Recent advances in nickel catalyzed Suzuki-Miyaura cross coupling reaction via C-O& C-N bond activation

https://doi.org/10.1016/j.scp.2022.100953Get rights and content

Highlights

  • Shreds light on the importance of Nickel catalyzed Suzuki-Miyamura coupling reaction.

  • Covered the alternatives for the drawbacks in the conventional Suzuki-Miyamura coupling reaction.

  • Assessed the recent advances in Nickel catalyzed Suzuki-Miyamura coupling via alternative coupling partners.

  • Discussed the mechanistic aspects involved in Nickel catalyzed C-O & C-N Bond activation.

Abstract

Research on transition metal catalysis is advancing day by day. Unlike the conventional precious metal based catalysis, the researchers are now focussing on exploring the applicability of earth-abundant first row transition elements as catalysts for C-C coupling reactions. Among them, the Suzuki-Miyaura coupling reaction greatly impacts the industrial catalytic processes. Nickel catalyzed Suzuki-Miyaura coupling is a great topic of interest for organic chemists over the last few decades. But, the mitigation of halogen waste is the major environmental concern associated with the traditional Suzuki coupling reaction. In this concern, various abundant and cheap electrophiles such as carboxylic acid derivatives, phenol derivatives, and amides have a remarkable priority in replacing the organic halides. In this strategy, the core C-O and C-N bonds were activated to form the C-C coupling products. Based on the advantages of Ni-catalysis, the research is directed towards exploring the various Ni-based catalytic systems for the successful Suzuki coupling reaction by activating C-O and C-N bonds. The recent reports are highly motivated toward green metrics. In the current review, we would like to summarize the recent advances in the Ni-catalyzed Suzuki coupling via C-O and C-N bond activation.

Introduction

Cross coupling reactions via transition metal catalysis hold an important place in the field of organic synthesis (Jana et al., 2011; Beletskaya and Ananikov, 2011; Nicolaou et al., 2005), in which their importance has been recognized by the scientific community by being rewarded with the Nobel prize (Johansson Seechurn et al., 2012). Suzuki-Miyaura (Suzuki, 1985), Heck (2005), Negishi (Milstein and Stille, 1979), Kumada (Miyaura, 2004), Sonagashira (Sonogashira et al., 1991), and Hiyama (Nakao and Hiyama, 2011) are the various cross-coupling reactions for the C-C and C-Hetero atom bond formation technologies with a high impact from lab scale to the industrial levels. Among them, Suzuki-Miyaura coupling is a highly versatile reaction for the C-C bond formation in which the Noble prize in chemistry was grabbed to its credit along with Heck and Negishi coupling reactions in 2010 (Thomas and Denmark, 2016). Suzuki coupling reaction has proved its potential since its discovery in 1979 to date in synthesizing a wide variety of pharmaceuticals, intermediates, specialty chemicals, supramolecules, polymers, etc (Blakemore, 2016). Suzuki coupling is a palladium-catalyzed cross-electrophile coupling reaction between organoboronic compounds and organic halides or pseudo halides. The reaction conditions include mild temperatures employing base and phosphine based ligands. The versatility of the reaction lies in the facile coupling between the organoboronic compounds and the organic halides that are comparatively having electrophilicity difference via three important steps such as oxidative addition, transmetalation, and reductive elimination which were easily facilitated by palladium catalysis (Devendar et al., 2018). Moreover, organoboron reagents are less toxic and easily available compounds with good stability in ambient conditions (Miyaura and Suzuki, 1995).

Palladium based catalysis for the Suzuki-Miyaura (SM) coupling reaction is highly studied by various research groups within the 40 years of its research history. Even, the improvements in nanotechnology have also contributed to the developments in the reaction by employing various palladium nanoparticles, heterogeneous palladium on various supports, etc (Hong et al., 2020; Wolfson and Levy-Ontman, 2020; Kalidindi and Jagirdar, 2012). Apart from this, successful ppm level palladium based catalysis is also established (Handa et al., 2015, 2016; Roy and Uozumi, 2018). Even the Pd-catalyzed symmetric versions of the SM coupling reaction are also available (Johansson Seechurn et al., 2012; Zhang and Wang, 2015; Shen et al., 2019). The sustainable developments in the Pd-based SM reaction with a broad range of tolerable substrates have attracted organic chemists. Later on, realizing the hazardous impacts of halide wastes different coupling partners such as pseudo halides, carboxylic acid derivatives, and amides are also explored by various researchers to replace the organic halides (Wang et al., 2016; Liu and Szostak, 2018; Zhang et al., 2021). These developments have made the reaction more sustainable and have become the preferential choice in terms of industrial scale up reactions also.

