Unified Strategy for Nickel-Catalyzed C-2 Alkylation of Indoles through Chelation Assistance

Regioselective Difluoroalkylation of 2-Pyridones with Fluoroalkyl Bromides Enabled by a Nickel(II) Catalyst

Regioselective C-H difluoroalkylation of diverse 2-pyridones with ethyl bromodifluoroacetates and bromodifluoroacetamides is accomplished by using a (dppf)NiCl2 catalyst under mild conditions. This efficient protocol could deliver a variety of C-3 difluoroalkylated pyridones with the tolerance of a range of highly susceptible functionalities, such as -Cl, -Br, -I, -COMe, -CN, -NMe2 and -NO2, including heteroarenes like pyridinyl, furanyl, thiophenyl and carbazolyl moieties. A preliminary mechanistic study suggests the radical pathway for the reaction involving fluoroalkyl radical intermediate.

Speciation and Reactivity of Mono- and Binuclear Ni Intermediates in Aminoquinoline-Directed C-H Arylation and Benzylation

This paper describes detailed organometallic studies of the aminoquinoline-directed Ni-catalyzed C-H functionalization of 2,3,4,5-tetrafluoro-N-(quinolin-8-yl)benzamide with diaryliodonium reagents. A combination of 19F NMR spectroscopy and X-ray crystallography is used to track and characterize diamagnetic and paramagnetic intermediates throughout this transformation. These provide key insights into both the cyclometalation and oxidative functionalization steps of the catalytic cycle. The reaction conditions (solvent, ligands, base, and stoichiometry) play a central role in the observation of a NiII precyclometalation intermediate as well as in the speciation of the NiII products of C-H activation. Both mono- and binuclear cyclometalated NiII species are observed and interconvert, depending on the reaction conditions. Cyclic voltammetry reveals that the NiII/III redox potentials for the cyclometalated intermediates vary by more than 700 mV depending on their coordination environments, and these differences are reflected in their relative reactivity with diaryliodonium oxidants. The oxidative functionalization reaction affords a mixture of arylated and solvent functionalization organic products, depending on the conditions and solvent. For example, conducting oxidation in toluene leads to the preferential formation of the benzylated product. A series of experiments implicate a NiII/III/IV pathway for this transformation.

Reactions of cyclonickelated complexes with hydroxylamines and TEMPO˙: isolation of new TEMPOH adducts of Ni(II) and their reactivities with nucleophiles and oxidants

This contribution describes a study on the reactivities of the complexes [{κP,κC-(i-Pr)2PO-Ar}Ni(μ-Br)]2, 1a-d (Ar: C6H4, a; 3-Cl-C6H3, b; 3-OMe-C6H3, c; 4-OMe-napthalenyl, d), with hydroxylamines in the presence of TEMPO˙ (TEMPO˙ = (2,2,6,6-tetramethylpiperidinyl-1-yl)oxyl). The results of this study showed that treating 1a-d with a mixture of Et2NOH and TEMPO˙ did not afford the desired oxidation-induced functionalization of the Ni-aryl moiety in 1a-d, giving instead the corresponding κO-TEMPOH adducts [{κP,κC-(i-Pr)2PO-Ar}Ni(Br)(κO-TEMPOH)], 3a-d (TEMPOH = N-hydroxy-2,2,6,6-tetramethylpiperidine). The TEMPOH moiety in these zwitterionic compounds 3 can be displaced by a large excess of acetonitrile (MeCN), 10 equiv. of morpholine, or 1-2 equivalents of imidazole. Although these reactions have given the authenticated products [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(NCMe)], 4a, [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(morpholine)], 5a, and [{κP,κC-(i-Pr)2PO-C6H4}Ni(imidazole)2]Br, 6a, a few other products were also detected by NMR, indicating that the observed reactivities are far more complex than simple substitution of the TEMPOH moiety. Similarly, treating 3a with AgOC(O)CF3 results in the isolation of [{κP,κC-(i-Pr)2PO-C6H4}Ni{OC(O)CF3}(κO-TEMPOH)], 7a, arising from the substitution of the bromo ligand, whereas spectroscopic evidence suggests further reactivity, possibly including displacement of TEMPOH and oxidative decomposition.

