Recent Progress in the Chemistry of Pyridazinones for Functional

JOCSynopsis Cite This: J. Org. Chem. 2018, 83, 1−11

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Recent Progress in the Chemistry of Pyridazinones for Functional Group Transformations Seohyun Kang,† Hyun Kyung Moon,† Yong-Jin Yoon,*,‡ and Hyo Jae Yoon*,† †

Department of Chemistry, Korea University, Seoul 02841, South Korea Department of Chemistry, Gyeongsang National University, Jinju 52828, South Korea

‡

ABSTRACT: While N-hetereocycles have received significant attention in organic synthesis and other research fields, the chemistry of pyridazine, a sixmembered aromatic ring with two adjacent nitrogen atoms, and its derivatives has been relatively little understood. This Synopsis describes recent progress made in the synthesis of pyridazine derivativesparticularly, pyridazin-3(2H)onesand their utility as efficient and recyclable functional group carriers for various important organic reactions.

1. INTRODUCTION Since the first synthesis by Fischer in 1886,1 the chemistry of pyridazinea six-membered aromatic ring with two adjacent nitrogen atomswas rarely investigated until Townsend,2−6 Castle,7−9 and others10,11 reported the synthetic routes to some pyridazine derivatives and their potential use in agricultural and medical chemicals in 1960s and 1970s. This Synopsis describes recent progress made for the synthesis, structural modification, and application of pyridazine derivatives, particularly 2-substituted pyridazin-3(2H)-ones. A prominent challenge in organic chemistry is to develop novel, environmentally acceptable and sustainable reagents and synthetic methodologies,12,13 and this challenge is at least in part resolved by the chemistry of 2-substituted pyridazin-3(2H)-ones. Pyridazin-3(2H)-ones could be electrophilic agents, especially halogen (X+) and cyano cations (NC+) equivalents, and readily form highly stable anions. These features make them good leaving groups and a novel synthetic auxiliary. Indeed, 2-substituted pyridazin-3(2H)-ones are used as recyclable functional group carriers (FGCs) for a variety of functional group transformations including acylation, nitration, cyanation, halogenation, carbonylation, benzenesulfonylation, and lactonization with various carbon, nitrogen, sulfur, and oxygen nucleophiles in chemo- and/or regioselective manners. Bistrycki et al. first synthesized 4,5-dichloropyridazin-3(2H)one.14,15 Mono- and disubstituted pyridazin-3(2H)-ones are useful starting compounds for the synthesis of multisubstituted pyridazin-3(2H)-ones and fused heterocycles involving pyridazin-3(2H)-one. In addition, 2-substituted pyridazin-3(2H)ones such as 2-acyl, 2-cyano, 2-halo, 2-nitro, and 2-arylsulfonyl derivatives are ecofriendly useful electrophilic reagents. Few reviews16−23 have been reported as chemical, biological, and pharmacological aspects; Lee et al.24 and Sung et al.25 also reviewed the chemistry of pyridazin-3(2H)-ones. © 2017 American Chemical Society

2. TAUTOMERISM AND THE RESONANCE OF PYRIDAZIN-3(2H)-ONES As shown in Scheme 1a, pyridazin-3(2H)-ones have three tautomers in water: pyridazin-3-ol (A), pyridazin-3(2H)-one (B), and pyridazin-1-ium 3-olate (C). These are stable cyclic amides, and the structure B is the main tautomer in water.26 Their tautomerism in the gas phase (Scheme 1b) takes place via two mechanisms, direct (mechanism A) and indirect (mechanism B) hydrogen transfers.27 Among two mechanisms, mechanism B via the eight-membered transition state by a double tautomerization of two molecules is more favorable than mechanism A. Mechanism B shows a low activation energy (14.66 kcal/mol).27 The tautomerization depends on the solvent’s dielectric constant and the electronic structure (e.g., the electron-donating vs electron-withdrawing groups) of substituents on the pyidazin3(2H)-one. Since pyridazin-3(2H)-one readily forms the stable anion, stabilized by the resonance, it could act as a good leaving group.24−34 Pyridazin-3(2H)-ones are also the ambient anions under basic conditions.35,36 The regioselectivity of their N- and O-alkylations depends on the nature of the base, the structure of alkyl halide, the substituents on the heterocycle, and the reaction conditions such as temperature and solvent.37,38 Pyridazin-3(2H)-ones can be easily removed and isolated for reuse during synthetic processes. Scheme 2a shows the general resonance structures of 2-substituted pyridazin-3(2H)-ones. The nitrogen of 2-position in the resonance structure I is positively charged, making the α-position atom of the Z moiety electron-deficient. Thus, 2-substituted pyridazin-3(2H)-ones behave as an electrophilic reagent. Received: September 30, 2017 Published: December 5, 2017 1

DOI: 10.1021/acs.joc.7b02481 J. Org. Chem. 2018, 83, 1−11

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The Journal of Organic Chemistry

acidity of NH than pyridazin-3(2H)-one. The regioselectivity of 4,5-dichloropyridazin-3(2H)-one at the C-4, C-5, and C-6 positions depends upon the substituent of N-2 position.50 Thus, 2-substituted 4,5-dichloropyridazin-3(2H)-ones could be used as a sustainable FGC for organic synthesis. One might think why not use pyridazine itself, rather than pyridazin-3(2H)-one, for functional group transformation. The N-2 position of pyridazine is not easily activated, whereas pyridazin-3(2H)-one can undergo tautomerization (as shown in Scheme 1a) and the N-2 is readily attacked by nucleophile, thereby leading to efficient functional group transfer. Furthermore, pyridazin-3(2H)-one shows good solubility over conventional organic solvents and is easily recovered by simple extraction with basic aqueous solution and/or recrystallization, as compared to pyridazine. Pyridazin-3(2H)-ones are usually recovered in >∼95% for most of reactions described in this Synopsis. Despite many attempts, we were not able to find the potential utility of pyridazine as functional group carrier. This remains challenging in the field of pyridazine chemistry.

