered ring chains These 2D sheets lying in the ac plane are connected together in the third dimension by axially coordinating BPY ligands to

give a 3D framework featuring with channel along b-axis With this structure, Cu2(OBA)2(BPY) has monoclinic crystal system The channels were believed too small to residue any guests The potential free volume was 281 Å per unit cell volume (3443 Å) equally to 8.1% of void per unit volume

as a strong base environment of t-BuOLi and high temperature The catalyst could be

reused at least 7 times under these reaction conditions

Scheme 1.1 Direct arylation of benzothiazole with aryl halide [50]

1.2.3 Synthesis, structure and physicochemical properties of Cu-MOF-74

Cu-MOF-74, having the molar composition corresponds to Cu2(DHTP)(CH3OH)2.73(DMF)0.25(H2O)1.33, was first synthesized by Sanz et al [51] Cu-MOF-74 is structurally homologous to the honeycomb-like M-MOF-74 (M=Ni, Co,

Mg ) series with large, one dimensional hexagonal channel of 11.7 Å in diameter It was prepared from the mixture of 2,5-dihydroxyterephthalic acid and copper nitrate

trihydrate in the presence of N,N-dimethylformamide Solvents used in MOF synthesis

were found to affect the physical properties of the final material The presence of isopropanol as co-solvent in the synthesis medium led to higher crystallinity, higher specific surface area and larger pore volume of Cu-MOF-74 Cu-MOF-74 was one of Cu-based MOF materials having the highest densities of Cu(II) sites per unit volume (4.7 nm−3) In Cu-MOF-74 structure, Cu(II) cations are coordinated to six oxygen atoms, resulting in octahedral Cu centers Three of them are from the oxygen atoms of carboxylate groups, two are from the oxygen atoms of the hydroxyl groups on the ligand and the sixth one is from the oxygen atom in a solvent (such as DMF, methanol or H2O) Due to the Jahn–Teller effect on the distortion of the coordination environment of the Cu2+ ions, the Cu2+ ions in Cu-MOF-74 exhibit a low partial positive charge [51, 52] leading to the weaker interaction between the Cu2+ sites and adsorbate molecules As a consequence, the solvent molecules can be easily and completely removed in vacuum or at high temperatures, providing a five-coordinate Cu(II) species and an unsaturated metal site [52, 53]

Figure 1.8 a) Coordination environment of Cu(II) centers in Cu-MOF-74 after thermal solvent removal (b) Inorganic SBUs crystalline framework (c) 3D honeycomb

structure of Cu-MOF-74 (Cu, blue; O, red; C, gray) [52-54]

The high density of exposed and unsaturated metal ions in Cu-MOF-74 has sparked

much interest in the field of adsorption and the adsorptive separation of compounds such as NH3, CO2, CH4 , H2, ethylene [53, 55-58]

In the catalytic application, Cu-MOF-74 has been used as a catalyst in many organic transformation reactions Owing to the structure containing open metal sites, Cu-MOF-74 has been used as a heterogeneous Lewis acid catalyst in the Friedel-Crafts acylation of anisole [59] or transformation of trans-ferulic acid into vanillin [60] Cu-MOF-74 was also used as a redox catalyst in oxidation of cyclohexene [61] It should be noted that the reaction conditions of using Cu-MOF-74 as catalyst were rather mild in most cases [62-64] The crystalline Cu-MOF-74 was believed to partially turn into amorphous state under the action of alkali (NaOH 1M) and high temperature (120oC) in the reaction of 4-methoxy benzene iodide with imidazole [59]

Scheme 1.2 Reaction of aryl iodides with N-H nucleophiles catalyzed by

[51, 65]

Cu2(OBA)2(BPY) [47]

MOF-235

[45, 48] Empirical

formula C11.17H18.02Cu2N0.25O10 C19H12CuNO5 C33H15Fe4Cl4N3O17

Crystal system Trigonal Monoclinic Hexagonal Unit-cell

dimensions a, b, c (Å) α, ,  (o)

a = b = 25.9972 c = 6.2587 α = 90  =90  =120

a = 22.1792 b = 13.6526 c = 13.0343 α = 90=  = 119

a =12.531 = b c = 18.476 α = 90  = 90  = 120 Coordination

geometry Octahedral Trigonal pyramid Octahedral

Thermal

BET specific surface area (m2/g)

