Palladium-Catalyzed Cross-Coupling Reactions: A Powerful Tool for

Jul 31, 2018 - Pd-catalyzed cross-coupling reactions have become essential tools for the construction of carbon–carbon and carbon–heteroatom bonds...
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Palladium-Catalyzed Cross-Coupling Reactions: A Powerful Tool for the Synthesis of Agrochemicals Ponnam Devendar,†,‡ Ren-Yu Qu,†,‡ Wei-Ming Kang,†,‡ Bo He,†,‡ and Guang-Fu Yang*,†,‡,# †

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Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University (CCNU), Wuhan 430079, P. R. China ‡ International Joint Research Center for Intelligent Biosensor Technology and Health, Central China Normal University (CCNU), Wuhan 430079, P. R. China # Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, P. R. China ABSTRACT: Pd-catalyzed cross-coupling reactions have become essential tools for the construction of carbon−carbon and carbon−heteroatom bonds. Over the last three decades, great efforts have been made with cross-coupling chemistry in the discovery, development, and commercialization of innovative new pharmaceuticals and agrochemicals (mainly herbicides, fungicides, and insecticides). In view of the growing interest in both modern crop protection and cross-coupling chemistry, this review gives a comprehensive overview of the successful applications of various Pd-catalyzed cross-coupling methodologies, which have been implemented as key steps in the synthesis of agrochemicals (on R&D and pilot-plant scales) such as the Heck, Suzuki, Sonogashira, Stille, and Negishi reactions, as well as decarboxylative, carbonylative, α-arylative, and carbon−nitrogen bond bond-forming cross-coupling reactions. Some perspectives and challenges for these catalytic coupling processes in the discovery of agrochemicals are briefly discussed in the final section. The examples chosen demonstrate that cross-coupling chemistry approaches open-up new, low-cost, and more efficient industrial routes to existing agrochemicals, and such methods also have the capability to lead the new generation of pesticides with novel modes of action for sustainable crop protection. KEYWORDS: agrochemical, herbicide, fungicide, cross-coupling, transition metal, palladium



INTRODUCTION The development and launch of new crop protection agents with novel modes of action remain important tasks for the research-based agrochemical industry to support global agriculture in its major goal to provide food security and to remain affordable for growers and farmers.1 Although sizable productivity improvements over the past 50 years have enabled an abundant food supply in many parts of the world, feeding the global population has reemerged as a critical issue. If current trends continue, by 2050, caloric demand will increase by 70%, and crop demand for human consumption and animal feed will increase by at least 100%.2,3 Additionally, the future of the global agrochemicals market (herbicides, fungicides, insecticides, etc.) faces major challenges: food driven by population growth and changes in diets, rising caloric consumption per person, increasing environmental stresses across the globe, demand for better quality food, diets richer in protein, a fast changing regulatory landscape, urbanization, freshwater supplies, the growth of pest resistance to existing active ingredients and traits, increasing R&D costs, shifting pest spectra, changing agricultural needs and practices, competition for intellectual property, etc.3−5 Therefore, the development of environmentally benign, short, and efficient synthetic methods over traditional methods and the discovery of new agrochemicals continue to be the central goal of current agrochemical research.6 The substitution of alkyl, aryl, and vinyl halides or pseudohalides by heteroatomic nucleophiles catalyzed by a transition-metal (TM) complex that lead to the formation of C−C and C−heteroatom bonds is broadly referred to as a © 2018 American Chemical Society

cross-coupling reaction if it follows the mechanistic course of oxidative addition, transmetalation, and reductive elimination.7 The nucleophilic cross-coupling partners are organometallic reagents,8−10 carboxylic acids,11,12 and C−H bonds (carboxylic acids and C−H bonds can be the source of the nucleophile under certain reaction conditions).13−15 The electrophilic cross-coupling partners are not only halides or pseudohalides16−20 but also diazonium salts21−24 and inert ethers or alcohols,25,26 which have been extensively used in these reactions. A wide range of ligands, bases, and additives can be utilized in cross-couplings. Generally, phosphines (e.g., trialkyl or triaryl phosphines: [P(t-Bu)3, PCy3, PPh3, and di(1adamantyl)-n-butylphosphine]),27−29 dialkylbiaryl phosphines (e.g., chelating bisphosphine ligands: [BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl)30,31 and dppf (1,1′-bis(diphenylphosphino)ferrocene)], 32,33 Hartwig’s QPhos [pentaphenyl(ditert-butylphosphino)ferrocene],34,35 Buchwald’s XPhos (2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl),36,37 secondary phosphine oxides,38 2,2-diphenylvinyl phosphines,39 etc., are widely used as ligands in cross-coupling reactions. The use of N-heterocyclic carbene (NHC, a cyclic carbene bearing at least one α-amino substituent) variants as alternative ligands in cross-coupling reactions has rapidly been gaining in popularity, widening the substrate scope and resulting in milder reaction conditions.8,40−44 The nature of Received: Revised: Accepted: Published: 8914

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2018 2018 2018 2018 DOI: 10.1021/acs.jafc.8b03792 J. Agric. Food Chem. 2018, 66, 8914−8934

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Journal of Agricultural and Food Chemistry

Figure 1. Heck−Matsuda cross-coupling reaction in the preparation of prosulfuron 4.

catalyzed cross-coupling reactions were highlighted by the Nobel Prize in chemistry in 2010, which was awarded jointly to Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki.18,68,117 Pdcatalyzed cross-coupling reactions are now the standard in the synthesis and derivatization of scaffolds. There are several examples of the successful application of cross-coupling chemistry in the large-scale synthesis of agrochemicals. Thus, in this review, we focus only on selected applications of the most commonly applied Pd-catalyzed crosscoupling reactions as they pertain to the synthesis of agrochemicals (plant and laboratory scale), namely, the Heck, Suzuki, Sonogashira, Negishi, and Stille cross-coupling reactions, as well as carbonylative, decarboxylative α-arylative, and C−N bond-forming cross-couplings. The synthetic routes presented herein have been taken from publications and patents; however, the exact manufacturing route for an agrochemical is often not fully disclosed.