Scheme 1. (a) Three Tautomers of Pyridazin-3(2H)-one in Water. (b) Tautomerization Mechanisms of Pyridazin3(2H)-one in the Gas Phase

3. PREPARATION OF 2-SUBSTITUTED 4,5-DICHLOROPYRIDAZIN-3(2H)-ONES A limited number of literature reports describe synthesis and functionalization of pyridazinones. Asif et al.51 reported the overview of the chemistry and synthesis of pyridazinones. Zare et al.52 demonstrated one-pot synthesis of pyridazinones from arenes, cyclic anhydrides and ArNHNH2 using HY-zeolite (a type of acidic zeolite). Mantovani et al.53 reported copper(I)catalyzed multicomponent cyclization reaction combining aldehyde, hydrazine and alkynylester. Recently, Parveen et al.54 showed rhodium-catalyzed alkylation on N-2 of pyridazin3(2H)-one with terminal allenes. We focused particularly on pyridazinones as electrophilic agents for functional group transformation and thus developed synthetic routes to various 2-substituted pyridazin-3(2H)-ones 3-10 (Scheme 3).28,31,51−56 2-Substituted 4,5-dihalopyridazin3(2H)-ones 3−10 were prepared from commercially available 4,5-dihalopyridazin-3(2H)-ones 1, easily prepared by the reaction of mucochloric and mucobromic acid with hydrazine sulfate (Scheme 4a).55,56 Regioselective functionalization on C-5 of compound 1 is achieved in different reaction conditions to form compounds 2 (Scheme 4b).57 2-Acyl-4,5-dichloropyridazin-3(2H)-ones 3 are prepared by the reaction of 4,5-dichloropyridazin-3(2H)-one with acyl chlorides in the presence of Et3N in methylene chloride in excellent yields (method A in Scheme 3).58 Lee et al.28 reported the preparation of 2-alkoxy (or aryloxy) carbonyl-4,5-dichloropyridazin-3(2H)-ones 4, which are a type of efficient carbonyl source, from 4,5-dichloropyridazin3(2H)-one and the corresponding chloroformates (method B in Scheme 3). 4,5-Dichloro-2-(p-nitrobenzenesulfonyl)pyridazin3(2H)-one (5) could be prepared by the reaction of 4,5-dichloropyridazin-3(2H)-one with p-nitrobenzenesulfonyl chloride in moderate to excellent yields (method C in Scheme 3).59 2-Nitro-4,5-disubstituted pyridazin-3(2H)-ones 6 were successfully synthesized through the reaction of the corresponding pyridazin-3(2H)-ones with Cu(NO3)2·3H2O and acetic anhydride at room temperature in good to excellent yields (method D in Scheme 3).60 Kim et al.61 reported for the first time the preparation of 2-cyano-4,5-disubstituted pyridazin-3(2H)-ones 7 and utilized them for chemoselective electrophilic cyanation. 2-Cyanopyridazin-3(2H)-ones were prepared by treating the corresponding 4,5-disubstituted pyridazin-3(2H)-ones with cyanogen bromide and triethylamine in THF at room temperature

Scheme 2. (a) Resonance Structures of 2-Substituted Pyridazin-3(2H)-ones. (b) Resonance Structures of 2-Acylpyridazin-3(2H)-ones

For example, 2-acylpyridazin-3(2H)-one as N-acylazinone has three resonance structures (Scheme 2b), and among them, the resonance structure I possesses the most electron-deficient carbonyl moiety at the N-2 position. Therefore, 2-acylpyridazin3(2H)-ones could be utilized as an excellent acylating agent in the presence of appropriate nucleophiles. Although 4,5-dihalopyridazin-3(2H)-one is weak acid, its direct functionalization can be achieved in a suitable solvent. Regioselective functionalization of 4,5-dichloropyridazin-3(2H)-one is directly achieved in polar solvents such as H2O, THF, MeOH, and mixed solvents of these in excellent yields.39 On the other hand, 2-hydroxy,40−43 2-oxopropyl, 1,1-dibromo-2-oxopropyl,44,45 1-bromo-2-oxopropyl,46 and 2-acetyloxymethyl-4,5-dichloropyridazin-3(2H)-one47,48 undergo easily the retro-ene reaction by heating and/or in the presence of base. The value of pKa is a great guide to the leaving group ability. The leaving group ability of pyridazin-3(2H)-one anion (pyridazin-3(2H)-one: pKa = 10.4626) is lower than that of p-toluenesulfonate (p-toluenesulfonic acid: pKa = −2.8).49 When it is halogenated, the leaving group ability significantly increases: 4,5-dichloropyridzin-3(2H)one shows higher stability of the corresponding anion and higher 2


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and metal halides such as ZnCl2, ZnBr2, FeCl3, CuCl, CuCl2, AlCl3 and SnCl4 in refluxing CH2Cl2 or CH3CN (Method G in Scheme 3). Interestingly, this reaction makes it possible to convert nucleophilic halides to electrophilic halides. Won et al.31 showed that (6-oxo-6H-pyridazin-1-yl)phosphoric acid diethyl esters 10 are an efficient coupling agent of carboxylic acid. The reaction of 4,5-disubstituted-pyridazin-3(2H)-ones with diethyl chlorophosphate in the presence of triethylamine in acetonitrile at room temperature afforded the corresponding (6-oxo-6Hpyridazin-1-yl)phosphoric acid diethyl esters 10 in excellent yields (method H in Scheme 3).