There were three basic approaches to achieve these benzo-fused heterocycles The first method was related to the intramolecular cross coupling of an anilides

Scheme 1.3 Cross coupling approaches to form benzazoles [73-76]

This approach included an intramolecular cyclization of anilides containing a leaving

group at the o-position promoted by a transition-metal catalyst such as Cu, Pd [73-76]

Thus, it was found inconvenient to synthesize benzimidazoles and benzothiazoles with this method due to the requirement of commercially unavailable starting materials such as amidines or thiocarbonyl Oxidative dehydrogenation reactions [77] were improved later, but these methods also had their own drawbacks involving the requirement of

reactive and toxic reagents, less-common substrates, transition-metal catalysts as well as strong oxidants

Scheme 1.4 Synthesis of 4H-3,1-benzoxazines [78]

Hence, Phillip reaction for the synthesis of these benzo-fused heterocycles showed

interest Those methods included the condensation of o-thio/hydroxy/aminoaniline with

either aldehyde or carboxylic acid and its derivatives (acyl chloride, ester, amide, nitrile and so forth)

Scheme 1.5 Phillip’s method in the synthesis of benzazoles [79]

Mineral acid, especially H3PO4, has been reported to be a highly effective reagent for promoting this type of condensation [79] Due to the importance of this method, the reaction was also studied in the presence of various Lewis acid catalysts such as ZrCl4, SnCl4, TiCl4, ZrOCl2.9H2O, In(Otf)3

Scheme 1.6 Plausible reaction for Phillip benzazole synthesis [80]

Despite significant advancements in this field, the construction of these benzo-fused

heterocycles was still a major challenge for organic chemists because these methods suffered from one or more of the disadvantages such as high thermal conditions (often more than 200oC), long reaction time (over 10 hours), difficulty in product separation and giving low yield in the preparation of 2-aryl substituted benzimidazoles [81-85] As a result, Cu-MOFs and Fe-MOFs have been used as catalysts for the synthesis of these fused heterocycles A porous MOF-537 with molecular formula Cu2(TPPB)2(DMF)6 was used as a catalyst for the transformation of amidines to 2-phenylbenzimidazoles [86]

Scheme 1.7 Conversion of amidine to 2-phenylbenzimidazole [86]

The target product was detected in 96% yield when a mixed solvent of DMSO/DMF was used Of note, benzoic acid was employed as acidic additive The use of benzoic acid was attributed to the suppression of decomposition products under these reaction conditions Substituent effect on the formation of 2-phenylbenzimidazoles was studied The results indicated that substituted amidines containing electron-withdrawing groups

or electron donating groups in ortho, meta, para position offered only moderate product

yields under optimized conditions For plausible mechanism, the author supposed that the CH functionalization of amidine underwent via two possible pathways

Figure 1.9 Two possible paths for the conversion of amidine [86]

In the first path, MOF-537 was coordinated to amidines and then followed by

electrophilic addition of copper center to N-phenyl ring to offer (B) After that, (B)

underwent reductive elimination and re-aromatization to give (D) In the second pathway, a copper nitrene intermediate (C) was formed instead of intermediate (B) via reaction of the amidine with MOF-537 Later, this intermediate underwent various steps, including insertion of the nitrogen into a C–H bond of amidine, electrocyclic ring closure and [1,3]-shift of a hydrogen to afford the final product (D) The moderate to high yields of benzimidazoles provided under these reaction conditions were attributed to the coordination ability as well as stabilizing intermediates (B) or (C) of MOF-537

Cu2(OBA)2(BPY) was used as a heterogeneous catalyst in one-pot domino reaction between 2-aminobenzyl alcohol and propiophenone to form phenyl(quinolin-3-yl)methanone [87]

Scheme 1.8 Reaction between 2-aminobenzyl alcohol and propiophenone [87] It should be noted that Cu2(OBA)2(BPY) displayed higher catalytic efficiency than Cu(BDC), Cu-MOF-74 and MOF-199 These results were attributed to the basic nitrogen atoms in BPY ligands which would facilitate the dehydrogenation on the alpha carbon in propiophenone in the catalytic cycle [87] As seen in the scheme 1.9, Ln-Cu(II)-enolate complex (A) was initially formed by coordination of copper species with ketone Then, (A) generated Cu(I) species and α-C-centered radical (B) via single electron transfer process This radical (B) reacted with TEMPO to form α-TEMPO-substituted ketone (C), which was then released TEMPOH to afford α, β-unsaturated ketone (D) β-Aminoketone (F) was formed by the reaction between (D) and aldehyde (E) Enaminone (G), which was formed by dehydrogenation of (F), was transformed into product via an intramolecular enamine–ketone condensation