the ligand is essential, as it has a significant impact on reactivity. The steric and electronic properties of the ligand can accelerate both the oxidative addition and reductive elimination steps in most of the cross-coupling reactions. The initial advancement in the field of cross-coupling was Fritz Ullman’s copper-catalyzed biaryl synthesis from aryl halides, reported in 1901.45 The discovery of modern TMcatalyzed cross-coupling reactions started in the 1970s, specifically, with the discovery of the Kumada coupling,46,47 followed by the Heck,48−50 Sonogashira,51,52 Negishi,53,54 Stille,55,56 Suzuki,57−59 Hiyama,60−62 Buchwald,63 and other cross-coupling reactions.9,64,65 The reactivities, scopes, limitations, and practical applications of transition metals, such as palladium,18,66−70 nickel,19,71−74 iron,9,75−78 copper,20,79−81 cobalt,82−85 ruthenium,86−89 rhodium,90−94 iridium,95−97 etc., in various types of cross-couplings have been documented. Having been established by incredible advances and applications, as well as a wide-ranging scope and compatibility, TM-catalyzed cross-couplings are an important and growing area of research due to the low cost, low toxicity, and exceptional synthetic versatility of the TMs. Compared with the widespread application of late and noble transition metals in this field, first-row transition metals, such as Fe, Co, Cu, and Ni, have become substantially more attractive due to their clear advantages, including high abundance on Earth, low price, little or no toxicity, and unique catalytic characteristics.98−100 A number of outstanding catalytic systems have been established in recent years, and these are applicable to the stereo-, regio-, chemo-, and enantioselective conversions of all classes of organic compounds. Provided the inhibition and deactivation of the TM catalyst is avoided, catalytic cross-couplings may offer several advantages such as total cost reduction, high reaction rate, high yield, high atom economy, simplified workup procedures with the potential for facile precious metal recovery, asymmetric catalysis, greater efficiency over linear and convergent synthetic strategies, expansive tolerance for functional groups, low waste, and low reaction temperatures.101 This approach has allowed us to produce a variety of compounds involving C−C and C−heteroatom bond constructions for academic and industrial purposes,66,102 such as in the syntheses of natural products,70,103 heterocycles, pharmaceuticals, agrochemicals,11,104,105 liquid crystalline conjugated polymers,106−108 the active components of organic lightemitting diodes (OLEDs),109,110 etc. Particularly, Pd-catalyzed coupling reactions represent some of the most powerful and versatile tools available to synthetic organic chemists and appear to be among the most popular reactions for the production of fine chemicals at the ton-scale.104,111 The recent advances in Pd-catalyzed C−C and C−heteroatom bond formations including α-arylation, direct arylation by C−H activation, and decarboxylative couplings have been extensively reviewed.67,69,112−116 The great success and significance of Pd-



HECK CROSS-COUPLING REACTIONS The arylation or vinylation of alkenes was independently discovered by Mizoroki and Heck in the early 1970s49,50 and is widely used in organic and organometallic syntheses for carbon−carbon bond formation. The Heck reaction (also called the Mizoroki−Heck reaction) is the Pd-catalyzed coupling reaction of an alkenyl or aryl halide with an alkene in the presence of a base to form a substituted alkene.118 In recent years, there have been numerous studies reporting on both the practical importance and scientific significance of this reaction.48,119,120 The approach has been applied to a huge variety of substrates and has been used in the syntheses of natural products and biologically active compounds in both small-scale and industrial applications.121 Heck reactions performed with arenediazonium salts are frequently referred to as Heck−Matsuda reactions.122 The Heck−Matsuda reaction involves the use of aryl diazonium salts as the electrophile, providing an alternative to traditional aryl halides or triflates. Some advantages include no requirement of phosphine ligands, anaerobic conditions, optional use of a base, and faster rates than the traditional Heck reaction.22,123 Indeed, the Heck reaction has become a particularly attractive method in the synthesis of crop-protecting chemicals, such as the synthesis of herbicides prosulfuron, 4, carfentrazone-ethyl, 11, and pyraflufen-ethyl, 15, as well as the fungicide fenpropimorph, 21. Prosulfuron 4. The sulfonylurea herbicide prosulfuron, 4 (Peak, launched in 1994, Figure 1) is used for the broadspectrum control of annual broad-leaved weeds in maize, and it acts by inhibiting plant amino acid synthesis-acetohydroxyacid synthase (AHAS). The first large-scale application of the Heck reaction was for the preparation of prosulfuron 4 by Ciba− Geigy (now Syngenta Crop Protection) via a Heck−Matsuda coupling as the key step (Figure 1).124,125 The process involves 8915

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Figure 2. Heck approaches toward carfentrazone-ethyl 11.

Figure 3. Synthesis of pyraflufen-ethyl 15 via the Heck cross-coupling reaction.

Carfentrazone-ethyl 11. The triazolinone class herbicide carfentrazone-ethyl 11 (marketed as QuickSilver by the FMC Corporation in 1997, Figure 2) targets the protoporphyrinogen oxidase enzyme (commonly abbreviated as protox or PPO), providing excellent postemergence broadleaf and sedge weed control in cereal and corn.127,128 Compound 11 is a chiral compound (administered as a racemate), and the buildingblock allylic alcohol ester is used to introduce the stereogenic center. Various synthetic methods have been reported for the synthesis of carfentrazone-ethyl 11,128−132 among them are two synthetic methods that include the Heck reaction as a key step. Early in 1999, Crispino et al. developed a synthetic method (Figure 2) that involved the Heck reaction.131 The triazolinone 5 was reacted with iodine in the presence of oleum to give the intermediate 6. The intermediate 6 was coupled with ethyl 2-(1-hydroxyethyl)acrylate 7 under the conditions of the Heck reaction and in the presence of a catalyst bis(benzonitrile)palladium(II) chloride [PdCl2(PhCN)2] and a tertiary amine (Bu3N), resulting in the ketone intermediate 8. Finally, carfentrazone-ethyl 11 was obtained by the substitution-elimination of ketone 8. Later, in 2015, Fan and coworkers reported an improved synthetic method (Figure 2) in which the inexpensive reagent ethyl acrylate 9 was employed to replace the commercially unavailable ethyl 2-(1-hydroxyethyl)acrylate 7.132 In this synthetic route, the intermediate 10 was synthesized by the Heck coupling of aryl iodide 6 with ethyl acrylate 9. The catalyst Pd(OAc)2 (1−2 mol %) gave the higher yield (85−88%) of product 10 over the catalysts