Scheme 3. Preparation of 2-Substituted Pyridazin-3(2H)ones

4. PYRIDAZIN-3(2H)-ONES AS RECYCLABLE FUNCTIONAL GROUP CARRIERS 4.1. 2-Acylpyridazin-3(2H)-ones as Acylating Agents. Several studies have established the utility of 2-acylpyridazin3(2H)-ones as efficient and atom-economical acylating agents. Kang et al.58 reported the chemoselective N-acylation of aliphatic and aromatic amines with 3 under neutral condition to afford the corresponding amides in good or excellent yields (Scheme 5a). The acyl-group carrier, 4,5-dichloropyridazin3(2H)-one (1a), is easily isolated from the reaction by recrystallization or extraction using basic aqueous solution. According to the theoretical study,32 the kinetics of such an aminolysis Scheme 5. Synthesis of (a) Amides and (b) 1,3,4Oxadiazoles with 2-Acyl-4,5-dichloropyridazin-3(2H)-ones Scheme 4. Synthetic Routes to Compounds (a) 1 and (b) 2

Scheme 6. (a) Esterification of Alcohols with Compound 3. (b) Synthesis of O-Substituted Hydroxamic Acids through the Reaction of O-Substituted Hydroxylamines with Compounds 3. (c) Synthesis of Symmetric Anhydride from 3

(method E in Scheme 3). The isolated yields of the reactions were good to excellent even in the presence of halide, phenolic, azide, and heteroatom substituents. Park et al.62 reported the synthetic route to 2,4-dichloro-5-substituted pyridazin-3(2H)ones 8 (X = Cl) as the electrophilic halogenation agents. 2,4,5Trichloro- and 2,4-dichloro-5-methoxypyridazin-3(2H)-ones 8 were straightforwardly prepared in 95−96% yields via the reaction of 4,5-dichloro- or 4-chloro-5-methoxypyridazin-3(2H)-ones with NaOCl in 50% acetic acid at room temperature (method F in Scheme 3). Kim et al.63 demonstrated the preparation of 2-bromo- and 2-chloropyridazin-3(2H)-ones 8 and 9 by the reaction of 4,5-dichloropyridazin-3(2H)-one with Pb(OAc)4 3


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The Journal of Organic Chemistry Scheme 7. Synthesis of 2-Substituted Benzo[d]thiazoles and Benzo[d]oxazoles Using 3

Scheme 10. One-Pot Synthesis of Benzo[d]azol-2(3H)-ones through Acylation Using Compound 4 and Intramolecular Cyclization

Scheme 8. (a) Esterification of Alcohols with 3 under Friedel−Crafts Conditions. (b) Comparison of Reaction Using 3 with Conventional Friedel−Crafts Reaction

Scheme 11. Acylation Reactions of Carboxylic Acids with Compound 4 To Achieve Coupling Reactions with Acids, Amines, Alcohols, and Thiols

Scheme 9. Synthesis of Carbamates, Symmetric, and Asymmetric Ureas with 4 (Ar = Ph)

Scheme 12. (a) Reactions of 4,5-Dichloro-2benzenesulfonylpyridazin-3(2H)-ones (5) with Amines. (b) Chemoselective and Efficient N-Benzenesulfonylation of Aliphatic Amines Using the Compound 5 Where X is Para-Substituted NO2

is influenced by the electronic structure of the substituents of amines and the acyl moiety in the compound 3: the reaction rate is faster with increasing the nucleophilicity of amines and making the acyl moiety more electron-deficient as found typically in nucleophilic substitution reactions. The aminolysis reaction could be extended to hydrazine. Reaction of 2-acyl(or aroyl)-4,5-dichloropyridazin-3(2H)-ones 3 or 4 with hydrazine hydrate in the presence of boron trifluoride diethyl etherate (BF3·OEt) or polyphosphoric acid (PPA) yields the corresponding symmetric and asymmetric 1,3,4-oxadiazoles (Scheme 5b).64 The substrate scope of 2-acylpyridazinones for acylation is not limited to amines; this reaction could be exploited for acylation of alcohols. Kim et al.33,65 described catalyst-free esterification of alcohols with 2-acyl-4,5-dichloropyrdazin-3(2H)ones 3 under microwave or reflux conditions. Reaction of aliphatic and aromatic alcohols with 3 gives the corresponding esters in good to excellent yields (Scheme 6a). The entire

process is atom-economic, cheap, and rapid. The compound 3 is also relevant to amidation. The reaction of 3 with O-substituted hydroxylamine hydrochlorides in refluxing water successfully 4


