Palladium‐Catalyzed CH Functionalization of Ferrocenecarboxylic Acid by using 8‐Aminoquinoline as a Removable Directing Group

Direct esterification of amides by the dimethylsulfate-mediated activation of amide C-N bonds

Amides are important intermediates in organic chemistry and the pharmaceutical industry, but their low reactivity requires catalysts and/or severe reaction conditions for esterification. Here, a novel approach was devised to convert amides into esters without the use of transition metals. The method effectively overcomes the inherent low reactivity of amides by employing dimethylsulfate-mediated reaction to activate the C-N bonds. To confirm the proposed reaction mechanism, control experiments and density functional theory (DFT) calculations were conducted. The method demonstrates a wide array of substrates, including amides with typical H/alkyl/aryl substitutions, N,N-disubstituted amides, amides derived from alkyl, aryl, or vinyl carboxylic acids, and even amino acid substrates with stereocentres. Furthermore, we have shown the effectiveness of dimethylsulfate in removing acyl protective groups in amino derivatives. This study presents a method that offers efficiency and cost-effectiveness in broadening the esterification capabilities of amides, thereby facilitating their increased utilization as synthetic compounds in diverse transformations.

Palladium-Catalyzed Direct C(sp2)-H Cyanomethylation of Arylamides using Chloroacetonitrile

In this study, we devised a palladium-catalyzed efficient and versatile method for C(sp2)-H ortho-cyanomethylation of arylamides with a broad substrate scope and moderate to excellent yields. An inexpensive and commercially available chloroacetonitrile was employed as a cyanomethylating source. This method is also compatible with the air atmosphere. Further, the synthetic feasibility of this technique is established by gram-scale synthesis and functional group transformation of the products.

Recent Advances in Transition-Metal-Catalyzed C-H Functionalization of Ferrocene Amides

During the past decades, in synthetic organic chemistry, directing-group-assisted C-H functionalization is found to be a key tool for the expedient and site-selective construction of C-C and hybrid bonds. Among C-H functionalization of ferrocene derivatives, the directed group strategy is undoubtedly the most commonly used method. Compared to the other directing groups, ferrocene amides can be synthesized easily and are now recognized as one of the most efficient devices for the selective functionalization of certain positions because its metal centre permits fine, tuneable and reversible coordination. The family of amide directing groups mainly comprises monodentate and bidentate directing groups, which are categorized on the basis of coordination sites. In this review, various C-H bond functionalization reactions of ferrocene using amide directing groups are broadly discussed.

Tetravalent Spiroselenurane Catalysts: Intramolecular Se···N Chalcogen Bond-Driven Catalytic Disproportionation of H2O2 to H2O and O2 and Activation of I2 and NBS

Chalcogen-bonding interactions have recently gained considerable attention in the field of synthetic chemistry, structure, and bonding. Here, three organo-spiroselenuranes, having a Se(IV) center with a strong intramolecular Se···N chalcogen-bonded interaction, have been isolated by the oxidation of the respective bis(2-benzamide) selenides derived from an 8-aminoquinoline ligand. Further, the synthesized spiroselenuranes, when assayed for their antioxidant activity, show disproportionation of hydrogen peroxide into H₂O and O₂ with first-order kinetics with respect to H₂O₂ for the first time by any organoselenium molecules as monitored by ¹H NMR spectroscopy. Electron-donating 5-methylthio-benzamide ring-substituted spiroselenurane disproportionates hydrogen peroxide at a high rate of 15.6 ± 0.4 × 10³ μM min^-1 with a rate constant of 8.57 ± 0.50 × 10^-3 s^-1, whereas 5-methoxy and unsubstituted-benzamide spiroselenuranes catalyzed the disproportionation of H₂O₂ at rates of 7.9 ± 0.3 × 10³ and 2.9 ± 0.3 × 10³ μM min^-1 with rate constants of 1.16 ± 0.02 × 10^-3 and 0.325 ± 0.025 × 10^-3 s^-1, respectively. The evolved oxygen gas from the spiroselenurane-catalyzed disproportion of H₂O₂ has also been confirmed by a gas chromatograph-thermal conductivity detector (GCTCD) and a portable digital polarographic dissolved O₂ probe. Additionally, the synthesized spiroselenuranes exhibit thiol peroxidase antioxidant activities for the reduction of H₂O₂ by a benzenethiol co-reductant monitored by UV-visible spectroscopy. Next, the Se···N bonded spiroselenuranes have been explored as catalysts in synthetic oxidation iodolactonization and bromination of arenes. The synthesized spiroselenurane has activated I₂ toward the iodolactonization of alkenoic acids under base-free conditions. Similarly, efficient chemo- and regioselective monobromination of various arenes with NBS catalyzed by chalcogen-bonded synthesized spiroselenuranes has been achieved. Mechanistic insight into the spiroselenuranes in oxidation reactions has been gained by ⁷⁷Se NMR, mass spectrometry, UV-visible spectroscopy, single-crystal X-ray structure, and theoretical (DFT, NBO, and AIM) studies. It seems that the highly electrophilic nature of the selenium center is attributed to the presence of an intramolecular Se···N interaction and a vacant coordination site in spiroselenuranes is crucial for the activation of H₂O₂, I₂, and NBS. The reaction of H₂O₂, I₂, and NBS with tetravalent spiroselenurane would lead to an octahedral-Se(VI) intermediate, which is reduced back to Se(IV) due to thermodynamic instability of selenium in its highest oxidation state and the presence of a strong intramolecular N-donor atom.