Pd(II) complexes bearing NNS pincer ligands: unveiling potent cytotoxicity against breast and pancreatic cancer

The continuously increasing rate of breast cancer is one of the major threats to female health worldwide. Recently, palladium complexes have emerged as impressive candidates with effective biocompatibility and anticancer activities. Hence, in the present study, we report a new series of palladium complexes bearing NNS pincer ligands for cytotoxicity studies. The reaction of thiophenol/4-chlorothiophenol/4-methylthiophenol/4-methoxythiophenol with 2-bromo-N-quinolin-8-yl-acetamide in the presence of sodium hydroxide in ethanol at 80 °C gave [C9H6N-NH-C(O)-CH2-S-Ar] [Ar = C6H5 (L1), C6H4Cl-4 (L2), C6H4Me-4 (L3), and C6H4-OMe-4 (L4)]. The corresponding reaction of L1-L4 with Na2PdCl4 in methanol at room temperature for 3 h resulted in complexes [(L1-H)PdCl] (C1), [(L2-H)PdCl] (C2), [(L3-H)PdCl] (C3), and [(L4-H)PdCl] (C4). All new compounds have been characterized by spectroscopic analyses. The structures of complexes C1, C3, and C4 have also been determined from single-crystal X-ray diffraction data. The cytotoxicities of L1-L4 and C1-C4 have been investigated for breast cancer 4T1 and pancreatic cancer MIA-PaCa-2 cells. The IC50 values for complexes C2 and C3 were observed to be comparable to or higher than those of cisplatin. The stressed morphology and cell death of cancerous cells were successfully observed through cellular morphology analysis and the assessment of cytoskeleton damage.

Recent Advances in the Nickel-Catalyzed Alkylation of C-H Bonds

Functionalization of C-H bonds has emerged as a powerful strategy for converting inert, nonfunctional C-H bonds into their reactive counterparts. A wide range of C-H bond functionalization reactions has become possible by the catalysis of metals, typically from the second row of transition metals. First-row transition metals can also catalyze C-H functionalization, and they have the merits of greater earth-abundance, lower cost and better environmental friendliness in comparison to their second-row counterparts. C-H bond alkylation is a particularly important C-H functionalization reaction due to its chemical significance and its applications in natural product synthesis. This review covers Ni-catalyzed C-H bond alkylation reactions using alkyl halides and olefins as alkyl sources.

Iridium catalysed C2 site-selective methylation of indoles using a pivaloyl directing group through weak chelation-assistance

Here we present an iridium catalysed C2-selective methylation of indoles using methyltrifluoroborate as a source of methyl group. The iridium catalyst selectively discriminates the indole C2 and C4 C-H bonds by coordination with a pivaloyl directing group.

Nickel-Catalyzed Asymmetric Hydrogenation for the Synthesis of a Key Intermediate of Sitagliptin

Nickel-catalyzed enantioselective hydrogenation of enamines leading to the efficient synthesis of 3-R-Boc-amino-4-(2,4,5-trifluorophenyl)butyric esters, the key intermediate of the blockbuster antidiabetic drug (R)-SITAGLIPTIN, is described. The sitagliptin motifs were isolated in more than 99% yield and with 75-92% ee using the earth-abundant nickel catalyst. Upon chiral resolution with (R)- and (S)-1-phenylethylamines, the partially enantioenriched (R)- and (S)-Boc-3-amino-4-(2,4,5-trifluorophenyl)butanoic acids provided >99.5% ee of the crucial sitagliptin intermediate. The asymmetric hydrogenation protocol was scaled up to 10 g with consistency in yield and ee, and has been reproduced in multiple batches.

Cyclometalated (NNC)Ru(II) complex catalyzed β-methylation of alcohols using methanol

Indolyl fragment containing phenanthroline based new ligands and their corresponding Ru(II) complexes were synthesized and fully characterized by various spectroscopic techniques. Catalytic activity of these newly synthesized cyclometalated (NNC)Ru(II) complexes...

Room temperature Z-selective hydrogenation of alkynes by hemilabile and non-innocent (NNN)Co(ii) catalysts

Room temperature chemo- and stereoselective hydrogenation of alkynes is described using a well-defined and phosphine-free hemilabile cobalt catalyst.

Nickel-catalyzed deaminative Sonogashira coupling of alkylpyridinium salts enabled by NN2 pincer ligand

Alkynes are amongst the most valuable functional groups in organic chemistry and widely used in chemical biology, pharmacy, and materials science. However, the preparation of alkyl-substituted alkynes still remains elusive. Here, we show a nickel-catalyzed deaminative Sonogashira coupling of alkylpyridinium salts. Key to the success of this coupling is the development of an easily accessible and bench-stable amide-type pincer ligand. This ligand allows naturally abundant alkyl amines as alkylating agents in Sonogashira reactions, and produces diverse alkynes in excellent yields under mild conditions. Salient merits of this chemistry include broad substrate scope and functional group tolerance, gram-scale synthesis, one-pot transformation, versatile late-stage derivatizations as well as the use of inexpensive pre-catalyst and readily available substrates. The high efficiency and strong practicability bode well for the widespread applications of this strategy in constructing functional molecules, materials, and fine chemicals.