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than 2-alkyl analogues, which was attributed to the different reactivity of intermediate amide’s carbonyl carbon in the cyclization step between them. Benzamide tautomers favor the enol configuration whereas the equilibrium for the acetamide tautomers lies toward the keto configuration. The reaction of 2-aminophenol with 3 in refluxing toluene gives the corresponding 2-substituted benz[d]oxazoles in 73−91% yields (Scheme 7). For the cyclization step, this reaction required dehydrating agent and a stoichiometric amount of POCl3. Compound 3 enabled the esterification of alcohols under Friedel−Crafts conditions (Scheme 8a).69 Typical reaction of aromatic alcohol with acyl halide under Friedel−Crafts condition affords a mixture of C-acylated products (Friedel−Crafts products) as major products and a trace amount of esterified product (Scheme 8b). However, according the synthetic method using compound 3, the aromatic alcohol could be esterified without C-acylation of benzene ring under Friedel−Crafts conditions (Scheme 8b). Interestingly, the limitations of acylation for the deactivated and amino group containing phenols under Friedel− Crafts conditions could be overcome by the reported method. 4.2. 2-Phenoxycarbonyl-4,5-dichloropyridazin-3-(2H)one as Phenoxycarbonyl Source. Lee et al.28 reported a study on an effective and selective carbonylation of amines using 2-phenoxycarbonyl-4,5-dichloropyridazin-3(2H)-one (4, Ar = Ph). The reaction of various aliphatic and aromatic amines with 4 (Ar = Ph) at room temperature affords the corresponding carbamates in 90−98% isolated yields. Furthermore, this work revealed the synthesis of symmetric and asymmetric ureas through the reaction of 4 (Ar = Ph) with various amines under catalyst-free conditions for asymmetric ureas or in the presence of organic catalyst (DMAP) for symmetric ureas (Scheme 9). Just as the other reactions above, the acyl carrier, 4,5-dichloropyridazin-3(2H)-one, is quantitatively isolated for reuse. Facile synthesis of benzo[d]azol-2(3H)-ones using compound 4 (Ar = Ph) as a green CO source has been recently reported.70 This reported synthetic methodology relies on a onepot reaction consisting of two sequential steps (Scheme 10): generation of carbamate intermediate through acylation reaction with compound 4, and intramolecular cyclization yielding a byproduct of alcohol derivative. Indeed, the reaction of 2-aminophenol, 1,2-phenylenediamine or 2-mercaptoaniline with 4 (Ar = Ph) enables the synthesis of the corresponding benzo[d]azol2(3H)-ones in 49−97% yields in one pot. The analogous reaction using 4 for the synthesis of carbonates has been further achieved.71 The reaction of alcohol derivatives with 4 under Friedel-Craft (i.e., in the presence of AlCl3) and ambient conditions affords symmetric and asymmetric carbonates in good to excellent yields. The universality of acylation using compound 4 as N-heterocycle-based acyl transfer agent has been further demonstrated. Moon et al.72 showed that reactions of carboxylic acids with compound 4 afforded mixed carbonic anhydride intermediates (Scheme 11), which was confirmed by in situ analysis of 13C NMR spectroscopy.73 The formed anhydrides are highly reactive and can be coupled with a wide range of substrates such as acids, amines, alcohols, and thiols. The wide substrate scope, ease of operation (no additive or catalyst), storage and handling stability, and atom-efficiency from recycling the pyridazinone carrier make the compound 4 attractive for acylation-based coupling reactions. 4.3. 2-Benzenesulfonyl-4,5-dichloropyridazin-3(2H)one as N-Arylsulfonylating Agent and Coupling Agents. N-Benzenesulfonylation of 1° and 2° amines with 2-benzenesulfonyl-4,5-dichloropyridazin-3(2H)-ones 5 has been reported.59

Scheme 13. (a) Synthesis of Symmetric Anhydride with 5 (X = p-NO2). (b) Esterification of Carboxylic Acids with 5 (X = 4-NO2). (c) Plausible Mechanism for the Reaction

Scheme 14. N-Nitration of Secondary Amines with 4-Chloro-5-methoxy-2-nitropyridazin-3(2H)-one (6, X = Cl, Y = OMe)

affords the corresponding O-substituted hydroxamic acids in 82−99% yields (Scheme 6b).66 The reaction of O-substituted hydroxylamines with 3 in the presence of Et3N or Amberlite IRA-67 in acetonitrile at room temperature also affords the corresponding O-substituted hydroxamide in excellent yields.67 For developing the synthetic methodology of amidation, these methods have advantages such as easy workup, use of water solvent, high yield, and/or the quantitative recovery of Amberlite resin and the acyl group carrier, pyridazinone. Park et al.68 demonstrated the ZnCl2-mediated synthesis of carboxylic anhydrides using 2-acyl-4,5-dichloropyridazin-3(2H)-ones. The treatment of 3 with ZnCl2 (0.5 equiv) in air in refluxing THF or acetonitrile affords the corresponding symmetric anhydrides in good to excellent yields (Scheme 6c). This homocoupling reaction only proceeds in aerobic condition and/or in the presence of trace amount of water. This has been proved by control experiment: the analogous reaction in dry acetonitrile and under an argon atmosphere does not give the desired product. Ecofriendly and atom-economical synthesis of 2-substituted benzo[d]thiazole and 2-substituted benzo[d]oxazoles using 3 has been reported.34 The reaction of 2-aminothiophenols with 3 under catalyst-free and neutral conditions affords the corresponding 2-substituted benzo[d]thiazoles in 50−94% yields (Scheme 7). 2-Arylbenzo[d]thiazoles exhibited lower yields 5