the reaction between o-diazoniumsulfonate 1 and 3,3,3trifluoropropene 2 using the catalyst Pd2(dba)3 [0.5−1.5 mol %, generated in situ from trans-dibenzylidenceacetone (dba) and palladium(II) chloride in a stainless steel vessel at 60 °C] and sodium acetate in n-pentanol at 70−75 °C. Charcoal was added after the arylation reaction to in situ produce the heterogeneous hydrogenation catalyst of Pd on charcoal that was able to catalyze the hydrogenation of the C−C double bond. Hence, Pd was used in two consecutive reactions and recovered by filtration to give the key intermediate 3 of prosulfuron 4. The process produces 2 kg of waste/kg of product for the sequential three steps (diazotization, alkenylation, and hydrogenation) in a one-pot sequence with an overall yield of 93%. This result corresponds to an impressive E-factor (i.e., environmental impact factor) of 2 and a turnover number (TON) of 100−200, which represents the absolute number of passes through the catalytic cycle before the catalyst becomes deactivated. The cost and recovery of the Pd catalyst, the linking of homogeneous and heterogeneous catalysis, the use of a one-pot procedure for three consecutive steps, the absence of phosphine ligands, solvent compatibility, and a benign nature are a few advantages of this method. Furthermore, the application of a diazonium salt instead of an arylhalide reduces the amount of salts generated, allowing for a particularly green reaction. This work, one of the few examples of a Pd-catalyzed coupling reaction working on an industrial scale, highlights the power of diazonium chemistry for process chemists.22,119,126 8916

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Figure 4. One-pot synthesis of the fungicide fenpropimorph 21 using Heck coupling in an ionic liquid.

Figure 5. Large-scale synthesis of boscalid 26 via Suzuki cross-coupling.

organic solvent is 51%. The ionic liquid 18 is useful to immobilize the Pd catalyst in an ionic phase of the biphasic reaction system.144−146

Pd(PPh3)4 and (PPh3)2PdCl2. Furthermore, the base Et3N is favored more than K2CO3 for the reaction, and N,Ndimethylformamide (DMF) displayed a better solvent effect than acetonitrile (MeCN) and DMF/H2O. Finally, a one-pot oxidative addition−elimination, followed by the reduction of compound 10, gave the desired target 11. The latter method exhibits some advantages over the former method including mild conditions, atom economy, low cost, and efficiency; however, these methods are not yet commercialized because of low yields. Pyraflufen-ethyl 15. The phenylprazole herbicide pyraflufen-ethyl 15 (Ecopart, Figure 3) was first developed by Nihon Nohyaku Co., Ltd. It is a potent protox-inhibiting cereal herbicide that controls a wide range of annual broad-leaved weeds including Galium apayine at a rate of 6−12 g a.i./ha in postemergence applications.133−135 It was introduced into the market in 1999 and is now being utilized as the active ingredient of a cereal herbicide. Recently, its synthesis has been reported via the Heck coupling (Figure 3).136 In this synthesis, the carbamate 12 reacted with 2-chloro-N-methylacetamide 13, followed by the Heck cross-coupling with ethyl acrylate 9 using palladium(II) acetate. This reaction produced the key intermediate 14 of pyraflufen-ethyl 15 in good yield. Fenpropimorph 21. The morpholine fungicide fenpropimorph 21 (Corbel and Volley, launched in 1983, Figure 4) is widely used as a sterol biosynthesis inhibitor to effectively control the powdery mildew diseases of cereals.137−139 Industrially, fenpropimorph 21 is produced as a racemic mixture with the cis-isomer, and several patents have been filed regarding the multistep synthesis.140−143 Recently, Forsyth et al. reported a one-pot route for the preparation of fenpropimorph 21 via Heck coupling, followed by a reductive amination reaction in an ionic liquid (Figure 4).139,144 The Heck coupling of 4-tert-butyl-iodobenzene 16 with methacryl alcohol 17 proceeded efficiently in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide 18 as the solvent and PdCl2 (0.05 equiv of metal) as the catalyst. This step was followed by one-pot reductive amination with cis-2,6dimethylmorpholine 20 to give fenpropimorph 21 in 81% yield (atom economy = 46% and E factor = 1.26); however, the yield without a solvent is only 14%, and the yield using an



SUZUKI CROSS-COUPLING REACTIONS The Suzuki coupling (also referred to as Suzuki−Miyaura reaction) is a TM-catalyzed reaction (typically Pd) between an (hetero)aryl, alkenyl, or alkynyl organoborane and organic halide or pseudohalide (e.g., triflate) under basic conditions. Over the past decades, it has become one of the most efficient methods for the construction of substituted biphenyls, poly olefins, and styrenes. This coupling reaction offers several additional advantages, such as being largely unaffected by the presence of water, tolerating a broad range of functional groups, mild conditions, the commercial availability and stability (to heat, oxygen and water) of boronic acids, the nontoxic inorganic boron-containing byproduct of the reaction that is easily removed from the reaction mixture, general regioand stereoselectivity, etc.147 Moreover, boronic acids are less toxic and safer for the environment than either of their counterparts, organostannane and organozinc compounds. Thus, the reaction is suitable for not only academic laboratories but also industrial processes. The scope of the Suzuki reaction for synthetic applications (particularly, the syntheses of natural products, biologically active pharmaceuticals, fine chemicals, and polymers for electronic applications) have been surveyed in several excellent reviews.57,103,147−149 Over the past decade, our group has engaged in pesticide discovery, and we have recently reported the synthesis of bioactive heterocycles, i.e., bulky 4-substituted isatins,150 functionalized 6-arylsalicylates,151 6-substituted-thiosalicylates,152 4-substituted pyrazoles,153 N-(pyridin-2-ylmethyl) substituent biphenyl-4-sulfonamides,154 and N-quinolyl biaryl carboxamides,155 via microwave-assisted Suzuki cross-couplings. The Suzuki coupling is widely applied in the synthesis of agrochemicals, with the most prominent examples being the synthesis of the fungicides boscalid 26, bixafen 33, fluxapyroxad 36, kresoxim-methyl 41, and pyriofenone 47; the insecticide bifenazate 52, the herbicide halauxifen-methyl 56, and the pheromone bombykol 59. 8917

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Figure 6. Large-scale synthetic route of bixafen 33 via Suzuki cross-coupling.

Figure 7. Suzuki biaryl coupling in the manufacture of fluxapyroxad 36.

