Common methodologies to access these CO 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].
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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 CH 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 CO 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
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containing exposed Cu (II) coordination sites [40]. The Ullmann type coupling of 4- iodoacetophenone and phenol was carried out in DMF, using NaOMe as base, in the presence of acetylacetone as a ligand [110].
Scheme 1.20. Ullmann coupling of phenol derivatives and aryl halides catalyzed by MOF-199 [110].
The conversion was found to be approximately 63% for the reaction of 4-chlorophenol, while it was only 8% for the coupling of 4-cyanophenol. It should be noted that the replacement of MeONa with Cs2CO3 could speed the reaction rate of 4-chlorophenol, 4-cyanophenol up to 99% and 60%, respectively. In the other hand, MOF-199 showed superior catalytic activity as compared to that of homogeneous catalysts such as Cu(NO3)2, Cu(Oac)2, CuCl2 and CuI. Interestingly, MOF-199 even showed higher catalytic performance than CuI, which was a homogeneous catalyst often used in various Ullmann type transformations. In addition to the outstanding activity, MOF-199 could be recycled at least five times under the reaction conditions. The P-XRD results showed that the crystallinity of the reused catalyst was retained. TEM micrograph also indicated that a porous material was still maintained, while BET measurement represented a slightly decrease in specific surface area as compared to that of fresh catalyst.
Diaryl ethers were synthesized by coupling reactions of nitroarenes with substituted phenols catalyzed by Cu2(BDC)2(DABCO) without the need of additional ligand [111].
Scheme 1.21. Coupling reaction of nitroarenes and substituted phenols catalyzed by Cu2(BDC)2(DABCO) [111].
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The Cu2(BDC)2(DABCO) was prepared from a mixture of 1,4-benzenedicarboxylic acid (H2BDC), 1,4-diazabicyclo[2.2.2]octane (DABCO), and Cu(NO3)2ã3H2O in the presence of DMF. Cu2(BDC)2(DABCO) is a copper-based MOF that connected through benzene fragment to produce a cubic closed pack structure. The coupling reaction between 4-nitrobenzaldehyde and phenol was carried out at a reactant molar ratio of 1:1.5 in the presence of K2CO3 and 3 mol% of Cu2(BDC)2(DABCO). The coupling reaction afforded more than 99% conversion. Moreover, this coupling reaction was readily carried out in the presence of a remarkable lower amount of Cu than the conventional Ullmann reactions. Indeed, several traditional Ullmann transformations usually required a copper concentration up to 10 mol% or more [100, 112-116]. The study was extended to the coupling reactions of 4-nitrobenzaldehyde with various substituted phenols, including 4-methoxyphenol, 4-methylphenol, 4-chlorophenol, and 4-cyanophenol. It was found that phenols containing electron-donating groups enhanced the reaction rate while the electron-withdrawing groups gave the lower conversion.
However, the conversion still reached up to 99% and 54% in the case of using 4- chlorophenol, 4-cyanophenol, respectively. It was interesting that the Ullmann coupling reactions between nitroarene and other electron-deficient phenols were failed in previous research [93, 114, 117]. These results showed the superior catalytic activity of Cu2(BDC)2(DABCO) in promoting the reaction of nitroarenes with electron-deficient phenols. Moreover, the catalyst was reused at least six times with negligible decrease in reaction conversion.
Cu2(BPDC)2(BPY) was prepared by a typical solvothermal method between cupric nitrate trihydrate and mixture of H2BPDC (4,4’-biphenylcarboxylic acid), BPY (4,4’
bipyridine). The catalyst synthesis was carried out at 120oC for 24 hours in a mixed solvent of DMF, pyridine and methanol. The H2-TPR experiment showed that the nature of copper species within Cu2(BPDC)2(BPY) framework was Cu2+ and Cu+ ions due to two broad reduction peaks at 340 oC and 410 oC. Cu2(BPDC)2(BPY) is a type of pillared- grid frameworks in which the BPDCs act as grid-forming ligands and BPYs act as pillars. Hence, the reactants are allowed to diffuse favorably to reach the active sites in this framework.
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Figure 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].
In one of these examples, Cu2(BPDC)2(BPY) was used as a catalyst for the cross- dehydrogenative coupling of ethers with 2-carbonyl-substituted phenols [119].
Complete conversion of 2-hydroxybenzaldehyde was observed when coupling reaction of 2-hydroxybenzaldehyde with 1,4-dioxane was carried out at a molar ratio of 1: 50 in the presence of 3 mol% of Cu2(BPDC)2(BPY) and 3 equivalents of TBHP.
[
Scheme 1.22. Reaction of 2-hydroxybenzaldehydes and dioxane catalyzed by Cu2(BPDC)2(BPY) [120].
To highlight the catalytic activity, various copper-based MOFs, including Cu3(BTC)2, Cu(BDC), and Cu(BPDC), were used under the identical coupling conditions. These MOFs showed lower catalytic activities than Cu2(BPDC)2(BPY). It was interesting that Cu(BPDC)-catalyzed coupling reaction only afforded 44% conversion in the absence of BPY whereas BPY itself was mostly inactive toward this coupling reaction. The combination of Cu(BPDC) and BPY was used as a catalyst system for the coupling reaction. It was found that the conversion was still lower as compared to that of using Cu2(BPDC)2(BPY) as catalyst. In addition, Cu2(BPDC)2(BPY) was found to show better catalytic activity compared to that of some homogeneous catalysts such as Cu(OAc)2, Cu(NO3)2. The superior catalytic performance was ascribed to the structural framework of Cu2(BPDC)2(BPY).
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Another cross dehydrogenative coupling between 2-hydroxybenzaldehyde and 1,4- dioxane in the presence of TBHP as oxidant was reported by Bharadwaj and coworkers.
Reaction was catalyzed by [Cu6(L)3(H2O)6].(14DMF).9(H2O). This robust MOF was synthesized from a bent amino-functionalized tetra-carboxylate ligand (H4L) and cupric nitrate trihydrate under solvothermal condition [121].
Figure 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].
Reaction mixture was stirred at 100oC for one hour. The product was obtained in 96%
isolated yield. The author assumed that the mechanism underwent radical pathway.
Initially, unsaturated copper metal sites coordinated with 2-hydroxybenzaldehyde to form a complex as well as interacted with TBHP to form tert-butoxyl radicals and hydroxyl radicals. Tert-butoxyl radicals abstracted a hydrogen atom adjacent to oxygen atom of dioxane to form dioxane radical species while hydroxyl radicals abstracted a hydrogen atom from chelate complex to from water. A desired product was formed when dioxane radicals collided with chelate complex radicals.
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Scheme 1.23. Plausible reaction mechanism of dioxane and 2-hydroxybenzaldehyde [121].
In general, Cu-MOFs have been used as catalysts for the synthesis of aryl ethers. Those transformations were often carried out between pre-functionalized reactants (halobenzenes, nitrobenzenes, boronic acids) and reactants bearing -OH group. These methods did not afford the requirement of atom-efficiency and sustainable development of green chemistry due to by-product and multi-step reaction problems. Hence, the cross dehydrogenative coupling has emerged as a profound method to form new CO bonds.
However, copper-based MOF-catalyzed cross dehydrogenative coupling reactions of aromatic CH bonds still have not been seen.