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Scheme 15. (a) Oxidative Conversion of Oximes with 2-Nitro-4,5-dichloropyridazin-3(2H)-one, 6 (X, Y = Cl). (b) Plausible Mechanism for the Conversion of Oxime to Carbonyl Facilitated by 6

Scheme 17. α-Halogenation of Active Methylenes and Methines with 2-Chloropyridazin-3(2H)-ones 8

Scheme 16. (a) Reaction of 7 (X, Y = Cl) with Some Nucleophiles. (b) Electrophilic Cyanation over Various Nucleophiles under Optimized Conditions. (c) 1,3-Diketones with 2-Cyanopyridazin-3(2H)-ones 7

Compounds 5 have the sulfonyl-transfer potential over the nucleophiles such as 1° and 2° amines, carboxylates, and alkoxides. The reaction of 2-benzenesulfonyl-4,5-dichloropyridazin-3(2H)-ones 5 with aliphatic amines under neutral condition affords 5-alkylamino-2-benzenesulfonyl-4-chloropyridazin-3(2H)-ones and/or the corresponding benzene sulfonamides (Scheme 12a). The product distribution of this reaction depends on the structure and the basicity of amines and the electronic structure and the position of substituent in benzene ring. Following this work, Kim et al.74 demonstrated the chemoselective and high-yielding N-benzenesulfonylation of aliphatic amines occurs upon use of 2-(4-nitrobenzenesulfonyl)-4,5-dichloropyridazin-3(2H)-one (5 where X = 4-NO2) (Scheme 12b). N-Benzenesulfonylation of 12 aliphatic amines with this compound 5 in refluxing 6


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A mixture of carboxylic acid (2 equiv), base (2 equiv) such as Et3N, K2CO3, and DMAP, and the acylating agent 5 (X = p-NO2) (1 equiv) in anhydrous THF or CH2Cl2 is refluxed until the acylating agent disappears to afford the corresponding symmetric anhydride in 70−98% yields. When alcohols are reacted with carboxylic acids in the presence of 5 (X = p-NO2) under basic conditions, efficient esterification of acids is achieved in excellent yields (Scheme 13b).76 This reaction shows excellent selectivity for primary or secondary alcohols in a mixture with secondary or tertiary alcohols. Plausible mechanisms for the reactions involving 5 (X = p-NO2) relies on two key intermediates (Scheme 13c): sulfonylcarboxylate (RCO2SO2R) and anhydride. 4.4. 2-Nitro-4,5-disubstituted Pyridazin-3(2H)-ones as Nitration Agents. Recently, Park et al.60 revealed the ability of pyridazinone to carry efficiently a nitro moiety for N-nitration. The authors surveyed 2-nitropyridazin-3(2H)-one derivatives containing various N-substituents such as chlorine, bromine, alkoxy, aryloxy, and azide and found 4-chloro-5-methoxy-2nitropyridazin-3(2H)-one (6, X = Cl, Y = OMe) as the optimal structure for N-nitration. According to this work, 5-alkoxy-4chloro-2-nitropyridazin(2H)-ones show excellent NO2 transfer potentiality for secondary amines (Scheme 14). In this reaction, the pyridazin-3(2H)-one derivative acts as a stable, mild, and efficient nitronium (NO2+) source functioning under mild, neutral, and homogeneous conditions. The work by Kim et al.30 found the other utility of 4,5-dichloro-2-nitropyridazin-3(2H)-one (6, X, Y = Cl). The compound 6 (X, Y = Cl) facilitates the conversion of oximes to carbonyl compounds; the conversion of 14 aliphatic and aromatic oximes has been demonstrated by the reaction in the presence of 6 (X, Y = Cl) under microwave irradiated conditions (Scheme 15a). A plausible mechanism for the cleavage of carbon− nitrogen double bond in oxime by compound 6 was proposed. As shown in Scheme 15b, the nitro group in compound 6 is transferred into oxime, and subsequent nucleophilic addition of water molecule leads to a hydrolysis process to afford carbonyl compound. The overoxidation byproduct, the corresponding carboxylic acid in the case of aldoxime did not form under this condition. 4.5. 2-Cyano-4,5-disubstituted Pyridazin-3(2H)-ones as the Electrophilic Cyanating Agent. Lee et al.50 investigated the regiochemistry for the reaction of 4,5-dichloro-2cyanopyridazin-3(2H)-one (7, X, Y = Cl) with some nucleophiles such as thiol, alcohol, amine, and azide. This reaction yields the corresponding cyanide and/or 4-chloro-2-cyano-5substituted pyridazin-3(2H)-one (Scheme 16a). According to this study, the regiochemistry in the reaction depends on the nucleophile and/or the solvent polarity. By optimizing reaction conditions and the structure of pyridazinone 7, chemoselective, electrophilic cyanation can be achieved. Kim et al.61 showed that the cyanation of various nitrogen and sulfur nucleophiles with 7 (X = Cl, Y = Cl or OMe) occurs in water under neutral, metal-free conditions to afford the corresponding N- or S-cyano derivatives in 78−98% yields (Scheme 16b). The cyanation of bifunctional nucleophiles such as 4-aminobenzenethiol and 4-aminophenol is chemoselective for nitrogen and sulfur. The reaction of 2-acetyl-3,4-dihydronaphthalene-1(2H)-one with 7 (X = Cl, Y = Cl or OMe) in THF mediated by ZnCl2 or NaH gives rise to deacetylated α-cyano ketone in excellent yields, and the reaction of 1-phenylbutane-1,3-dione with 7 (X = Cl, Y = Cl or OMe) under the same conditions affords the corresponding α,α-dicyano derivative in excellent yield (Scheme 16c). 4.6. 2-Halopyridazin-3(2H)-ones as the Electrophilic Halogenation Agents. Pyridazinone derivatives act as efficient halogenation agents for various substrates. Park et al.62 reported