Alkynes are amongst the most valuable functional groups in organic chemistry, however, the preparation of alkyl-substituted alkynes still remains elusive. Here the authors show a nickel-catalyzed deaminative Sonogashira coupling of alkylpyridinium salts.

Silver-Catalyzed Direct Synthesis of Trifluoromethylated Enaminopyridines and Isoquinolinones Employing Trifluorodiazoethane

This Letter reports a Ag-catalyzed three-component approach for the N-alkenylation of 2-aminopyridines employing aldehydes and trifluorodiazoethane. Unlike the known reactions of trifluorodiazoethane with imines, which generate Mannich adducts, aziridines, or triazolines depending on the substrates and conditions, this reaction, after Mannich addition, proceeds via a carbene formation and 1,2-aryl migration sequence to afford (E)-enaminopyridines. This surprising selectivity, which is effective for a wide range of aldehydes and 2-aminopyridines, has been subsequently explored to access trifluoromethylated isoquinolinones.

Nickel-Catalyzed C-H Bond Functionalization of Azoles and Indoles

Direct C-H functionalization of privileged and biologically relevant azoles and indoles represents an important chemical transformation in molecular science. Despite significant progress in the palladium-catalyzed regioselective C-H functionalization of azoles and indoles, the use of abundant and less expensive nickel catalyst is underdeveloped. In the recent past, the nickel-catalyzed regioselective C-H alkylation, arylation, alkenylation and alkynylation of azoles and indoles have been substantially explored, which can be applied to the complex organic molecule synthesis. In this Account, we summarize the developments in nickel-catalyzed regioselective functionalization of azoles and indoles with a considerable focus on the reaction mechanism.

Achiral and chiral NNN-pincer nickel complexes with oxazolinyl backbones: application in transfer hydrogenation of ketones

NNN-based achiral and chiral (oxazolinyl)amido-pincer nickel complexes are developed and employed for the catalytic transfer hydrogenation of ketones.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.

Removal and modification of directing groups used in metal-catalyzed C–H functionalization: the magical step of conversion into ‘conventional’ functional groups

This feature review is focused on recent approaches for removing versatile directing groups.

Nickela-electrocatalyzed Mild C-H Alkylations at Room Temperature

Direct alkylations of carboxylic acid derivatives are challenging and particularly nickel catalysis commonly requires high reaction temperatures and strong bases, translating into limited substrate scope. Herein, nickel-catalyzed C-H alkylations of unactivated 8-aminoquinoline amides have been realized under exceedingly mild conditions, namely at room temperature, with a mild base and a user-friendly electrochemical setup. This electrocatalyzed C-H alkylation displays high functional group tolerance and is applicable to both the primary and secondary alkylation. Based on detailed mechanistic studies, a nickel(II/III/I) catalytic manifold has been proposed.

MnBr2-Catalyzed Direct and Site-Selective Alkylation of Indoles and Benzo[h]quinoline

Manganese-catalyzed regioselective C-H alkylation of indoles and benzo[h]quinoline with a variety of unactivated alkyl iodides is reported. Unlike other Mn-catalyzed C-H functionalization, this protocol does not require a Grignard reagent base and employs a simple and inexpensive MnBr₂ as a catalyst. This method tolerates diverse functionalities, including fluoro, chloro, bromo, iodo, alkenyl, alkynyl, pyrrolyl, and carbazolyl groups. The alkylation proceeds through a single-electron transfer pathway comprising reversible C-H manganesation and involving an alkyl radical intermediate.

Synthesis of 2,3-disubstituted indoles via a tandem reaction

A wide array of 2,3-disubstituted indoles were accessed in modest to good yields via a tandem reduction/condensation/fragmentation/cyclization sequence. Differential fragmentation made the reaction more complicated.

The copper(ii)-catalyzed and oxidant-promoted regioselective C-2 difluoromethylation of indoles and pyrroles

A novel and efficient approach for the highly selective C-2 difluoromethylation of indole derivatives was developed by using sodium difluoromethylsulfinate (HCF₂SO₂Na) as the source of difluoromethyl groups and a Cu(ii) complex as the catalyst.