Although, the developments in palladium catalysis are appreciable the cost of palladium is too high than the easily available first row transition metals (Peng et al., 2019). This made the researchers work on the best alternatives for palladium and the various reports have evaluated the efficiency of nickel on the SM coupling reaction successfully. Even though there are numerous developments in the SM coupling reaction, Nickel catalysis had a great impact in the last decade. After a decreased interest in conventional SM coupling reactions incorporation of Nickel as a catalyst has again demanded the chemists make advancements in this field. This growing factor is highly impactful in terms of industrial chemistry, which has grasped our attention. In fact, Ni is a highly earth abundant metal and is found to be much cheaper than that palladium (Du and Eisenberg, 2012), which has grasped the attention of organic chemists. Comparing the chemistry of Pd and Ni, palladium is commonly exhibited in Pd(0)/Pd(II) redox states (Johansson and Colacot, 2010; Hickman and Sanford, 2012) and the applicability of high valent palladium is currently under investigation. But, nickel is a first row transition metal with relatively less size than that of palladium, which makes it more nucleophilic and typically exhibits Ni(0)/Ni(II) as well as high valent states such as Ni(I)/Ni(III) (Rosen et al., 2011). These salient features of the nickel are more interesting in exploring its catalytic activity and the researchers have successfully established the Ni-catalyzed SM cross-coupling reaction with high yields compatible with the palladium (Ramgren et al., 2013). Apart from SM, the versatility of nickel based catalysts was successfully demonstrated by extensive studies on C-C and C-Hetero atom bond forming reactions employing various electrophiles that which were less reactive even in the presence of Pd (Diccianni and Diao, 2019), apart from the advantages in the case of the Ni as an alternative for the Pd, the discouraging factor is the loading of high amounts of Ni catalysts in all the earlier reports. Rather, the current status of research on Pd based catalysts has achieved the level of employing ppm levels of palladium for the SM coupling reaction. Although significant growth was observed in the case of Ni-catalysis, but the high catalyst loading is found to be the major concern to be focussed on. In fact, 5 mol% to 10 mol% levels of the catalyst loading was observed when Ni-catalyst was employed, wherein 0.1 mol% of catalyst loading instances were reported with the Pd (Akporji et al., 2020; Takale et al., 2019; Landstrom et al., 2018; Chatterjee and Ward, 2016). But as it is certain that the research progresses day by day and it is astonishing that the different versions of SM coupling reaction are now designed with the various Ni as a catalytic source. In the same way we may expect the ppm level Ni-catalyzed SM reaction from the dynamic research groups in the scientific community.

In the case of electrophiles employed in the SM coupling reaction, organo halides are the well-established coupling partners with the organo boronic acids. Commonly, organo chlorides, bromides, and iodides are used where the attention has to be kept on halogen recovery as they are environmental pollutants and also costly (Chen et al., 2020; Xu et al., 2021). The synthesis of various organic halides usually needs hectic processes which are industrially not effective and make them costlier. Even the production processes also generate halogen wates that are harmful. In this concern, in the search for alternative electrophiles, scientists have successfully evaluated various abundant feedstocks such as naturally available phenols and their derivatives, carboxylic acid derivatives, and amides (Boit et al., 2020). Even, the by products are CO, CO2, amines, etc, which are value added products and could be easily captured and recycled. Ni-catalyzed C-O and C-N bond activation has emerged as an impactful tool in creating C-C bonds, different research groups are working on this branch of Ni-catalysis and considerable growth in terms of academic research was observed. Various number of remarkable and exciting transformations contributed by various independent research groups, Garg (Hie et al., 2016; Simmons et al., 2016, 2017; Dander et al., 2017), Szostak (Shi et al., 2016; Shi and Szostak, 2016a, 2016b, 2017), Shi (Hu et al., 2016, 2017), Zou (Li and Zou, 2015), and Rueping (Yue et al., 2017a, 2017b; Srimontree et al., 2017), have paved a way for the Ni-catalyzed C-C bond formations via C-N and C-O bond activation. This strategy was found to be a green approach as it is more atom economical, and environmentally friendly and replaces halides with easily available and cheap starting materials such as electrophiles. Although initially palladium catalysis was explored for C-O and C-N bond activation for SM coupling, realizing the versatility and abundance of Ni, Ni-based catalysts were successfully employed for C-O and C-N bond activation for the C-C coupling reaction with better results than that of Pd (Rosen et al., 2011; Wang et al., 2020; Liu and Szostak, 2017; Liu et al., 2021). In this concern, this article summarizes the recent advances in the nickel catalyzed SM coupling reaction through C-O and C-N bond activation. The different substrates concerned in this review are carboxylic acid derivatives, phenols, and their derivatives and amides. For the carboxylic acid derivatives and phenols, the catalytic systems developed for C-O bond breaking were discussed along with the electronic effects. Even, the C-N bond breaking under Ni-catalysis was discussed from the demonstrated examples of amides including their mechanistic aspects.

Apart from carboxylic acids and their salts, anhydrides, esters, and acid halides are the frequently utilized family of carboxylic acid derivatives that are prevalent in nature. They are among the least reactive electrophiles (Zheng and Newman, 2019). The use of carboxylic acids and their derivatives as coupling counterparts in nickel catalysis has drawn a lot of attention in comparison to conventional cross-coupling reactions with aryl halides and sulfonates (Guo and Rueping, 2018a). This is because it prevents the production of corrosive halide-containing wastes and makes a variety of synthetically and commercially available COOH-containing molecules accessible for functionalization. Additionally, the replacement of these groups is helpful in retrosynthetic analysis because before replacement, they can also be employed as directing groups in metal-catalyzed C-H functionalizations by activating the C(aryl)O bond through transition metal -catalysis (Park et al., 2011; Xiao et al., 2010). A wide range of nucleophiles could be coupled with these carboxylic acid derivatives under nickel catalysis via decarbonylative and decarboxylation strategies with minimal waste generation, even supporting maximum atom economy. Albeit, there are several reports in the same fashion as palladium, there are various drawbacks involving the high cost of the catalyst and safety issues.