Scheme 18. (a) Electrophilic Halogenations of 2,2′-Bithiophene Using Compounds 8 or 9. (b) Halogenation of 2,2′,5′,2″-Terthiophene with 8 and 9

Scheme 19. (a) Esterification of Carboxylic Acids with Alcohols with (6-Oxo-6H-pyridazin-1-yl)phosphoric Acid Diethyl Ester. Synthesis of (b) Amides and (c) Dipeptides with (4,5-Dichloro-6-oxo-6H-pyridazin-1-yl)phosphoric Acid Diethyl Ester 10. (d) Plausible Mechanism of Reaction Using 10a

a

A key intermediate is acyl phosphate.

cyclohexane gives the corresponding sulfonamides in 80−98% yields. In this reaction, the N-sulfonylation of less hindered primary amine shows high yields than the secondary amine and the hindered amine. Facile synthesis of carboxylic anhydrides using 5 (X = 4-NO2) has been reported by Kim et al. (Scheme 13a).75 7


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Scheme 20. Synthesis of Lactones with (4,5-Dichloro-6-oxo-6H-pyridazin-1-yl)phosphoric Acid Diethyl Ester and the Yield of Products

a

The yield depends on reaction conditions (base and solvent); all reactions were done at r.t.

the α-chlorination of active methylenes and methines using 2-chloropyridazin-3(2H)-ones 8. α-Chlorination of active methylene and methine compounds with 8 in the presence of either Lewis acid such as AlCl3, ZnCl2, FeCl3, CuCl2, or protonic acid such as p-toluenesulfonic acid, H3PO4, HCl, and H2SO4 in dichloromethane for Lewis acid or in water for protonic acid and none at room temperature affords selectively α-monochlorides and/or α,α-dichlorides in good to excellent yields (Scheme 17). The great stability and sustainability of pyridazinone-based functional group carrier and mild and ecofriendly reaction conditions for the chlorination make the compound 8 attractive as an electrophilic chlorinating agent. Common methods for halogenating bithiophenes include coupling reaction of halothiophene using transition-metalcontaining catalysts.77−81 For bromination, reactions based on N-bromosuccinimide, quaternary ammonium polyhalides, and thionyl chloride have been previously reported.82−85 These previous methods except for N-bromosuccinimide often suffer poor regioselectivity, need additives, and/or show low reaction yields. For chlorination of bithiophene using N-chlorosuccinimide, the undesirable products and low yields are often observed. Jung et al.86 demonstrated the utility of pyridazinone derivative for selective and efficient halogenations over 2,2′bithiophene (Scheme 18a). The halogenations of 2,2′-bithiophene and halogenated 2,2′-bithiophenes with 2-halo-4,5-dichloropyridazin-3(2H)-ones 8 and 9 in the presence of zinc halide give selectively the corresponding dihalo-, trihalo, and tetrahalo-2,2′bithiophenes involving the same or different halogens in excellent yields, respectively. This methodology allows one to control the degree of halogen substitution through a straightforward control of stoichiometry of the halogenation agents 8 or 9. For example, monosubstituted product is formed in the reaction containing 1 equiv of halogenation agent, while the analogous reaction of 2 equiv of the agent yields disubstituted product. More interestingly, this methodology makes it possible to introduce different halogen atoms on different positions with good regioselectivity in high yields. Such a regioselectivity is explained by Wheland intermediates.87 The delocalized cationic

character in the intermediate makes the electrophilic attack on 2-position more favorable than that on 3-position. Recently, the performance of compounds 8 and 9 was further investigated for multihalogenation. Reactions of 2,2′,5′,2″-terthiophene with 8 or 9 under ambient conditions afforded mono-, di-, tri-, tetra-, penta-, or hexahalogenated terthiophenes in moderate to excellent yields (Scheme 18b).88 This study confirms the good regio- and chemoselectivities of compounds 8 and 9 for halogenation of thiophene oligomers. All of the halogenation reactions occur in ambient conditions. Therefore, the methodology enables consecutive halogenations under environmentally friendly conditions and is useful for electron-rich aromatics. 4.7. (6-Oxo-6H-pyridazin-1-yl)phosphoric Acid Diethyl Ester as a Coupling Agent of Carboxylic Acid. Won et al.31 demonstrated the esterification of aliphatic and aromatic carboxylic acids with alcohols using (6-oxo-6H-pyridazin-1yl)phosphoric acid diethyl ester (10) as a novel coupling agent to give the corresponding esters in good to excellent yields (Scheme 19a). In the case of bifunctional alcohol, p-aminophenol, and 2-mercaptoethanol, the chemoselectivity of NH2 and SH groups is higher than that of the OH group under these conditions. This is selective for primary alcohol in the reaction of 1°/2° alcohol mixture or 1°/3° alcohols mixture, for phenol in the reaction of cyclohexanol/phenol mixture. Because the esterification occurs via acyl phosphate as a key intermediate, this is more effective than the method using 4,5-dichloro-2-[4nitrobenzenesulfonyl]-pyridazin-3(2H)-one (5) as a coupling agent. Kang et al.89 demonstrated the amidation of carboxylic acids with amines using phosphoric acid diethyl ester 10. The amidation of aliphatic and aromatic carboxylic acids with amines using 10 in the presence of potassium carbonate in THF at room temperature affords chemoselectively the corresponding amides in good to excellent yields (Scheme 19b). The authors further employed the method to form a peptide bond: as a proof-of-concept three protected dipeptides were prepared from N-BOC-Ph and O-Me-amino acid hydrochlorides using 10 and triethylamine in THF at room temperature (Scheme 19c). Note that this methodology enables an efficient 8