Figure 8. Synthesis of kresoxim-methyl 41 via Suzuki cross-coupling.

procedure is the first example of the transfer of a Pd-catalyzed coupling reaction to large-scale agrochemical synthesis (>1000 tons per year), and it is currently one of the largest-known industrial applications of the Suzuki−Miyaura reaction.170 Bixafen 33. The fungicide bixafen 33 (Aviator, Figure 6), a pyrazole carboxamide fungicide for corn, soybeans, cereals, canola, peanuts, and potatoes, was launched by BASF in 2011. It is developed specifically for foliar applications to control important cereal diseases such as strobilurin-resistant septoria leaf blotch (Septoria tritici).171−173 The mixture of bixafen 33 with prothioconazole is used as Aviator Xpro, which has a broad spectrum of activity in the cereal segment and controls all major leaf spots and brown rust. It blocks the fungal respiratory chain by inhibition of succinate dehydrogenase (SDHI).174,175 At Bayer CropScience, an elegant route via the Suzuki cross-coupling was worked out for the tetrasubstituted biphenyl derivative 31, which is a key intermediate in the synthesis of bixafen 33 (Figure 6).176 The key step involves the Suzuki coupling of 3,4-dichlorophenylboronic acid 29 with bromide 30 catalyzed by palladium(II) acetylacetonate [Pd-

Boscalid 26. The fungicide boscalid 26 (Cantus, Pristine, Figure 5) belongs to the carboxamide (nicotinamide) class of fungicides, was developed by BASF in 2003, and is the most significant member of the SDHI (succinate dehydrogenase inhibitor) group of fungicides.156−158 It is a systemic and broad-spectrum fungicide, being active against nearly all types of fungal diseases, and it has excellent control of gray mold and powdery mildew. 159,160 Numerous methods have been developed for the synthesis of boscalid 26 and its intermediates;161−167 however, the large-scale synthesis (three steps) involves the Suzuki cross-coupling (Figure 5).104,168−170 The Suzuki−Miyaura cross-coupling of 4chlorophenylboronic acid 22 with 1-chloro-2-nitrobenzene 23, facilitated by Pd(PPh3)4 (0.25−0.5 mol %) with the addition of tetrabutylammonium bromide (TBAB) as a phase transfer catalyst, afforded the nitrobiphenyl intermediate 24 in excellent yield. In the second step, the nitro group of the biphenyl intermediate 24 was selectively reduced to the corresponding aniline, followed by condensation with 2chloronicotinoyl chloride 25 to produce boscalid 26. This 8918

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Figure 9. Synthesis of pyriofenone 47 using Suzuki cross-coupling.

powdery mildew fungicides introduced by ISK (Ishihara Sangyo Kaisha, Ltd.) to control powdery mildews in cereals, grapes, and vegetables.188,189 The chemical structure (benzoylpyridine) of this product is developed by replacing the benzene ring in metrafenone (Vivando, BASF) with a pyridine ring and assuming that they share the same mode of action.190,191 An efficient synthesis of pyriofenone 47 (Figure 9) involves a Suzuki coupling in the final step, i.e., the coupling of chloro-intermediate 45 and trimethylboroxin 46, using the catalyst Pd(PPh)4 in 1,4-dioxane as a solvent under refluxing conditions, to give pyriofenone 47 in good yield.192 Bifenazate 52. The insecticide bifenazate 52 (Floramite, Figure 10), launched by Uniroyal Chemical Company, Inc.

(acac)2] with the additions of K2CO3 as a base and tritertbutylphosphine tetrafluoroborate ([(t-Bu)3PH]BF4) as a phase transfer catalyst, affording the key biaryl amide intermediate 31 in excellent yield. Bixafin 33 was given by the further basemediated deacetylation of 31, followed by amide bond formation with acid chloride 32. Fluxapyroxad 36. The fungicide fluxapyroxad 36 (Xemium, Figure 7), presented by BASF in 2012, is a broadspectrum pyrazole carboxamide fungicide (SDHI fungicide). It is efficient against all major cereal diseases including leaf spot (especially, that caused by Ascomycetes species) and rusts in wheat and barley and seed treatments; it has been announced to be a mixing partner for epoxiconazole and pyraclostrobin.173 Fluxapyroxad 36 is structurally related to boscalid 26; it contains the difluoromethyl-substituted pyrazole acid building block and the biphenyl parts. Several different methods for the construction of fluxapyroxad and its intermediates have been found in the literature,167,177 and its synthesis has also been reported via the Suzuki cross-coupling (Figure 7).178−181 The synthesis involves the Suzuki cross-coupling of aryl chloride 23 and boronic acid 34, using a Pd(PPh3)4 catalyst, followed by nitro group reduction; the product 35 is then coupled with an acid chloride 32 to give fluxapyroxad 36 in good yield. Kresoxim-methyl 41. The strobilurin fungicide kresoximmethyl 41 (Stroby, Figure 8), launched by BASF in 1996, belongs to the oximino-acetate chemical group as a QoIfungicide (quinone outside inhibitor). It is intended for use as an agricultural spray in the control of scab and other fungal infections on crops and fruits.182 The (E)-methyl-methoxyiminoacetate building block of kresoxim-methyl 41 is isosteric with the (E)-methyl-β-methoxyacrylate unit of azoxystrobin (Syngenta Crop Protection, 1996), and both of these are firstgeneration strobilurin fungicides.183 Various methods have been reported for the synthesis of kresoxim-methyl 41,184−186 and one of them incorporates the Suzuki cross-coupling (Figure 8).187 This synthesis involves the Suzuki crosscoupling of boronate 38 and bromo-imimonoacetate 40, in the presence of Pd(PPh3)4 as a catalyst and Na2CO3 as a base in toluene under reflexing conditions, to give kresoxim-methyl 41 in moderate yield. However, the method is not commercially viable because of low yields. Pyriofenone 47. The fungicide pyriofenone 47 (IKF-309, Property, provisionally approved in 2010, Figure 9) belongs to the aryl phenyl ketone chemical group, a new generation of

Figure 10. Synthesis of benfenazate 52 using Suzuki cross-coupling.