A stoichiometric nickel-mediated decarbonylation process of aryl carboxylates was reported by Yamamoto and co-workers in 1980 (Yamamoto et al., 1980). The reaction was hypothesized to happen through an oxidative addition mechanism, started by the nucleophilic attack of the electron-rich nickel complex at the carbonyl group, and was promoted by a monodentate phosphine nickel complex (for example, triphenylphosphine). The isolation and characterization of Ni(CO)Ln complexes provided evidence for extrusion of the carbon monoxide, in the support of the decarbonylation mechanism. Based on the ground-breaking work of Yamamoto, catalytic decarbonylative cross-couplings of esters and other carboxylic acid derivatives with diverse nucleophiles have been further investigated to produce carbon–carbon and carbon-heteroatom bonds (Dermenci and Dong, 2013; Takise et al., 2017).

Both the decarbonylation and decarboxylation strategies (Dzik et al., 2012), almost have a similar mechanistic aspect with minor variations. In the first process, the oxidative addition of Ni(0) species to the C(acyl)O bond of COOX substrates causes the decarbonylative cross coupling to occur. Following a decarbonylation step involving the expulsion of carbon monoxide, the resultant acylnickel (II) complex produces the arylnickel (II) species. Different nucleophiles enable transmetalation, which takes place following the extrusion. The following and last stage entails a reductive elimination reaction that releases the associated coupling product and regenerates the active Ni(0) species. Shi and coworkers (Pu et al., 2016) conducted mechanistic studies by isolating and characterizing critical nickel intermediates, which helped to illustrate this chain of processes. The plausible mechanism for the lateral process begins with the oxidative addition of Ni(0) into ArX, followed by the transmetalation of Ar'[M] produced by decarboxylation onto the aryl Ni intermediate and (iii) reductive elimination to liberate the product and regenerate the Ni0 catalyst. In the latter case, the decarboxylation occurs via transmetalation, whereas in the previous case decarbonylation occurs in the oxidative addition step.

The use of aroyl compounds in metal-catalyzed decarbonylative or decarbonylative coupling processes has gained popularity recently (Zhou et al., 2020). In this concern, by applying different types of second row transition metals such as Pd, Rh, etc., decarbonylative reactions were devised efficiently using esters as electrophiles (Guo and Rueping, 2018b). But, it should be realized that the availability of these costly second row transition metals is a standing drawback. Then, the research has begun using the first row of transition metals. Among the recent reports, the Nickel-based catalytic approaches have a prominent significance since nickel could be the best alternative for costly metals such as Pd. Recently, Itami and co-workers (Muto et al., 2015) (scheme 1) have developed a Nickel based catalytic system comprising Ni(OAc)2 as a nickel source, tri-n-butylphosphineas ligand, and sodium carbonate as a base for the decarbonylative SM coupling of benzoyl esters and phenylboronic acids for the synthesis of biaryl compounds. During their initial trials using Ni(cod)2, the yield was found to be low, and then after employing Ni(OAc)2, the yield switched to 95%. The optimal loading of the Ni source was found to be 5 mol% and in fact, the cost of Ni(OAc)2 is considerably low when compared to the different Ni sources.

When it comes to both coupling partners, the scope is fairly vast. It was discovered that p-anisyl boronic acid can cross-couple with a variety of electronically and sterically different arene carboxylic acid esters in good to excellent yields. Smooth reactions produced the respective hetero biaryls from the esters of heterocycles including thiophenes, furans benzothiophenes, oxazoles, thiazoles, pyridines, and quinolines. The majority of heteroaryl groups were transferred into the product, except 2-pyridyl, 2-pyrazyl, and 2-quinolinyl groups. Apart from the various aromatic counterparts, the coupling with aliphatic esters resulted in low yields. The mechanistic studies of this decarbonylative SM coupling reaction were found to proceed via acylnickel (II) intermediate which was created by the oxidative addition of the C(acyl)-O bond of some carboxylic acid esters to Ni (0) complexes. This intermediate can then go through a transmetalation reaction with a boronic acid and a decarbonylation reaction to produce a diorganonickel (II) intermediate. The decarbonylative cross-coupling product is released by further reductive elimination, which also regenerates Ni(0) species.

In the same period, Love and co-workers (LaBerge and Love, 2015) (Scheme 2) also devised a novel nickel based catalytic system for the decarbonylative SM coupling of phenyl nicotinate with phenylboronic acids. During their screening studies, they found Ni(cod)2 was the suitable nickel source for the reaction. Even, the same reaction yielded 0% of the required product with the Pd and Rh. Then, among the ligands, compared to the bidentate ligands, mono dentate ligands have resulted in a decarbonylative coupling product in which the bidentate ligands have resulted in <5% yields. Tri-cyclohexyl phosphine was found to be the suitable ligand, coordinating with the Ni(cod)2. Screening of different bases has revealed the requirement of a strong base such as cesium carbonate for successful transmetalation. Finally, the optimal conditions suggest the 10 mol% of Ni(cod)2, tri-cyclohexyl phosphine (20 mol%), and 2.0 equiv of cesium carbonate in toluene under reflux for 24 h.