A Guide to Directing Group Removal: 8-Aminoquinoline

The use of directing groups allows high levels of selectivity to be achieved in transition metal-catalyzed transformations. Efficient removal of these auxiliaries after successful functionalization, however, can be very challenging. This review provides a critical overview of strategies used for removal of Daugulis' 8-aminoquinoline (2005-2020), one of the most widely used N,N-bidentate directing groups. The limitations of these strategies are discussed and alternative approaches are suggested for challenging substrates. Our aim is to provide a comprehensive end-users' guide for chemists in academia and industry who want to harness the synthetic power of directing groups-and be able to remove them from their final products.

Synthesis of Chiral-Substituted 2-Aryl-ferrocenes by the Catellani Reaction

A palladium-catalyzed and norbornene-mediated methodology has been developed for the synthesis of chiral 2-aryl-ferroceneamides from chiral 2-iodo-N,N-diisopropylferrocencarboxamide, iodoarenes, and alkenes using a JohnPhos ligand and potassium carbonate as a base in dimethylformamide at 105 °C. The developed three-component coupling protocol allows the compatibility of electron-withdrawing fluoro, chloro, ester, and nitro and electron-donating methyl, methoxy, dimethoxy, benzyl ether-substituted iodo-benzenes, other iodoarenes, such as iodo-naphthalene, heteroarenes, such as iodothiophene, and terminating substrates, such as methyl, ethyl, tert-butyl acrylates, and substituted styrenes with 2-iodo-N,N-diisopropylferrocencarboxamide. Furthermore, the developed three-component Catellani method proceeded with the retention of the configuration of the planar chiral ferrocene, which depends on the role of the participating carbon-iodine bond in ferrocene. Consequently, the developed protocol enabled the formation of densely substituted chiral 2-aryl ferroceneamides, exhibiting good to excellent enantioselectivity. The conversion of an ester of the synthesized chiral 2-aryl ferroceneamides has also been carried out to further accommodate the easily expendable acid and alcohol functionalities.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

8-Aminoquinoline-Assisted Synthesis and Crystal Structure Studies of Ferrocenyl Aryl Sulfones

A copper-catalyzed 8-aminoquinoline-directed oxidative cross-coupling of the C-H bond of ferrocene with sodium arylsulfinates has been achieved. The robust copper catalyst tolerates a range of methyl, tert-butyl, bromo, chloro, iodo and nitro functional groups in the phenyl ring, and set the stage for the synthesis of substituted ferrocene sulfones. Furthermore, X-ray crystal structure study on several ferrocenyl sulfones reveals the tetrahedral geometry around sulfur; interestingly, the O-S-O angle is larger than the electropositive substituent C-S-C angle which could be explained by Bent's rule. Further, unusual intramolecular O(S)⋅⋅⋅N(amide) short contacts (2.925-3) and O(S)⋅⋅⋅C=O were also noticed in ferrocenyl sulfones.

Versatile coordination and C-H activation of a multi-donor phosphinoferrocene carboxamide ligand in Pd(ii) complexes

A multi-donor phosphinoferrocene carboxamide, FcCONHCH₂CH₂PPh₂ (1, Fc = ferrocenyl), was prepared, converted into the corresponding phosphine oxide 1O and phosphine selenide 1Se and, mainly, studied as a ligand in Pd(ii) complexes. In its native form, amide 1 preferentially coordinated soft Pd(ii) as a simple phosphine, giving rise to mixtures of cis and trans-[PdX₂(1-κP)₂] (2; X = Cl (a), Br (b), and I (c)), wherein the isomer ratios depended on the auxiliary halide ligand or, alternatively, to the complex [(L^NC)PdCl(1-κP)] (6, L^NC = 2-[(dimethylamino)methyl-κN]phenyl-κC¹). This coordination mode was nevertheless easily changed when creating a vacant coordination site at the palladium. Thus, treatment of 2a with NH₄[PF₆] in the presence of free 1 produced [PdCl(1-κP)₃][PF₆] (3), while complete halogen removal with a Ag(i) salt led to cationic complexes cis-[Pd(1-κ²O,P)₂]X₂ (4, X = CF₃SO₃ (a), ClO₄ (b), BF₄ (c)) or [(L^NC)Pd(1-κ²O,P)]X (7a and 7b), containing seven-membered O,P-chelate rings. In contrast, amide nitrogen deprotonation with KOt-Bu followed by spontaneous intramolecular halogen substitution resulted in the transformation of 6 into the chelate complex [(L^NC)Pd{(1- H)-κ²N,P}] (8) featuring a five-membered N,P-chelate ring, and in the conversion of 2a and 2b into the product of C-H bond activation [Pd{Fe(η⁵-C₅H₃CONCH₂CH₂PPh₂-κ³C,N,P)(η⁵-C₅H₅)}(1-κP)] (5), with doubly chelating deprotonated 1. Importantly, complexes 2-4-5 and 6-7-8 were mutually interconverted in triads (by protonation/deprotonation and by halide addition/abstraction), which highlights the flexible coordination and chemical stability of ligand 1. The crystal structures of 1O·½H₂O, trans-2a·MeCN, trans-2b·3C₂H₄Cl₂, trans-2c·2.5C₂H₄Cl₂, 4a·CH₂Cl₂, 5·3CHCl₃·Et₂O, and 8 were determined by single-crystal X-ray diffraction analysis, and the representative compounds were studied by cyclic voltammetry.