(now Chemtura Corporation) in 1999, belongs to the group of carbazate acaricides (hydrazine carboxylate) and is widely used as a nonsystemic acaricide against phytophagous mites (such as spider mites, pecan leaf mites, European red mites, and brown almond mites) infesting agricultural and ornamental crops.193 Although it was first thought to be a neurotoxin, genetic evidence has pointed to the action being that of a mitochondrial complex III electron transport inhibitor.194 Different synthetic approaches have been reported for the preparation of bifenazate 52;195−197 however, its synthesis can also be achieved through the Suzuki cross-coupling (Figure 10).198,199 Starting from economical 4-fluoro-3-nitroaniline 48, the aromatic nucleophilic substitution of the fluorine atom with sodium methoxide was followed by the diazotization of aniline to provide the diazonium tetrafluoroborate salt 49. The 8919

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Figure 11. Synthesis of halauxifen-methyl 56 via Suzuki cross-coupling.

Figure 12. Regioselective synthesis of bombykol 59 via Suzuki cross-coupling.

Figure 13. Synthesis of bixafen 33 via the Sonogashira reaction.

the biaryl system (Figure 11).188,203,204 Halauxifen-methyl 56 is prepared from the 2-chloro-6-fluoroanisole 53 by metalation and then borylation to yield the boronic acid 54. The Suzuki reaction of 54 with chloride 55, using the Pd(OAc)2/PPh3 catalytic system followed by a subsequent deprotection under acidic conditions, gave the title product 56. Bombykol 59. The pheromone bombykol 59 [(10E,12Z)10,12-hexadecadien-1-ol, Figure 12], released by the female silkworm moth (Bombyx mori) to attract mates, was the first pheromone to be characterized chemically by Butenandt et al. in 1959.205−207 Different synthetic approaches have been documented for bombykol 59;208−210 however, its regioselective synthesis can also be attained via the Suzuki crosscoupling (Figure 12).211,212 The Suzuki cross-coupling of alkenylboronate 57 with vinylic halide 58, in the presence of Pd(PPh3)4 (5 mol %) as a catalyst and NaOEt as a base in benzene, can give bombykol 59 in excellent yields.

salt 49 was reacted with phenylboronic acid 50, using Pd immobilized on BaCO3 as a catalyst, to give the cross-coupled product, which was reduced in situ to give aniline 51. The transformation of aniline 51 into the required hydrazine occurred via diazotation, then reduction with SnCl2, followed by carbamate formation with isopropyl chloroformate to afford bifenazate 52 in an overall 18% yield. Halauxifen-methyl 56. The first commercial 6-arylpicolinate herbicide halauxifen-methyl 56 (Arylex, Quelex, Figure 11), a new herbicide launched by Dow AgroSciences in 2014 for use in cereals and other crops, is a synthetic auxin of the picolinic acid class of herbicide.200,201 Halauxifen-methyl 56 provides a potent and broad-spectrum herbicidal activity with excellent safety to cereal crops, and it is active at significantly lower rates than classical auxin herbicides in the control of broadleaf weeds. Its corresponding carboxylic acid form exhibits a commercially acceptable soil half-life range of 10− 30 days; however, the specific mechanism of action is not known. The large-scale synthesis of halauxifen-methyl 56 has been reported via the Suzuki coupling.200 Two synthetic routes have been patented by Dow AgroSciences: one of these involves the de novo synthesis of the pyridine ring,202 while the other utilizes the Suzuki cross-coupling reaction to generate



SONOGASHIRA CROSS-COUPLING REACTIONS The cross-coupling of an aryl or vinyl halide or pseudohalide with terminal acetylene promoted by a palladium(0) or other transition metal catalyst, a Cu(I) cocatalyst, and an amine is 8920

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Figure 14. Stille cross-coupling in the manufacture of pinoxaden 75.

Figure 15. Synthesis of fluxapyroxad 36 via Negishi cross-coupling.

groups without requiring protection.223 Several reviews have been published describing the advancements and development of the Stille reaction,224,225 and the method has been used in organic synthesis, specifically, in the syntheses of a variety of polymers226,227 and natural products.228,229 The main disadvantage is the toxicity of the tin compounds used, while the low polarity of these reactants makes them poorly soluble in water. To date, the prominent example of the Stille crosscoupling is the synthesis of pinoxaden 75. Pinoxaden 75. The herbicide pinoxaden 75 (Axial, Figure 14), introduced by Syngenta Crop Protection in 2006, is a novel aryl-1,3-dione or phenylproazoline class (DEN) of acetyl CoA carboxylase inhibitor (ACCase; it acts on plant lipid synthesis) and provides postemergence activity against a broad spectrum of grass weed species in both wheat and barley.230−233 The ethyl side chains were successfully incorporated into pinoxaden 75 via the Stille coupling (Figure 14).234,235 The synthesis involves the dibromination of ptoluidine 65, the Meerwein arylation with vinylidene dichloride 67, and a one-pot methanolysis and acid treatment, which allowed efficient access to the ester 69. Malonate elaboration was followed by a combination of the Stille cross-coupling, using the vinyl tin 72 reagent and Pd(PPh3)4 as a catalyst, and then hydrogenation to provide 73. Finally, cyclization with oxadiazepane 74 and esterification with pivaloyl chloride gave

commonly termed as the Sonogashira cross-coupling reaction.213 It is one of the most reliable and widely used C(sp2)− C(sp) bond formation reactions (the construction of arylalkynes and conjugated alkenynes) in organic synthesis,214 particularly, in the syntheses of natural products,215 pharmaceuticals and agrochemicals,104 and macrocycles, as well as the construction of conjugated polymers, nanostructures, etc.216−219 The alkyne intermediate 62 of bixafen 33 has been reported via a Pd-catalyzed Sonogashira coupling (Figure 13).220,221 The alkylation of disubstituted anilide 60 with 2methylbut-3-yn-2-ol 61 under Sonogashira reaction conditions, i.e., Pd(PPh3)2Cl2 as catalyst and CuI as a cocatalyst in NEt3 as a base/solvent, leads to alkyne 62, which is cleaved to the phenylacetylene 63 under basic conditions. The tandem cycloaddition−cycloreversion of 63 with 3,4-dichlorothiophene-1,1-dioxide 64 provides the biphenyl derivative 31, which after deacetylation and amidation, leads to bixafen 33.