In consideration of the extent of the reaction between aryl ester and boronic acid, different aryl boronic acids with various electronic and steric characteristics were screened. The best yields (among the screened and reported by them) were from boronic acids that were neutral and electron-rich. Ortho substrate phenyl boronic acids have been found to produce considerably low yields. It was simple to cross-couple naphthyl- and biphenyl-based boronic acids to phenyl nicotinate, which enabled the production of substituted polyarenes at moderate yields. The reaction can also be carried out on a gram scale, and 3-phenyl pyridine was produced with an isolated yield of 50%. The scope of the esters was studied completely studied on the phenyl nicotinate derivatives only. But, the yields of all the screened substrates are relatively lower and were in the range of 30–50%. The attempts on the acid chloride resulted in a very low yield of the desired product. Overall, the attempted method is good for the synthesis of pyridyl biaryl compounds. Even though the yields are low, the work holds a good example for further studies.

As proposed by them, the plausible mechanism of the reaction proceeds through the oxidative addition of Ni-complex with the phenyl nicotinate to form an acylnickel(II) intermediate, followed by the transmetalation and reductive elimination to generate the desired product. Unlike the previous reports, the CO extrusion was observed after the reductive elimination, which was evidenced by the formation of ketone along with biaryl in the reaction of phenyl benzoate.

Recently, using aromatic esters and alkyl organoboron reagents as coupling partners, a ligand-controlled and site-selective SM cross-coupling process was developed by L. Cavallo and co-workers (Chatupheeraphat et al., 2018) (Scheme 3). By effectively suppressing the undesirable ß-hydride elimination process, this technique offers a simple route for Csp3-Csp2 bond generation in a straightforward manner. The ester substrates are transformed into the alkylated arenes and ketone products by simply altering the phosphorus ligand. It has been observed that while nickel complexes with mono-dentate phosphorus ligands favor activation of the C(acyl)-O bond, which ultimately produces the ketone product, nickel complexes with bidentate ligands favor C(aryl)-C bond cleavage in the oxidative addition step leading to the alkylated product via a decarbonylative process. It is noteworthy to mention that the versatility of Ni-sources for different transformations could be noticed from this example.

Complete optimization studies were done with a fixed Ni-source, Ni(cod)2 by varying the ligands. During their initial trials, several trialkyl-monodentate phosphine ligands, including PnBu3 and PCy3, were used in preliminary tests that produced a mixture of ketone and decarbonylative products. Even a trace of the product was not produced by carbene ligands such as 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr). It was observed that the generation of unwanted ketone by products was completely suppressed, according to further analysis of a variety of bidentate phosphine complexes. Among the bidentate ligands tried by them, Dcype [1,2-bis (dicyclohexylphosphino) ethane] was found to be the most effective ligand, yielding the product in a 75% yield. Then, various bases were screened and they found that cesium fluoride has the best ability to smoothen the process of transmetalation, among others. The optimized catalytic system for the decarbonylative SM coupling of aryl esters and alkyl boronic species is Ni(cod)2 (10 mol%), bidentate ligand type (20 mol%), and the fluoride salt, cesium fluoride (1.0 equiv) as a base in toluene at 150 °C for 65 h.

The scope of this catalytic system is widely applicable, esters with a variety of heterocyclic patterns, such as benzofuran, benzothiophene, and indole, were examined, and the respective products were obtained in high yields. Homoaromatic substrates with non-substituted, electron-rich, and electron-deficient phenyl esters were also reported to produce the resulting products in moderate to good yields. Additionally, the study of substrates with π-extended aromatic rings, such as biphenyl, 2-naphthyl, and 1-naphthyl esters, yielded positive results.

Concerning the mechanistic aspects of this reaction, they have observed that after the oxidative addition of Ni-complex to the C(acyl)-O, CO extrusion occurs via decarbonylation. Then transmetalation and reductive elimination occur for the product formation. According to their DFT studies, in the case of the Ni-bidentate system, the Ni(dcype) activation of the C(aryl)-O bond is favoured over the activation of the C(acyl)-C bond due to the requirement of less activation energy for the C(aryl)-O bond activation, resulting in the suppression of competitive C(acyl)-O bond activation and thereby keto product formation.

In the same paper, they further extended their study toward the SM coupling for ketone formation by varying different ligands (Scheme 4). As discussed earlier, ketone is the by product in their initial study which is found to be dominating when tri-alkyl monodentate ligands were employed. Based on this observation, they moulded the reaction pattern by optimizing the reaction with different tri-alkyl monodentate phosphine ligands and found it successful in both the cases of PCy3 and PnBu3. On the other side, screening of different bases revealed that apart from cesium carbonate, most of the bases are not able to give good yields. Finally, for the C(acyl)-C bond activation the developed catalytic system comprised of Ni(cod)2 (10 mol%), monodentate ligand PnBu3, or PCy3(20 mol%), cesium carbonate (1.0 equiv) as a base in toluene at 80 °C for 10 h. The designed catalytic system is found to be widely applicable for different substrates such as electron rich, electron deficient aromatics, and heterocyclics. ranging from good to excellent yields.