JOCSynopsis


for the reactions is acyl phosphate resulting from the reaction of acid with 10 (Scheme 19d). The lactonization of α-hydroxycarboxylic acids using 10 has been reported.90 The lactonization of α-hydroxycarboxylic acids with 10 in the presence of equimolar amounts of K2CO3 or DMAP affords the corresponding monoolides, diolides, triolides, and/or tetraolides (Scheme 20). This lactonization is applicable to lactones of various ring sizes ranging from 6-membered to 26-membered rings.

Table 1. Summary of Some Previous Examples of Functional Group Transfer Carriers and Reactions That Are Relevant to Those Described in This Synopsis

5. CONCLUSION While N-heterocycles have played a central role in not only organic and organometallic synthesis, and pharmaceuticals but also materials chemistry and nanoscience in the last decades, pyridazine derivatives were not fully explored in the context of synthesis, functionalization, and application. In this Synopsis, we have described the studies done over the last two decades about synthetic methodologies to prepare and modify pyridazinone derivatives and how understanding of the chemical and electronic structures of newly developed pyridazinones provided insight that enabled a wide range of functional group transformations in an efficient, convenient, and atom-economical manner. For comparison, Table 1 summarizes some of literature examples for FGCs and reactions63,91−111 relevant to those described in this Synopsis. (Note that due to a large number of papers we could not cite all the previous work associated with acylation.) Although the types of reactions discussed in this Synopsis have been widely investigated and used for various applications, they often depend on limited FGC resources that are not recyclable for achieving atom-economical reaction, require catalyst and/or harsh reaction conditions, show the narrow substrate scope, and generate environmentally harmful byproduct. Despite the advances shown here, we believe pyridazinone (and pyridazine) chemistries are still in early stages of developments, and the huge potential remains unexploited in many fields. Further study will open a new era of scientific developments in pyridazine chemistry, in particular to functional group carriers for novel reactions and applications in organic synthesis, and to new core moieties for functional materials in various research fields.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yong-Jin Yoon: 0000-0002-6061-0682 Hyo Jae Yoon: 0000-0002-2501-0251 Notes

The authors declare no competing financial interest. Biographies

Seohyun Kang received B.Sc. in chemistry from the Catholic University of Korea, South Korea, in 2016. She joined the Chemistry Department at Korea University in 2016 to pursue a doctoral degree. Her work currently focuses on the development of functional polymers by exploiting heterocyclic chemistry.

and selective coupling for primary or aliphatic amines in the presence of secondary or aromatic amines. The key intermediate 9