STILLE CROSS-COUPLING REACTIONS The Stille cross-coupling reaction is the organic reaction of an organohalide or its equivalent (e.g., acetate, triflate, or boronate) with an organostannane compound via a Pd catalyst to give the coupled product.222 The major advantages of the Stille reaction are that it works under mild and neutral reaction conditions and therefore can tolerate different functional 8921

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Figure 16. Decarboxylative cross-coupling in the syntheses of boscalid 26 and bixafen 33.

the formation of metal carboxylates, and then the different organometallic species, being formed by the release of carbon dioxide gas from the carboxylate complexes, can further be intercepted in diverse coupling reactions, depending on the catalyst system and the reaction conditions employed. The use of reasonably economical and stable carboxylic acids to replace organometallic reagents facilitates the decarboxylative crosscoupling reactions to proceed with good selectivities and functional group tolerance. The remarkable development attained in this field has been recently reviewed.240−243 In particular, Pd-catalyzed decarboxylative cross-coupling reactions have emerged within the last years following the discovery of the decarboxylative biaryl synthesis. These couplings rely on a lower catalyst loading and have been successfully applied to large-scale syntheses of commercially important biaryls, e.g., key synthetic intermediates for the agrochemicals boscalid 26 and bixafen 33 (Figure 16). The key intermediate 81 of bixafen 33 (50 g scale) was synthesized via a Goossen-type Pd/Cu-catalyzed decarboxylative cross-coupling. The synthesis involves the decarboxylative cross-coupling of the carboxylate 80 with aryl chloride 27, using a catalyst system consisting of CuBr/1,10phenanthroline along with Pd(acac)2 and PPh3, to afford the corresponding biaryl 81 (72% HPLC purity).244−246 The Cu/ phenanthroline complex mediates the release of CO2 from the aromatic carboxylate to give an arylcopper species, and the Pd complex catalyzes the cross-coupling of the arylcopper species with aryl halide. Goossen’s group also synthesized the key intermediate 24 of boscalid 26 using a bimetallic complex.247−249 The synthesis involves the decarboxylative crosscoupling of acid 82 with aryl bromide 83 in the presence of a bimetallic catalyst system, i.e., CuI/1,10-phenanthroline and Pd(acac)2, K2CO3 as a mild base, and N-methylmorpholine (NMP) as a solvent at 160 °C to give the corresponding biaryl 24 in excellent yield.

pinoxaden 75 in good yield. Despite some value in applying the Stille reaction to prepare vinyl arenes as precursors for their corresponding ethyl derivatives, use of the tin reagent was prohibitive for upscaling purposes due to noticeable causes of toxicity, byproduct formation, purification, and cost.



NEGISHI CROSS-COUPLING REACTIONS The Negishi cross-coupling reaction is the organic reaction of an organic halide or triflate with an organozinc compound using a palladium or nickel catalyst to give the coupled product.53,54 The Negishi cross-coupling has been widely applied in modern organic synthesis, and its applications in the synthesis of organic molecules of chemical, biological, or medicinal importance have been extensively reviewed.53,71,236−238 The Negishi coupling is not as commonly engaged in industrial applications as its cousins, the Suzuki and Heck reactions, mostly as a result of the water and air sensitivity of the required alkyl Zn reagents; however, organozincs are more reactive than both organostannanes and organoborates, which correlates to faster reaction times. For the synthesis of the key amine intermediate 35 of fluxapyroxad 36, Suzuki and Goossen-type Pd/Cu-catalyzed decarboxylative cross-couplings could be applied; however, an alternative synthetic strategy for fluxapyroxad 36 has been reported and involves the Negishi cross-coupling (Figure 15).188,239 It utilizes the Negishi cross-coupling of a trifluorophenylzinc species, which is generated in situ from its corresponding Grignard 77, with the Schiff base 79 and Pd with bulky electron-donating ligand, such as Pd-PEPPSI-iPr (PEPPSI = pyridine-enhanced precatalyst preparation stabilization and initiation), followed by hydrolysis to give 2-(3,4,5trifluorophenyl) aniline 35. The subsequent amidation of 35 provided fluxapyroxad 36.



DECARBOXYLATIVE CROSS-COUPLING REACTIONS Decarboxylative couplings are chemical transformations in which carboxylic acids release CO2 and form new bonds in the place of the former carboxylate group. In recent years, TMcatalyzed decarboxylative cross-couplings have emerged as a powerful strategy to form C−C or C−heteroatom bonds starting from carboxylic acids. Decarboxylative cross-couplings constitute beneficial replacements to traditional cross-couplings.11,240 Carboxylic acids can react with metals resulting in



CARBONYLATIVE CROSS-COUPLING REACTIONS The Pd-catalyzed carbonylation of aryl and vinyl halides was first described more than 30 years ago by Richard Heck.250,251 Various catalytic (mainly Pd, Ni, and Cu) carbonylative crosscoupling reactions (Suzuki, Heck, Negishi, Sonogashira, and Hiyama types) of aromatic halides (or pseudohalides) in the presence of various nucleophiles and CO have undergone a rapid development during recent years.72,252−257 The function8922

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Figure 17. Pd-catalyzed carbonylative coupling in the production of imazamox 86.

Figure 18. Nonreductive carbonylation in the pilot-plant preparation of CGA 308 956.

Figure 19. Synthesis of tolpyralate 94 via Pd-catalyzed carbonylative cross-coupling.

double ethoxycarbonylation of 2,3-dichloro-5(methoxymethyl)pyridine 84, and the optimized conditions are Pd(OAc)2 (0.4 mol %) as a catalyst, dppf as the most active ligand, sodium acetate as an acid acceptor, and EtOH as a solvent and reagent; the synthesis results in the key intermediate of imazamox 86, compound 85, to be produced in excellent yield. CGA 308 956. 2-Sulfo-4-methoxybenzoic acid 88 (SMBA) is a key intermediate in the synthesis of CGA 308 956 (Figure 18), which is a development herbicide of the former CibaGeigy AG.268 A laboratory process was developed for the preparation of SMBA via the diazotization of 2-amino-4methoxybenzenesulfonic acid 87, followed by PdCl2-catalyzed nonreductive carbonylation. Acid 88 was obtained using PdCl2 (1 mol %) under an initial CO pressure of 8 bar and water as a nucleophile in acetonitrile at 60 °C. The majority of the catalyst (85−95%) was removed via adsorption on carbon in the presence of hydrogen followed by filtration.269 Tolpyralate 94. The racemic proherbicide tolpyralate 94 (development code SL-573, Figure 19) was discovered by ISK and controls a wide range of weeds, not only broadleaves but also grasses, with excellent selectivity on corn fields.188,270 It is