The mechanistic aspects of this reaction were studied based on the DFT, they observed that after the oxidative addition of Ni-complex to the C(aryl)-O, transmetalation and the reductive elimination occur for the product formation, and the CO extrusion was not observed. According to their DFT studies, in the case of the Ni-monodentate system the, Ni(PnBu3) activation of the C(acyl)-O bond is favoured over the activation of the C(aryl)-C bond due to the requirement of less activation energy for the C(acyl)-O bond activation, resulting in the suppression of competitive C(aryl)-O bond activation.

From all the above-reported examples it is clear that base has an important role in the transmetalation of active intermediates which then participate in fast transmetalation with the boronic acids. Contrary to all the above reports, recently M.S. Sanford and co-workers (Malapit et al., 2018) (Scheme 5) have reported a base free nickel catalyzed decarbonylative SM coupling reaction for the synthesis of various biaryl compounds. As per the traditional SM coupling reaction, the base accelerates the transmetalation step to form the coupling products (Amatore et al., 2012), on the other side it was also reported that the base also mediates the off-cycle formation of organoboronate intermediates which can form the homocoupling products as the side products (Cox et al., 2016, 2017). To overcome these setbacks, the authors have designed a new catalytic system where the reaction did not need a base, meaning a base-free transmetalation intermediate could be generated. As per their report, it is obvious that the combination of Ni with acid fluoride leads to the formation of transmetalation active intermediate directly. The exact nickel-based catalytic system developed by them comprises a nickel source, Ni(cod)2 (10 mol%), and monodentate phosphine ligand PPh2Me (20 mol%) in THF as a solvent at 100 °C for 16 h. The acid fluoride was generated in situ from the carboxylic acids using fluorine sources. Their initial studies started with the stoichiometric amounts of Nickel source which confirmed the formation of transmetalation active intermediate which upon catalytic scale trials using PCy3 as a ligand yielded biaryl product along with ketone by product. Then switching the ligand from PCy3 or PPh2Me has improved the selectivity of biaryl product to 95% with the undetectable amounts of the ketone.

Concerning towards the scope of nickel-catalyzed coupling of arylboronic acids involves several aromatic and heteroaromatic carboxylic acids. Tolerable substances include esters, nitriles, trifluoromethyl groups, methyl- and phenyl ethers, amides, alkenes, imidazoles, oxazoles, and pinacol boronate esters. From their reports, it is observed that acid fluorides with electron-donating substituents and those with ortho-substituents both produced moderate yields. For the former, gas chromatography and mass spectrometry investigation of the crude reaction mixtures revealed ketone side products, showing that decarbonylation is somewhat sluggish with electron-rich substrates. With the latter, the unreacted starting material was still present, indicating that when the acid fluoride is sterically inhibited, oxidative addition proceeds slowly.

Towards the investigations on the mechanism of the reaction, they have reported that the first step is common ie, oxidative addition of Ni (0) to the aryl fluoride substrate followed by the decarbonylation and then base free generation of the transmetalation active intermediate. Then the as usual reductive elimination for the formation of the biaryl product. It is noteworthy to mention that the protocol developed by them is novel in comparison with the mechanistic aspects of all the reported methods so far. The electron deficient nature of the acid fluorides is the driving force for the rapid oxidative addition followed by decarbonylation upon which has undergone base free transmetalation.

Decarboxylative cross-couplings offer a more cost-effective and environmentally friendly method of creating biaryl linkages than other common cross-coupling processes (Goossen et al., 2010; Kaur et al., 2020). In these reactions, aromatic carboxylic acids are coupled with aryl halides (or pseudohalides) or organometallic compounds. The carboxylic acids used in these reactions can be in either native or latent form, and they are bench stable and work with multistep reaction sequences (e.g., from esters, amides, etc.). Additionally, copious biomass feedstocks commonly contain structurally varied carboxylic acids, which are cheap. Realizing this, research is advancing well in this direction, among such reports, recently D. Kalyani and co-workers (Sardzinski et al., 2015) (Scheme 6) have reported a nickel-catalyzed decarboxylative cross-coupling reaction of perfluorobenzoate with the aryl halides and sulfonates. The developed catalytic system is widely accessible for most of the halides and pseudo halides and penta, tetra, and trifluoro compounds are efficiently synthesized. As per the previous reports, it is obvious that elevated temperatures are needed for CO2 extrusion. As it was known that perfluorobenzoates are the type of molecules that can extrude CO2 at moderate temperatures, they were selected as a model substrate for this reaction. The authors have devised a novel nickel-based method using Ni(cod)2 as a nickel source for the cross coupling between benzoates and aryl halides, pseudo halides.