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(5) Mourad, A. E.; Wise, D. S.; Townsend, L. B. J. Heterocycl. Chem. 1993, 30, 1365. (6) Katz, D. J.; Wise, D. S.; Townsend, L. B. J. Org. Chem. 1983, 48, 3765. (7) Castle, R. N.; Kaji, K. J. Heterocycl. Chem. 1965, 2, 463. (8) Castle, R. N. Pyridazines; Wiley: New York, 1973 (9) Castle, R. N. Condensed Pyridazines Including Cinnolines and Phthalazines; John Wiley & Sons: New York, 1973. (10) Stanovnik, B.; Tišler, M. Tetrahedron Lett. 1966, 7, 2403. (11) Kaji, K.; Nagashima, H.; Hirose, Y.; Oda, H. Chem. Pharm. Bull. 1985, 33, 982. (12) Wender, P. A.; Miller, B. L. Organic Synthesis: Theory and Applications; JAI: Greenwich, 1993; Vol. 2. (13) Gaich, T.; Baran, P. S. J. Org. Chem. 2010, 75, 4657. (14) Bistrzycki, A.; Simonis, H. Ber. Dtsch. Chem. Ges. 1899, 32, 534. (15) Bistrzycki, A.; Herbst, C. Ber. Dtsch. Chem. Ges. 1901, 34, 1010. (16) Dury, K. Angew. Chem., Int. Ed. Engl. 1965, 4, 292. (17) Coates, W. J. Comprehensive Heterocyclic Chemistry, 1st ed.; Pergamon: Oxford, 1996; Vol. 6. (18) Mátyus, P.; Czakó, K. Trends Heterocycl. Chem. 1993, 3, 249. (19) Dal Piaz, V.; Ciciani, G.; Giovannoni, M. P. Acta Chim. Slov. 1994, 41, 189. (20) Heinisch, G.; Frank, H. Progress in Medicinal Chemsitry; Elsevier Science Publishers: Amsterdam, 1990; Vol. 27. (21) Heinisch, G.; Frank, H. Progress in Medicinal Chemsitry; Elsevier Science Publishers: Amsterdam, 1992; Vol. 29. (22) Kappe, T. J. Heterocycl. Chem. 1998, 35, 1111. (23) Bansal, R.; Thota, S. Med. Chem. Res. 2013, 22, 2539. (24) Lee, S. G.; Kim, J. J.; Kim, H. K.; Kweon, D. H.; Kang, Y. J.; Cho, S. D.; Kim, S. K.; Yoon, Y. J. Curr. Org. Chem. 2004, 8, 1463. (25) Sung, G. H.; Kim, B. R.; Lee, S. G.; Kim, J. J.; Yoon, Y. J. Curr. Org. Chem. 2012, 16, 852. (26) Katritzky, A. R.; Pozharskii, A. F. Handbook of Heterocyclic Chemistry, 2 ed.; Pergamon: New York, 2000. (27) Emamian, S. R.; Domingo, L. R.; Tayyari, S. F. J. Mol. Graphics Modell. 2014, 49, 47. (28) Lee, H. G.; Kim, M. J.; Park, S. E.; Kim, J. J.; Kim, B. R.; Lee, S. G.; Yoon, Y. J. Synlett 2009, 2009, 2809. (29) Kim, S. K.; Kweon, D. H.; Cho, S. D.; Kang, Y. J.; Park, K. H.; Lee, S. G.; Yoon, Y. J. J. Heterocycl. Chem. 2005, 42, 353. (30) Kim, B. R.; Lee, H. G.; Kim, E. J.; Lee, S. G.; Yoon, Y. J. J. Org. Chem. 2010, 75, 484. (31) Won, J. E.; Kim, H. K.; Kim, J. J.; Yim, H. S.; Kim, M. J.; Kanga, S. B.; Chunga, H. A.; Lee, S. G.; Yoon, Y. J. Tetrahedron 2007, 63, 12720. (32) Hwang, J.; Hwang, Y.; Yang, K.; Yoon, Y. J.; Koo, I. S. Bull. Korean Chem. Soc. 2009, 30, 2779. (33) Kim, B. R.; Sung, G. H.; Lee, S. G.; Yoon, Y. J. Tetrahedron 2013, 69, 3234. (34) Sung, G. H.; Lee, I. H.; Kim, B. R.; Shin, D. S.; Kim, J. J.; Lee, S. G.; Yoon, Y. J. Tetrahedron 2013, 69, 3530. (35) Kornblum, N.; Smiley, R. A.; Blackwood, R. K.; Iffland, D. C. J. Am. Chem. Soc. 1955, 77, 6269. (36) Kim, S. K.; Cho, S. D.; Kweon, D. H.; Yoon, Y. J.; Kim, J. H.; Heo, J. N. J. Heterocycl. Chem. 1997, 34, 209. (37) Comins, D. L.; Jianhua, G. Tetrahedron Lett. 1994, 35, 2819. (38) Chung, N. M.; Tieckelmann, H. J. Org. Chem. 1970, 35, 2517. (39) Chung, H. A.; Kweon, D. H.; Kang, Y. J.; Park, J. W.; Yoon, Y. J. J. Heterocycl. Chem. 1999, 36, 905. (40) Kim, S. K.; Cho, S. D.; Kweon, D. H.; Lee, S. G.; Chung, J. W.; Shin, S. C.; Yoon, Y. J. J. Heterocycl. Chem. 1996, 33, 245. (41) Kim, S. K.; Cho, S. D.; Moon, J. K.; Yoon, Y. J. J. Heterocycl. Chem. 1996, 33, 615. (42) Kim, S. K.; Cho, S. D.; Yoon, Y. J. J. Heterocycl. Chem. 1997, 34, 1135. (43) Chung, H. A.; Kang, Y. J.; Chung, J. W.; Cho, S. D.; Yoon, Y. J. J. Heterocycl. Chem. 1999, 36, 277. (44) Choi, S. Y.; Shin, S. C.; Yoon, Y. J. J. Heterocycl. Chem. 1991, 28, 385.

Hyun Kyung Moon received a B.Sc. in chemistry from the Chungnam National University, South Korea, in 2015. She recently completed her master degree in the Chemistry Department at Korea University. Her thesis work focused on functional group transformations utilizing N-heterocyclic compounds for small molecules and polymeric materials.

Hyo Jae Yoon received his Ph.D. from Northwestern University in 2010 where he studied functional supramolecules for developing allosteric enzyme mimics. After postdoctoral study at Harvard University, he joined the Department of Chemistry at Korea University as assistant professor in 2014. His research is focused on synthesis of organic and organometallic molecules for applications in surface engineering, electronics, and energy harvest and conversion.

Yong-Jin Yoon obtained his Ph.D. in 1983 from Sungkyunkwan (SKKU) University and accepted a position at Gyeongsang National University as an Organic Chemistry Instructor. He studied with Prof. L. B. Townsend in the Department of Medicinal Chemistry at the University of Michigan in 1984. His research has been focused on exploring the chemistry of pyridazine, synthesis of heterocycles, synthetic methodology, and development of novel materials involving drugs, agrochemicals, and solar cell dyes.

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ACKNOWLEDGMENTS H.J.Y. acknowledges support from the Future Research Grant at Korea University, and National Research Foundation of Korea (NRF-2017M3A7B8064518). Y.-J.Y. sincerely thanks all his former co-workers involved in the projects related to pyridazine chemistry and Prof. L. B. Townsend, Emeritus Professor of Medicinal Chemistry at the University of Michigan, who motivated him to dedicate his entire carrier to the exploration of pyridazine chemistry.

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Recent Progress in the Chemistry of Pyridazinones for Functional

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