Nickel-catalyzed C-H alkylation of indoles with unactivated alkyl chlorides: evidence of a Ni(i)/Ni(iii) pathway

A mild and efficient nickel-catalyzed method for the coupling of unactivated primary and secondary alkyl chlorides with the C-H bond of indoles and pyrroles is described which demonstrates a high level of chemo and regioselectivity. The reaction tolerates numerous functionalities, such as halide, alkenyl, alkynyl, ether, thioether, furanyl, pyrrolyl, indolyl and carbazolyl groups including acyclic and cyclic alkyls under the reaction conditions. Mechanistic investigation highlights that the alkylation proceeds through a single-electron transfer (SET) process with Ni(i)-species being the active catalyst. Overall, the alkylation follows a Ni(i)/Ni(iii) pathway involving the rate-influencing two-step single-electron oxidative addition of alkyl chlorides.

Nickel-Catalyzed C(2)-H Arylation of Indoles with Aryl Chlorides under Neat Conditions

Nickel-catalyzed regioselective C(2)-H arylation of indoles and pyrroles with aryl chlorides is achieved under neat conditions. This method allows the efficient coupling of diverse aryl chlorides employing a user-friendly and inexpensive Ni(OAc)₂/dppf catalyst system at 80 °C. Numerous functionalities, such as halides, alkyl ether, fluoro-alkyl ether, and thioether, and substituted amines, including heteroarenes like benzothiazolyl, pyrrolyl, indolyl, and carbazolyl, are well tolerated under the reaction conditions. The preliminary mechanistic study highlights a single-electron transfer (SET) pathway for the arylation reaction.

Benzothiazole Nickelation: An Obstacle to the Catalytic Arylation of Azoles by Cyclopentadienyl Nickel N-Heterocyclic Carbene Complexes

NiCp†L(NHC)](+) complexes (Cp† = Cp (η5-C5H5), Cp* (η5-C5Me5); NHC = N-heterocyclic carbene; L = Cl− or NCMe) have been tested as pre-catalysts for the direct arylation of benzothiazole in the presence of an alkoxide. Only the pentamethylcyclopentadienyl derivative, [NiCp*Cl(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenylimidazol-2-ylidene), enabled low conversion to the desired coupling product with phenyl iodide as the electrophilic coupling partner. In contrast, all cyclopentadienyl complexes proved to be inactive. 1H NMR studies of the “catalytic” reaction mixtures demonstrate that they cleanly convert to an unreactive C(2)-benzothiazolyl derivative, whose identity has been confirmed by an independent synthesis and characterization. The latter constitutes a potential energy well that quenches all further reactivity, and provides a rare example of C(2)-metallated azolyl complex.

Cobalt-Catalyzed Direct C-H Thiolation of Aromatic Amides with Disulfides: Application to the Synthesis of Quetiapine

A direct C(sp²)-H thiolation of aromatic amides with disulfides was developed. The coupling reaction proceeds between the thioether radical and cobaltacycle intermediate. This method exhibits a relatively broad substrate scope and high functional group compatibility. A mechanistic study indicates that the cobalt(IV) intermediate is probably formed during the course of the reaction. The thiolation product can be transformed to Quetiapine, which is an atypical antipsychotic agent approved for the treatment of schizophrenia and bipolar disorder.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Cobalt-catalyzed C(sp3)-H/C(sp2)-H oxidative coupling between alkanes and benzamides

A direct cobalt-catalyzed oxidative coupling between C(sp2)-H in unactivated benzamides and C(sp3)-H in simple alkanes, ethers and toluene derivatives was explored. This protocol achieves direct C-C formation without using alkyl or aryl halide surrogates and exhibits high practicality with ample substrate scope. The method provides a new way to construct linear and five- or six-membered ring moieties in bioactive molecules.

Nickel-catalyzed direct C-H trifluoroethylation of heteroarenes with trifluoroethyl iodide

A highly selective nickel-catalyzed C-H trifluoroethylation of heteroarenes was developed with the assistance of a monodentate directing group. This protocol provides efficient access to various trifluoroethyl-substituted heteroarenes, including indoles, pyrroles, furans, and thiophenes, with commercially available CF₃CH₂I as an alkylation reagent. This robust catalytic procedure is scalable and tolerates a broad range of functional groups. Moreover, multifluoroalkylation of indoles is also viable.