Their initial studies started with the potassium pefluorobenzoate and 4-iodoanisole, using 10 mol% of Ni(cod)2. Keeping the nickel salt constant, the reaction profile was screened using different phosphine ligands and among them, they found that Pt Bu3 is much more efficient than PCy3 and 20 mol% of the ligand is found to be the optimal amount in diglyme as solvent at 120–140 °C. The scope of the reaction was applied initially to the different derivatives of aryl iodides. Under the optimized conditions, all the electron rich, electron deficient, and neutral substrates of aryl iodides have afforded decent yields. The scope of various aryl bromides and aryl chlorides has resulted in modest to good yields. In the same context, screening of different pseudo halides such as aryl triflates and aryl tosylates gave the coupled products in good yields. But, it is observed that comparatively, aryl triflates have given better yields than aryl tosylates. Coming to the scope of perfluorobenzoates, tetra fluoro and tri-fluoro phenyl carboxylates were examined and found to give good yields. But the reactivity of tetra fluoro benzoates is observed to be better than those of trifluoro benzoates. Overall, the aforementioned nickel based catalytic system is found to be efficient for the synthesis of various perfluoro based biaryl compounds using various aryl halides and pseudo aryl halides.

It was proposed that the mechanism of the reaction undergoes initially the oxidative addition of Ni (0) to the aryl halides or pseudo halides. Then the decarboxylation of benzoate proceeds and subsequently the transmetalation. Finally, reductive elimination regenerates the catalyst and affords a biaryl compound. In comparison with decarbonylation which occurs in the oxidative addition step, decarboxylation is found to happen after the oxidative addition step. As proposed by the authors it is to be noted that decarboxylation is the driving force for the transmetalation step.

Further, P. S. Baran and co-workers (Wang et al., 2016) (Scheme 7) have reported a nickel catalyzed decarboxylative SM cross coupling of redox active esters with the aryl boronic acids for the synthesis of Csp3-Csp2 coupled products. The uniqueness of this work could be observed in the nickel catalyzed activation of Alkyl carboxylic acids by generating the redox-active ester derivatives, notably by using N-hydroxy-tetrachlorophthalimide. Unlike the different decarboxylation reactions reported so far, this reaction proceeds via a single electron transfer process for the decarboxylation which is found to be the crucial step for this transformation. As devised by them, initially the carboxylic acids are converted into the respective esters of N-hydroxy-tetrachlorophthalimide. Both the isolated ester and even the in-situ generated are found to be effective for the transformation. The protocol comprises the inexpensive nickel source NiCl2.6H2O (10 mol%) and 4,4′-di-tert-butyl-2,2′-bipyridine (20 mol%) as a ligand and triethylamine as a base which promotes the transmetalation, in a 1,4-dioxane, DMF (10:1) solvent system at 75 °C for 12 h. During their initial optimization studies, different inorganic strong bases such as cesium carbonate, etc. were used which resulted in unsuccessful attempts. Screening of different organic bases such as tributyl amine, triethylamine, and DIPEA has given good yields, among them triethylamine is found to proceed with the smooth transmetalation process.

The scope of this Csp3-Csp2 SM coupling is found to be wide and affords the respective coupling products good to excellent yields. The reaction seems to be unaffected by a wide range of functional groups, including aryl halides, common protecting groups, esters, nitriles, ketones, carbamates, and even medicinally important heterocycles. With or without radical stabilizing groups, primary and secondary alkyl carboxylic acids are both found to be viable, and amino acid side chains protected by N-Boc are observed to not affect their optical activity. Apart from aryl boronic acids, vinyl boronic acids also effectively participated in the reaction. The mechanism of this reaction is completely different from the various reported approaches, as proposed by the authors, initially the Ni (I) -ligand complex undergoes the transmetalation with the aryl boronic acid promoted by the base Et3N to form the Ni (I)-Ligand-Aryl complex. The carboxylic acid ester undergoes the reduction by the single electron process which on decarboxylation provides Alkyl radical. The alkyl radical undergoes the addition of the Nickel (I) complex to form the Ni(II) complex which rapidly undergoes reductive elimination to form the product.

X. Liao and co-workers (Chen et al., 2018) (Scheme 8) have described the nickel-catalyzed anhydride-based decarboxylative alkylation of aryl iodides. This method of decarboxylative coupling accepts aromatic substituents that are helpful for synthetic purposes and works with a variety of aliphatic carboxylic anhydrides. The current process takes place under mild conditions supported by a redox system of pyridine N-oxide and zinc additives. A notable aspect of this approach is its strong chemo selectivity for alkyl migration using a combination of aliphatic and aromatic anhydrides. It is a Csp3-Csp2 coupling reaction where a Ni (II) precatalyst was used and was reduced in situ using additive zinc. Thus formed Ni (0) catalyst chelates with ligands and proceeds further. During their optimization studies, among the different ligands screened, simple bipyridyl without any substituent worked well and among the Ni (II) sources, all the sources gave more than 50% yield, but NiCl2 is found to be both fruitful and commercially cheap.

During their studies on various substrates, it was observed that neutral, electron-rich, and electron-deficient substituents of aryl iodides were all well tolerated. The common conditions were compatible with a variety of functional groups, including ester, ketone, cyano, and protected nitrogen. Ortho- and meta-position substitutes did not affect the yields. But, it was observed that the heteroaryl iodides resulted in low yields. Among the acid anhydrides, primary and secondary had shown good results whereas the tertiary carboxylic acid anhydrides showed sluggish yields. Similar to the mechanism reported by P. S. Baran and co-workers (Wang et al., 2016), X. Liao and co-workers also reported the SET supported the decarboxylative generation of the alkyl radical. Initially, Ni (II) was reduced to Ni (0) followed by complexation with the ligand. Ni (O)-Ligand complex then undergo oxidative addition with the aryl iodide followed by reduction of Ni (II) to Ni (I). The alkyl radical generated was added to Ni (I) to form Ni (II) complex which upon subsequent reductive elimination affords the product.