Scheme 1.9 Referred mechanism for reaction between 2-aminobenzyl alcohol and propiophenone [87]

Another benzo-fused heterocycle, 2,3-dihydro-2,2,4-trimethyl-1H-1,5-benzodiazepine,

was formed in the cyclocondensation of 1,2-phenylenediamine with acetone using MOF-235 as catalyst [88]

Scheme 1.10 Reaction between 1,2-phenylenediamine and acetone to form

2,3-dihydro-2,2,4-trimethyl-1H-1,5-benzodiazepine [88]

It was observed that metal sites played significant roles in the activity of MOF-235 The catalytic performance of MOF-235 was compared with those of other MOFs containing different metal sites, including MOF-5, Mn(BDC) and Ni2(BDC)2(DABCO) The nickel-based MOF, Ni2(BDC)2(DABCO), gave no trace amount of product after 180

min Less than 10% conversion was detected after 180 min in the case of the zinc-based MOF-5 and manganese-based Mn(BDC)

Reaction between 1,2-phenylenediamine and acetone, which was catalyzed by 100(Fe) to form benzodiazepine, was reported by Jhung [89] Reaction mechanism was proposed involving the catalysis of Lewis acid sites in the frameworks

MIL-Scheme 1.11 Proposed mechanism for reaction between 1,2-phenylenediamine and acetone catalyzed by MIL-100 (Fe) to form benzodiazepine [89]

Among benzo-fused heterocycles, quinazolines and quinazolinones are an important class of compounds with large spectrum of therapeutic potentials 2-Phenylquinazolin-

4(3H)-one was synthesized via a two-step process including iron-catalyzed the

decarboxylation of phenylacetic acid, and subsequent oxidative cyclization with aminobenzamide [90]

2-Scheme 1.12 Synthesis of 2-phenylquinazolin-4(3H)-one via a one-pot, two-step

process [90]

The first step was catalyzed by VNU-21, which was constructed form FeCl2 and

mixed-linkers of 1,3,5-benzenetricarboxylic acid and 4,4’-ethynylenedibenzoic acid valence iron species of Fe2+and Fe3+, resulted in VNU-21 framework, were both active

Mixed-for oxidative Csp3H bond activation of phenylacetic acid to produce benzaldehyde After that, the catalyst was removed by filtration, and followed by adding a solution of 2- aminobenzamide to the reactor It was interesting that the transformation starting with phenylacetic acid under these reaction conditions gave no trace amount of benzoic acid while a large amount of benzoic acid as a by-product was formed in a catalyst-free cyclization of benzaldehyde with 2-aminobenzamide Probable reaction mechanism was proposed via radical pathway

Scheme 1.13 Probable reaction mechanism [90]

2-Phenylquinazolin-4(3H)-one was also synthesized via the reaction of benzyl alcohol with o-aminobenzamide, which was catalyzed by Fe(BTC) [91]

Scheme 1.14 Tandem process for the conversion of

benzyl alcohol to 2-phenylquinazolin-4(3H)-ones [91]

The target product was obtained in 82% isolated yield when TBHP was employed as an oxidant The decrease of product yield was observed in the case of using H2O2 as oxidant It was attributed to the instability of H2O2 compared with that of TBHP under these catalytic reaction conditions

As can be seen in the proposed mechanism, there were three pathways to obtain the benzaldehyde product The pathway (1) occurred via the carbocation formation The pathway (2) included the formation of a gem-diol-like structure which was then dehydrated to form benzaldehyde The other pathway passed through a hydrogen-atom

abstraction from (C) by the t-BuOO· or t-BuO· radicals After that, activated aldehyde reacted with o-aminobenzamide to give 2-phenylquinazolin-4(3H)-ones

Scheme 1.15 Plausible reaction mechanism [91]

