Palladium-Catalyzed Direct Dehydrogenative Annulation of Ferrocenecarboxamides with Alkynes in Air

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.

Rhodium(III)-Catalyzed C(sp2)-H Chemoselective Annulation to O-Cyclized Isochromen-imines from Benzamides

Through the development of ligands and reaction conditions, the Rh(III)-catalyzed selective annulation of benzamides with internal alkynes has been achieved to the formation of O-cyclized isochromen-imines. Various substituents are well-tolerated under mild reaction conditions. Density functional theory calculations indicate that silver carbonate could act as a Lewis acid to assist the ligand to improve the chemical selectivity of the reaction in a catalytic system.

Fast construction of isoquinolin-1(2H)-ones by direct intramolecular C-H/N-H functionalization under metal-free conditions

The general protocol for the synthesis of isoxazolidine-fused isoquinolin-1(2H)-ones was established with the help of bench stable hypervalent iodine reagent PIDA. Polycyclic six-, seven- and eight-membered N-heterocycles can be rapidly synthesized from available amides under metal-free conditions within 1 min at room temperature through C-H/N-H functionalization. Moreover, the protocol has the merits of broad substrate scope, atom economy and operational simplicity.

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.

Micellar Catalysis for Ruthenium(II)-Catalyzed C-H Arylation: Weak-Coordination-Enabled C-H Activation in H2 O

Chemoselective C-H arylations were accomplished through micellar catalysis by a versatile single-component ruthenium catalyst. The strategy provided expedient access to C-H-arylated ferrocenes with wide functional-group tolerance and ample scope through weak chelation assistance. The sustainability of the C-H arylation was demonstrated by outstanding atom-economy and recycling studies. Detailed computational studies provided support for a facile C-H activation through thioketone assistance.

Thiocarbonyl-enabled ferrocene C-H nitrogenation by cobalt(III) catalysis: thermal and mechanochemical

Versatile C–H amidations of synthetically useful ferrocenes were accomplished by weakly-coordinating thiocarbonyl-assisted cobalt catalysis. Thus, carboxylates enabled ferrocene C–H nitrogenations with dioxazolones, featuring ample substrate scope and robust functional group tolerance. Mechanistic studies provided strong support for a facile organometallic C–H activation manifold.

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.

Synthesis of Planar Chiral Ferrocenes via Transition-Metal-Catalyzed Direct C-H Bond Functionalization

Ferrocenes are of great interest in the fields of materials science, organic synthesis, and biomedical research. Of particular significance is the fact that ferrocenes bearing planar chirality have been demonstrated to be highly efficient ligands or catalysts in asymmetric catalysis, some of which have been employed in the industrial synthesis of pharmaceuticals and agrochemicals. So far, the main methods for the synthesis of planar chiral ferrocenes involve diastereoselective directed ortho-metalation (DoM), enantioselective DoM, and chiral resolution. Despite the fact that these approaches are well developed and widely applied, the use of chiral auxiliaries or external stoichiometric chiral bases is required in most cases. Additionally, the practicality of these processes is hampered by the requirement of sensitive organometallic reagents, the poor compatibility with functional groups, and the low atom economy in some cases. Therefore, the development of highly efficient strategies to introduce planar chirality on the backbone of ferrocene that do not possess these limitations is highly desirable. Meanwhile, transition-metal-catalyzed asymmetric C-H bond functionalization reactions have attracted much attention over the past few years owing to their emerging potential for providing a straightforward approach for the preparation of chiral molecules. In addition to the majority of the work focusing on the installation of central chirality, methods for the catalytic asymmetric synthesis of planar chiral compounds via C-H bond functionalization have also been explored. In this Account, we summarize our recent efforts aimed at the development of novel methods to synthesize planar chiral compounds via asymmetric C-H bond functionalization and also highlight related achievements by other groups. First, we briefly introduce the precedent examples of diastereoselective and enantioselective synthesis of planar chiral ferrocenes. Subsequently, asymmetric syntheses of structurally diverse planar chiral ferrocenes via Pd [Pd(II), Pd(0)]-, Ir-, Rh-, Au-, and Pt-catalyzed C-H bond functionalization are described. These methods have impressive advantages over traditional approaches for the synthesis of functionalized planar chiral ferrocenes in terms of both step- and atom-economies. Notably, the products of these processes are easily transformed into a variety of new catalysts or ligands, which have been demonstrated to promote efficient asymmetric reactions. Moreover, DFT calculations have been conducted to explore the origin of the excellent enantioselectivity of Pd-catalyzed enantioselective C-H bond functionalization reactions. Progress made in the area of asymmetric C-H bond functionalization provides an effective platform for the design and synthesis of planar chiral ferrocenes.