The same group has also studied the above reaction by replacing aryl iodides with aryl triflates without any change in the conditions and was reported separately (Scheme 9) (Chen and Liao, 2019). In the case of aryl triflates, unlike the aryl iodide, neutral functionality containing aryl triflates was found to afford the lower yields and the electron withdrawing containing groups yielded the products in good amounts. Where the scope of anhydrides has shown the tolerability of both cyclic and acyclic counterparts but the heteroatomic molecules have resulted in modest yields. As reported by them, the mechanism of this reaction also proceeds the same as the earlier one with the SET mechanism provided pyridine N-Oxide was used as an additive in which the decarboxylation occurs to generate the alkyl radical.

It is well known that one of the most effective ways to create carbon-carbon bonds is by cross-coupling reactions between aryl halides and organometallic reagents, which are catalyzed by transition metals (Shi et al., 2011). A lot of work has lately been put into creating catalytic systems that let innocuous but easily accessible phenol derivatives be used in place of aryl halides, to increase the usability of cross-coupling technology. According to studies in this area, inert phenol derivatives such as aryl ethers, carboxylates, carbamates, and naphthols, which are completely inactive under the conditions employed for typical palladium-catalyzed cross-coupling processes, can be activated exceptionally well by nickel-based catalysts (Tobisu et al., 2017). In this context, recently, M. Rueping and co-workers (Guo et al., 2016) (scheme 10) have reported a novel nickel based catalytic system for the SM coupling of ethers and alkyl boronates for Csp2-Csp3 coupling via decarboxylative strategy. As per the previous studies on Csp2-Csp3 coupling via different nucleophilic centers such as alkyl magnesium and alkyl zinc reagents, β-hydride elimination is found to be the liming factor for successful coupling (Jana et al., 2011; Cross Coupling Reactions: A Practical Guide Edited by N. Miyaura, 2002). Wherein, in this work, the authors have exclusively mentioned the versatility of alkyl boron reagents in minimizing the β-hydride elimination leading to the broad scope of the catalytic system. During their optimization studies, it was observed that different nickel salts apart from Ni(cod)2 have resulted in unsuccessful attempts where in the initial attempt of Ni(cod)2 with IPr.HCl has resulted in a 90% isolated yield of the coupling product. Then the screening of different phosphine and NHC based ligands were found to give very low yields. Further, they have screened the different bases such as K3PO4, tBuOK, and CSF which resulted in low yields. Then employing cesium carbonate was found to take up the smooth coupling. Finally, the optimized condition was Ni(cod)2 (8mol%) as nickel salt in combination with IPr.HCl (16 mol%) as ligand and Cs2CO3(1.5 equiv) as a base in isopropyl ether as a solvent for 12 h @ 100 °C.

The optimized nickel-catalytic system was reported to have a good scope of coupling for both the partners such as various phenol derivatives and alkyl boronates. It was observed that in the scope of phenol pivalates, the scope of naphthyl phenol pivalates was exclusively studied rather than the phenyl derivatives. Napthyl, phenanthrayl, and biphenyl pivalates were found to undergo the reaction smoothly to yield the corresponding coupling products in good yields. The optimized protocol was found to be chemo selective as ketones, esters and TMS functionalities remained unaffected. The scope of alkyl boronates with naphthyl pivalate has shown good to moderate yields and the various functional groups on the alkyl boronate remained unchanged. As mentioned by the authors, so far, the mechanism of the reaction was under study. But the reports that are published later have revealed the dealkoxylative mechanism of the reaction which could be discussed further.

Similarly, in the same period, M. C. Schwarzer and co-workers (Schwarzer et al., 2017) (Scheme 11) have reported the mechanistic aspects of nickel catalyzed SM cross coupling of methoxy arenes and boronate esters for the synthesis of various biaryl compounds via C-O bond cleavage based on their previous study. They have proposed a DFT supported mechanism for the nickel catalyzed dealkoxylative coupling of methoxy arenes. The hallmark of the optimized condition doesn't require the base for mediating the reaction, which was observed due to the ligand Icy. It was found that the coupling products with the aryl ethers are mostly limited to aryl magnesium halides or the highly lewis acidic compounds such as AlR3. But, the coupling of aryl ethers with boronic esters is very few. To break this limiting factor, the authors have exclusively studied the mechanistic aspects of the C-O bond cleavage for C-C bond formation using boronate esters. Their initial studies began with Ni(cod)2 as a nickel salt and PCy3 as a ligand source where no yields were obtained without the base. Then after employing CsF as a base, the reaction yielded the cross coupled product in quantitative yields. Then, employing ICy instead of PCy3 has resulted in very good yields without the base. Initially, the reaction of Icy.HCl (20 mol%) + NaOtBu (20 mol%) has resulted in the product in 72% yield which indicated the role of base neutralizing the Icy.HCl to generate ICy in situ. Then employing the ICy, has directly yielded the product in 87% yield indicating the base free mechanism of the reaction in the presence of ICy as a ligand.