A bimetallic (Zr, Fe) MOF denoted as Fe@PCN-222(Fe) was prepared by modification of PCN-222(Fe) with FeCl3 The synthesized Fe@PCN-222(Fe) was used as an efficient catalyst for a one-pot reaction between benzyl alcohol and 2-aminobenzamide through a tandem oxidation/cyclization/oxidation under visible light The target product, 2-

phenylquinazolin-4(3H)-one, was obtained in 79% isolated yield when the reaction was

carried out in the presence of a mixed solvent of DMSO/water for 32 hours under oxygen atmosphere [92]

Scheme 1.16 Reaction of benzyl alcohol and 2-aminobenzamide [92]

The catalytic activity of the bimetallic MOF was superior as compared to that of homogeneous catalysts (FeTCPPCl, iron and zirconium salts) and related heterogeneous catalysts (PCN-222(Fe), UiO-66, Fe(BTC)) These results were attributed to the presence of the hexagonal mesochannels in MOF framework The mesoporosity enabled the reaction to occur in the interior of Fe@PCN-222(Fe) pores [92]

Scheme 1.17 Plausible reaction mechanism [92]

In general, copper and iron-based MOFs have been used as catalysts in some limited protocols for the synthesis of benzo-fused heterocycles Benzimidazoles were synthesized by oxidative dehydrogenation of amidines catalyzed by MOF-537

Quinazolinone derivatives were synthesized by the reaction between

o-aminobenzamides and carboxylic acids/aldehydes/alcohols catalyzed by some MOFs with Lewis acid sites such as Fe@PCN-222(Fe), VNU-21, Fe-BTC

Fe-As could be seen that the synthesis of quinazoline, benzoxazine, benzazole derivatives catalyzed by copper, iron-based MOFs was quite limited Therefore, different synthetic approaches need to be further developed to obtain these valuable skeletons

1.4 Carbonheteroatom bond forming reactions for the synthesis of aryl ethers

CO bond forming reactions have shown significant attraction after CC and CN bond forming reactions Among the compounds containing CO bonds, aryl ethers are key constituents in the structure of many biologically important compounds [93, 94] About one fifth of the top 200 pharmaceuticals contain the key structure of alkyl aryl ethers [71] For examples, some methyl aryl ethers are found in drugs using for vasospasm, dyspepsia, anticancer and eczema treatment

Common methodologies to access these CO bonds involved the use of reactions under transition-metal catalysis such as Ullmann coupling or the Chan–Evans–Lam coupling [95-97] However, the utilization of pre-functionalized starting materials such as aryl halides or aryl boronic acids limited their applications Further, these couplings usually required harsh reaction conditions [98-100]

Transition-metal catalyzed C−H etherification reactions have recently been recognized as the most direct way and have attracted considerable attention However, developing a procedure for this reaction has been quite challenging One successful version of this

reaction includes the utilization of directing group to promote the O-Csp2 bond formation at a specific position In early stage, most studies of direct etherification of arenes bearing directing groups have been performed using palladium or ruthenium as catalysts in the presence of a strong oxidant such as K2S2O8 or PhI(OAc)2 [101-104]

Scheme 1.18 Palladium-catalyzed alkoxylation of N-methoxybenzamides [103]

Despite the efficiency of the precious metal salts, copper-catalyzed dehydrogenative alkoxylation of arenes has been developed (CuOH)2CO3 was first used as a copper-

based catalyst for the direct alkoxylation of Csp2-H bonds by Daugulis in 2013 [105] In addition to cost, the main advantage of using copper-based catalysts is that they tend to show their best in promoting reactions of substrates that contain coordinating functional groups Some active intermediates, such as organocopper (I) [106] and organocopper (III) [106-109], have been found in CH activation reactions

Scheme 1.19 Copper-catalyzed phenoxylation of N-(quinoline-8-yl)benzamide

derivatives [105]

The reaction was promising due to the utilization of either copper-based catalyst or air as an oxidant However, the yield of target product was rather low in the case of benzamide grafted with the electron-donating groups such as methoxy or methyl As a result, new efficient copper or copper/ligands systems have been discovered to create the CO bonds in mild conditions, but there still remain some challenges in catalyst reutilization and product separation after reaction

Along with the development of MOFs, MOF-catalyzed reactions for the synthesis of these aryl ethers have been shown significant progress in recent years MOF-199, which consisted of cupric nitrate trihydrate and benzene tricarboxylic acid, was used as an effective catalyst for coupling reaction between aryl iodides and phenol to form diaryl ethers MOF-199, also known as Cu3(BTC)2, is a rigid MOF with a zeolite-like structure