alization of aryl/heteroaryl molecules with carbon monoxide to yield arylcarbonyl derivatives (e.g., acids, ketones, amides, esters, etc.) that are important intermediates in the manufacture of dyes, pharmaceuticals, agrochemicals, and other industrial products have been reviewed.257−262 The usage of gaseous CO impedes the application of carbonylation reactions in academic laboratories and at the industrial scale. This type of reaction has been explored in the synthesis of the herbicides imazamox 86, CGA 308 956, tolpyralate 94, and topramezone 101. Imazamox 86. The herbicide imazamox 86 (Raptor, Figure 17) was developed by the American Cyanamid Co. (Princeton, NJ) in 1991 and is a broad-spectrum and postemergence herbicide in the imidazolinone family that is associated with inhibition of acetohydroxyacid synthase (AHAS).263 Imazamox 86 is selective toward controlling grass and broadleaf weeds (including aquatic situations) in many leguminous crops such as soybeans, alfalfa, and peanuts.264 The ethyl ester of 5-(methoxymethyl)pyridine2,3-dicarboxylic acid 85 is the main intermediate of imazamox 86 and has great value in research and industrial production (Figure 17).265−267 The synthesis involves the Pd-catalyzed 8923

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Figure 20. Synthesis of topramezone 101 via Pd-catalyzed carbonylative cross-coupling.

Figure 21. Synthesis of sedaxane 109 via Pd-catalyzed C−N bond formation.

of the benzene ring, and subsequent S-oxidation affords the sulfone 99. Finally, topramezone 101 is obtained by the carbonylative cross-coupling of sulfone 99 with 1-methyl-5hydroxypyrazol 100 in the presence of carbon monoxide, bis(triphenylphosphine)palladium(II) dichloride as a catalyst, and 1,4-dioxane as a solvent, with a Pd recovery rate of more than 85%.

a member of the benzoylpyrazole class of HPPD (4hydroxyphenylpyruvate dioxygenase)-inhibiting herbicides, which ultimately cause the destruction of chlorophyll and lead to the death of sensitive plants. The synthesis of the key intermediate of tolpyralate 94, the carboxylic acid 93, has been reported via a Pd-catalyzed carbonylative cross-coupling (Figure 19).271 The synthesis involves the Friedel−Crafts reaction of chloride 89 with mesyl chloride, followed by the regioselective aromatic substitution reaction of the resultant chloride 90 with alcohol 91 under basic conditions, and finally the Pd-catalyzed carbonylation of 92 under high pressure to give acid 93, which is further converted to tolpyralate 94. Topramezone 101. The benzoylpyrazolone herbicide topramezone 101 (Impact, Clio, HPPD inhibitor, Figure 20), launched by BASF in 2006, is a new, highly selective herbicide for the postemergence control of broadleaf and grass weeds in corn with rates of 12−75 g/ha (the selectivity between corn and the weed species is above 1000-fold).272,273 It is usually mixed with low rates of photosystem (PS) II-inhibitors such as atrazine or terbutylazine, which show a very prominent HPPD−PSII synergism. Various synthetic methods have been reported for topramezone 101, one of which includes the carbonylative cross-coupling as a key step (Figure 20).274−277 Beginning with 3-nitro-o-xylene 95, dihydroisoxazole 97 is prepared via chloro-oximination and base-initiated cycloaddition, followed by catalytic reduction. The replacement of the amino group of 97 by methyl sulfide, bromination



CARBON−NITROGEN BOND FORMING CROSS-COUPLINGS Many biologically active and widely distributed natural products, pharmaceutical agents, agrochemicals, and materials have heterocyclic rings as part of their structures, and their syntheses have been attained through the formation of C−N, C−S, and C−O bonds by employing TM-catalyzed crosscoupling reactions. In the last few decades, this cross-coupling chemistry has received particular attention, which has brought about important progress in this field. The Pd-catalyzed C−S bond formation has been the least studied transformation out of the corresponding C−C, C−N, and C−O bond formations. Amide bond formation is a fundamentally important reaction in organic synthesis, and the amide functionality is a common feature in small and complex, synthetic and natural molecules, which widely exist in pharmaceuticals and agrochemicals.278 The cross-coupling reactions that form C−N bonds have become useful methods to synthesize anilines and amides, which are an important class of compounds throughout 8924

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Figure 22. Synthesis of mepanipyrim 115 via Buchwald−Hartwig amination.

Figure 23. Synthesis of bifenazate 52 via Buchwald−Hartwig amination.

chemical research.279 The applications of Pd-catalyzed C−N cross-coupling reactions in the synthesis of heterocycles and pharmaceuticals, materials science, and natural product synthesis have been reviewed.69,280−283 The prominent examples are the synthesis of the fungicides sedaxane 109 and mepanipyrim 115 and the acaricide bifenazate 52. Sedaxane 109. The fungicide sedaxane 109 (Figure 21) is a new broad-spectrum seed treatment pyrazole carboxamide fungicide (SDH inhibitor) developed by Syngenta Crop Protection for the control of seed- and soil-borne diseases in a broad range of crops; in particular, it is used to control Rhizoctonia spp. but also has documented growth-enhancement effects on wheat.284 For the synthesis of the key intermediate of sedaxane 109, the aniline 107, several synthetic routes have been reported in process patents.285,286 The most favorable (large-scale) methodology is the Pd-catalyzed benzylamination (PhCH2NH2) of the chlorobenzene precursor 105 in the presence of an appropriate carbene ligand 106 (Figure 21).69,287,288 After catalytic cleavage (using Pd/C, H2) of the protecting group, o-biscyclopropyl aniline 107 was obtained in excellent yield (>85% yield over two steps). Finally, the aniline 107 was treated with the appropriate acyl chloride 108 in the presence of a cheap base such as triethylamine to give sedaxane 109 in very good yield. Mepanipyrim 115. The anilinopyrimidine fungicide mepanipyrim 115 (Frupica, Cockpit, Figure 22), introduced by Kumiai Chemical Industry in 1995, is widely used as a methionine biosynthesis inhibitor.289−291 It is used to control a wide range of fungal pathogens, such as Botrytis cinerea (gray mold) on vines and vegetables, Venturia sp. (scab) on apples and pears, and Monilinia f ructicola (brown rot) on peaches. Several different methods were found in the literature for the