Ferrocenyl palladacycles derived from unsymmetrical pincer-type ligands: evidence of Pd(0) nanoparticle generation during the Suzuki-Miyaura reaction and applications in the direct arylation of thiazoles and isoxazoles

A new family of ferrocenyl-palladacycle complexes Pd(L¹)Cl (Pd1) and Pd(L²)Cl (Pd2) were synthesized and characterized by UV-visible, IR, ESI-MS, and NMR spectral studies. The molecular structures of Pd1 and Pd2 were determined by X-ray crystallographic studies. Palladacycle catalyzed Suzuki-Miyaura cross-coupling reactions were investigated utilizing the derivatives of phenylboronic acids and substituted chlorobenzenes. Mechanistic investigation authenticated the generation of Pd(0) nanoparticles during the catalytic cycle and the nanoparticles were characterized by XPS, SEM and TEM analysis. Direct C-H arylation of thiazole and isoxazole derivatives employing these ferrocenyl-palladacycle complexes was examined. The reaction model for the arylation reaction implicating the in situ generation of Pd(0) nanoparticles was proposed.

Palladium-Catalyzed C-H Diarylation of Ferrocenecarboxylic Acids with Aryl Iodides

Palladium-catalyzed C-H diarylation of ferrocenes is described. In the presence of 10 mol % Pd(OAc)₂, direct C-H diarylation reactions of commercially available ferrocenecarboxylic acid with aryl iodides proceeded smoothly to afford diarylated ferrocenes bearing a variety of functional groups in moderate to good yields. The carboxylic group could also act as a remote directing group to result in the third arylation on the other Cp ring of ferrocene.

Nickel-catalyzed oxidative C-H/N-H annulation of N-heteroaromatic compounds with alkynes

The reaction of N-heteroaromatic compounds, such as 2-aryl-pyrrole, benzimidazole, imidazole, indole, and pyrazole derivatives, with alkynes in the presence of a catalytic amount of a nickel complex results in C-H/N-H oxidative annulation. The reaction shows a high functional group compatibility. While both Ni(0) and Ni(ii) complexes show a high catalytic activity, Ni(0) is proposed as a key catalytic species in the main catalytic cycle. In the case of the Ni(ii) system, the presence of a catalytic amount of a strong base, such as KOBu t , is required for the reaction to proceed. In sharp contrast, a base is not required in the case of the Ni(0) system. The proposed mechanism is supported by DFT studies.

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.

Synthesis of unsymmetrical multi-aroyl derivatives of ferrocene using palladium catalysed oxidative C–H aroylation

Palladium catalysed oxidative C–H aroylation of ferrocene: accessing the multi-aroyl derivatives of ferrocene.

Palladium-Catalyzed Removable 8-Aminoquinoline Assisted Chemo- and Regioselective Oxidative sp2-C-H/sp3-C-H Cross-Coupling of Ferrocene with Toluene Derivatives

The coupling of the C(sp²)-H bond of ferrocene with toluene avoiding in situ functionalization of both coupling partners has been achieved using palladium and iron dual catalysis. This cross oxidative coupling features regioselectivity to primary and secondary C(sp³)-H bonds and chemoselectivity toward the -C(sp³)-H over C-I bond of toluene with gram-scale production. It seems Fe(II) catalyst not only stabilizes the generated benzyl radical but also tames its oxidizing behavior, and consequently transfers it to the palladacycle for C-C coupling.

Borylation, silylation and selenation of C–H bonds in metal sandwich compounds by applying a directing group strategy

Carbon–heteroatom bond formation in metal-sandwich compounds using C–H activation by selective directing groups.

Palladium-catalyzed direct C–H arylation of ferrocenecarboxamides with aryl halides

A simple and facile protocol for palladium-catalyzed ortho-arylation of ferrocenecarboxamides with aryl halides was developed with the assistance of the bidentate directing group.