The scope of the base free Ni-cat, dealkoxylative reaction is wide, as they reported, both the electrons realizing group and electron withdrawing group containing methoxy arenes have tolerated providing the yields in good numbers under base-free conditions. But here, there arises a question on the role of ICy in catalytic amounts to generate the products without yields, which they have answered these questions based on the theoretical studies. In comparison with the base free system, the PCy3 + CsF condition was also studied by them to evaluate the role of the base when PCy3 was employed.

In the case of PCy3 as a ligand, the reaction proceeds with the exchange of ligands from Ni(cod)2 to form Ni(PCy3)2 which is found to be the lowest energy pathway. Then, the ligand exchanged complex undergoes the coordination with the C-O bond of methoxy arene to allow oxidative addition which subsequently undergoes the transmetalation and reductive elimination to form the product. But, here it should be noted that their studies on the activation energies of these transition state complexes with and without the base have revealed that, in the case of PCy3 without a base, the activation energies of transition states leading to C-O bond cleavage are high when compared to that of the transition states in the presence of a base. This suggests the valid pathway for C-O bond cleavage occurs in the base mediated condition.

The theoretical studies in the case of ICy without base have revealed that the reaction here also proceeds via ligand exchange initially from Ni(cod)2 to Ni(ICy)2. Then undergoes oxidative addition, transmetalation, and reductive elimination. In this case, the oxidative addition where the initial interaction of Ni-Ligand complex with C-O bond occurs, the relative activation energy was found to be low when compared to that of Ni(PCy3)2. Indicating the smoother initial interaction in the case of ICy. Followingly, the transition states of the transmetalation step without a base in the case of Ni(ICy) 2 are considerably lower than that of Ni(PCy3)2 with a base, which indicates the efficiency of the ICy ligand. As mentioned by the authors, the electronic structure of ICy ligand comprises of rich electron cloud which is relatively poor than that of PCy3. Hence, the electron density of the ICy ligand makes the Ni-Ligand backbone strong enough which made the transition state complexes relatively lower leading to the base free mechanism. From there in deep mechanistic insights, it is obvious that a stronger Ni-Ligand backbone is sufficient enough for C-O bond cleavage rather than employing the bases.

Very recently, W. Su and co-workers (Liu et al., 2022) (Scheme 12) have reported a nickel catalyzed deoxygenative borylation of phenols using uronium-based reagents for the C-O activation. In this paper, they have also demonstrated an example of deoxygenative SM coupling of phenols with aryl boronic acids in which Ni(cod)2 was used as a nickel source and PCy3 was used as a ligand along with the stochiometric equivalence of uronium based additive, fluoro-N,N,N′,N′-tetramethylformamidinium hexafluorophosphate (TFFH), in solvent dioxane at very mild temperature 60 °C for 24 h. The isolated yield of the SM coupled product is found to be 75%. In this work, they have widely studied the scope of deoxygenative borylation but apart from the single example of the SM reaction the scope was not studied much. But, the optimized conditions are apt for the SM coupling of the phenols with aryl boronic acids via C-O bond activation. For the coupling between phenols and aryl boronic acids, initially, the C-O bond of phenol was activated using TFFH using the base K3PO4, under a nitrogen atmosphere. Then the uronium activated phenol was subjected to coupling by a nickel source which was further continued under the nitrogen atmosphere. Ultimately, the reaction conditions optimized for the cross coupling are comparatively mild when compared to all the reported procedures wherein the temperature is very mild around 60 °C in which the reflux conditions of the respective solvents are observed in most of the previous reports. Further studies on this particular reaction are highly appreciated. Even the chemistry of phenol activated by the uronium based reagents is the first example.

Regarding the mechanistic aspects, the plausible mechanism as proposed by the authors starts with the reaction of TFFH with the phenol in the presence of a base to form the O-phenyl-uronium by releasing KF. The ligand exchange happens between the PCy3 with Ni(cod)2 to form Ni(PCy3)2 complex. The Ni-ligand complex then reacts with the O-Phenyl uronium complex to undergo oxidative addition and undergoes deoxygenation along with the recombination of F to the nickel complex. Thus obtained Ni-complex undergoes dissociation of one ligand followed by transmetalation and reductive elimination to form the product.

Section snippets

Via C-N bond activation

Given the advantages that nickel catalysis may have, it is not unexpected that numerous academic and corporate groups have worked to create novel nickel-based techniques. Additionally, nickel has been deliberately used in SM coupling, aminations, Heck reactions, and reductive couplings (Magano and Monfette, 2015; Campeau and Hazari, 2019). Along with work in C-H and C-F bond activation (Shiota et al., 2011; Nohira et al., 2020), other prominent examples of nickel catalysis include its

Conclusion and outlook

Realizing the importance of Ni-catalysis, the present review summarizes the recent developments in the contribution of Ni-catalysis to the highly important SM coupling reaction. C-O and C-N bond activation for the C-C bond activation is an intellectual aspect that breaks the origins of traditional SM coupling. Although the developments in the Pd-catalysis concerning SM coupling of the same fashion are encouraging but suffers from the high cost of various commercially available Pd catalysts. In

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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