synthesis mepanipyrim 115,221,289,292,293 and its synthesis has also been reported via Buchwald−Hartwig amination (Figure 22).294 The treatment of 2-chloropyrimidine 110 with the Turbo-Hausar base TPPMgCl·LiCl (TPP = 2,2,6,6-tetramethylpiperidinyl) and transmetalation with ZnCl2, followed by quenching with I2 leads to the 4-iodopyrimidine 111. The subsequent magnesiation of 111 (at position 6) can be readily achieved with the base TPPMgCl·LiCl and provides the intermediate 112 after trapping with (BrCCl2)2. The Negishi cross-coupling of 112 with CH3MgCl in the presence of Pd(PPh3)4 and ZnCl2 in THF gave 113. The cross-coupling of 113 and propyne under Sonogashira conditionsPd(dba)3 as a catalyst, CuI as a cocatalyst, P(o-furyl)3 as a ligand, and NEt3 as a basegave compound 114 in excellent yield. Finally, the Buchwald−Hartwig amination of 114 with aniline, in the presence of Pd(OAc)2 as a catalyst, xantphos as a ligand, and K2CO3 as a base in 1,4-dioxane as a solvent, gave mepanipyrim 115 in good yield. Bifenazate 52. The synthesis of bifenazate 52 (Figure 23) was patented by using the Buchwald−Hartwig amination reaction.197 It involves the coupling of aryl bromide 116 with diarylhydrazone 117, using Pd(OAc)2 as a catalyst, the bidentate BINAP as a ligand, and sodium t-butoxide as a base and is followed by hydrolysis to give the corresponding hydrazine 119. In the final step, 119 was reacted with isopropyl chloroformate 120 using pyridine as a base to give bifenazate 52.



Pd-CATALYZED α-ARYLATION OF ENOLATES One of the most challenging problems in organic synthesis is the formation of a C−C bond between an aromatic carbon and the carbon alpha to a carbonyl group.295,296 Pd-catalyzed 8925

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Figure 24. Efficient scale-up synthesis of pinoxaden 75 using Pd-catalyzed malononitrile α-arylation.

reactions to industry is that homogeneous catalysts are difficult to recycle, which leads to the loss of expensive metals and the need to remove impurities from the products. The real limitations of some cross-couplings are the limited catalyst availability, substrate scope, operational stability, and anaerobic reaction conditions. Despite commercial success, these Pdcatalyzed cross-couplings in agrochemical synthesis still pose many challenges to be addressed: for example, catalyst costs remain high, catalyst loadings still need to be reduced, catalysts for certain specific couplings are still inadequate, catalysts with lower environmental impact and higher turnover number are still needed, and ligands are needed for excellent yield under mild conditions, while lower energy reactions and less byproduct waste are also required. Recent scientific breakthroughs in cross-couplings based on microwave irradiation, nanomaterials, and continuous flow conditions with alternative solvents such as water, compressed carbon dioxide, and ionic liquids, as well as the usage of abundant and inexpensive firstrow TMs (Fe, Ni, and Co) to supersede the thus far predominant Pd catalysts, would help to overcome some of these limitations and challenges. The choice of metal complex, ligand, additives, solvent, and temperature are all extremely important for the success of the coupling and, in many cases, should be optimized for the individual substrate. This review has highlighted the past and recent developments, as well as some future prospects, of Pd-catalyzed cross coupling reactions in the synthesis of agrochemicals. This review provides an overview of the most significant developments in this important area of research, and it is hoped that it will be an essential text for researchers at academic institutions and professionals at pharmaceutical/agrochemical companies.

enolate arylation is the cross-coupling of an enolate and aryl halide in the presence of a Pd catalyst. Several new Pdcatalyzed cross-coupling methods have recently been developed for the α-arylation of enolates, such as carbonyls, esters, amides, imides, nitriles, etc.114−116,297−300 α-Arylation is useful for the preparation of a variety of carbon skeletons commonly found in natural products, pharmaceuticals, and agrochemicals. The representative example for this type of cross-coupling reaction is the synthesis of the herbicide pinoxaden 75. Pinoxaden 75. A wide variety of synthetic methods for pinoxaden 75 (Figure 24) have been reported.234,301−304 In this procedure, an efficient malononitrile arylation via crosscoupling to access a sterically hindered aryl malononitrile is the key step in the large-scale synthesis of pinoxaden 75 (Figure 24).232,305 In the first step, the bromide 122 was synthesized from the commercially available amine 121 using the Sandmeyer reaction, followed by bromination. The crosscoupling of aryl bromide 122 with malononitrile 124 (enolate nucleophile) was achieved in excellent yield in the presence of PdCl2/PCy3 and using NaOtBu as a base. Then, the key intermediate 124 underwent hydration, cyclocondensation with [1,4,5]oxadiazepane 74 (synthesized in three steps from hydrazine) and was followed by esterification with pivaloyl chloride to efficiently afford pinoxaden 75.232,305



PERSPECTIVES AND CHALLENGES The agrochemical industry is continuously probing for modern agrochemicals with optimal efficacy, environmental safety, user friendliness, and economic viability. Pd-catalyzed crosscouplings have appeared as an essential tool in the synthesis of agrochemical intermediates, and today, synthetic chemists can readily access carbon−carbon and carbon−heteroatom bonds from a vast array of starting compounds. Despite the progress made to date, improving the versatility and practicality of these reactions in agrochemistry remains a tremendous challenge. Although a number of agrochemicals are prepared via cross-coupling reactions, relatively few applications have been explored on a large scale (prosulfuron 4, boscalid 26, bixafen 33, pinoxaden 75, etc.). The lack of homogeneous catalysis in first-generation processes is very apparent in the production of agrochemicals. A large challenge in the application of homogeneous Pd-catalyzed cross-coupling



AUTHOR INFORMATION

Corresponding Author

*Tel: +86-27-67867800. Fax: +86-27-67867141. E-mail: [email protected]. ORCID

Ponnam Devendar: 0000-0003-3079-1491 Guang-Fu Yang: 0000-0003-4384-2593 Notes

The authors declare no competing financial interest. 8926

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ACKNOWLEDGMENTS The authors are grateful to the National Key R&D Program (2017YFD0200507) and National Natural Science Foundation of China (No. 21332004 and 21672079) for generously funding programs in cross-coupling chemistry.



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