Iron-catalysed enantioconvergent Suzuki-Miyaura cross-coupling to afford enantioenriched 1,1-diarylalkanes

Electrochemical Enantioselective Nickel-Catalyzed Cross-Coupling of Aldehydes with Aryl Iodides

The preparation of enantioenriched diarylmethanol derivatives is described using nickel-catalyzed electrochemical cross-couplings between various alkyl/aryl aldehydes and aryl iodides. Performed in an electrochemical cell equipped with an iron anode and a nickel cathode, this electrocatalytic variant led to the scalemic targeted products in the presence of 2,2-bis((4 R,5S)-4,5-diphenyl-4,5-dihydrooxazol-2-yl)acetonitrile (L2), as enantiopure cyano-bis(oxazoline) ligand. X-ray structure analysis of a pre-catalyst, for instance the [Ni(II)(L2)2] complex, with L2 as an anionic bisoxazolinate ligand, confirms the chemical formulation of one nickel surrounded by two ligands. The redox behavior of the new Ni complexes generated in situ was first assessed by cyclic voltammetry showing a redox wave at ca. -1.5 V that can be assigned to the two-electron reduction of the Ni(II) center to the Ni(0) state. Oxidative addition between the electrogenerated Ni(0) complex and aryl iodide was evidenced. An intense current was observed in presence of a mixture of the two substrates pertaining an electrocatalytic process. Interestingly, we found that the sacrificial iron anode plays a crucial role in the catalytic mechanism.

Recent progress in asymmetric radical reactions enabled by chiral iron catalysts

Transition-metal-catalyzed radical asymmetric reactions offer a versatile and effective platform for accessing chiral organic molecules with high enantiopurity. Given that iron is the most abundant and less toxic transition metalic element available, the application of iron catalysts is considered to be a more sustainable and attractive approach. Over the last decade, several exciting and notable achievements have been witnessed. In this highlight, we aim to provide an overview of the progress in ligand-enabled iron-catalyzed asymmetric radical reactions, with an emphasis on the reaction mechanisms.

Recyclable ionic iron-substituted ceria: a precious metal-free, ligand-free, and versatile catalyst for C-C coupling and ipso-hydroxylation of arylboronic acid

Fluorite-type nanocrystalline 5% iron-substituted ceria (Ce0.95Fe0.05O2-δ) was synthesized by the solution combustion method with an average crystallite size of 15 nm. The catalyst was well characterized by SEM, EDX, XPS, XRD, and H2-TPR studies, which confirmed the substitution of Fe3+ in the CeO2 lattice. This precious metal-free catalyst was developed for the selective conversion of various arylboronic acids to the corresponding biphenyls and phenols employing K2S2O8 and H2O2, respectively. This catalyst could be coated on cordierite monolith, which acts as a catalyst cartridge employed efficiently in recycling experiments up to seven cycles. The scale-up reaction was also carried out with a catalyst cartridge, affording a good yield of the desired product.

A Rationally Designed Iron(II) Catalyst for C(sp3)-C(sp2) and C(sp3)-C(sp3) Suzuki-Miyaura Cross-Coupling

Despite the paramount importance of the Suzuki-Miyaura coupling (SMC) in academia and industry, and the great promise of iron to offer sustainable catalysis, iron-catalyzed SMC involving sp3-hybridized partners is still in its infancy. We herein report the development of a versatile, well-defined electron-deficient anilido-aldimine iron(II) catalyst. This catalyst effectively performed C(sp3)-C(sp2) and C(sp3)-C(sp3) SMC of alkyl halide electrophiles and (hetero)aryl boronic ester and alkyl borane nucleophiles respectively, in the presence of a lithium amide base. These couplings operated under mild reaction conditions and displayed wide functional group compatibility including various medicinally relevant N-, O- and S-based heterocycles. They also tolerated primary, secondary and tertiary alkyl halides (Br, Cl, I), electron-neutral, -rich and -poor boronic esters and primary and secondary alkyl boranes. Our methodology could be directly and efficiently applied to synthesize key intermediates relevant to pharmaceuticals and a potential drug candidate. For C(sp3)-C(sp2) couplings, radical probe experiments militated in favor of a carbon-centered radical derived from the electrophile. At the same time, reactions run with a pre-formed activated boron nucleophile coupled to competition experiments supported the involvement of neutral, rather than an anionic, (hetero)aryl boronic ester in the key transmetalation step.

Iron-catalyzed stereoselective C-H alkylation for simultaneous construction of C-N axial and C-central chirality

The assembly of chiral molecules with multiple stereogenic elements is challenging, and, despite of indisputable advances, largely limited to toxic, cost-intensive and precious metal catalysts. In sharp contrast, we herein disclose a versatile C-H alkylation using a non-toxic, low-cost iron catalyst for the synthesis of substituted indoles with two chiral elements. The key for achieving excellent diastereo- and enantioselectivity was substitution on a chiral N-heterocyclic carbene ligand providing steric hindrance and extra represented by noncovalent interaction for the concomitant generation of C-N axial chirality and C-stereogenic center. Experimental and computational mechanistic studies have unraveled the origin of the catalytic efficacy and stereoselectivity.

Fenton-mediated thermocatalytic conversion of CO2 to acetic acid by industrial waste-derived magnetite nanoparticles

Iron oxide dust, discarded as industrial waste, has been used here to fabricate magnetic iron oxide nanoparticles (Fe3O4-NPs). We have proposed the thermo-catalytic reduction of carbon dioxide (CO2) using Fe3O4-NPs in the presence of H2O2 to get acetic acid (AcOH) at near ambient conditions (100 °C, 10 bar) with a maximum yield of ∼0.4 M in a batch-reactor. The importance of H2O2 can be described as it facilitates the production of higher concentrations of OH˙ and H+/˙, which consequently supports the synthesis of AcOH.

Iron-Catalyzed Enantioselective Multicomponent Cross-Couplings of α-Boryl Radicals

Despite recent interest in the development of iron-catalyzed transformations, methods that use iron-based catalysts capable of controlling the enantioselectivity in carbon-carbon cross-couplings are underdeveloped. Herein, we report a practical and simple protocol that uses commercially available and expensive iron salts in combination with chiral bisphosphine ligands to enable the regio- and enantioselective (up to 91:9) multicomponent cross-coupling of vinyl boronates, (fluoro)alkyl halides, and Grignard reagents. Preliminary mechanistic studies are consistent with rapid formation of an α-boryl radical followed by reversible radical addition to monoaryl bisphosphine-Fe(II) and subsequent enantioselective inner-sphere reductive elimination. From a broader perspective, this work provides a blueprint to develop asymmetric Fe-catalyzed multicomponent cross-couplings via the use of alkenes as linchpins to translocate alkyl radicals, modify their steric and electronic properties, and induce stereocontrol.

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Mechanistic Investigations of Phenoxyimine-Cobalt(II)-Catalyzed C(sp2)-C(sp3) Suzuki-Miyaura Cross-Coupling

The mechanism of phenoxyimine (FI)-cobalt-catalyzed C(sp2)-C(sp3) Suzuki-Miyaura cross-coupling was studied using a combination of kinetic measurements and catalytic and stoichiometric experiments. A series of dimeric (FI)cobalt(II) bromide complexes, [(4-CF3PhFI)CoBr]2, [(4-OMePhFI)CoBr]2, and [(2,6-diiPrPhFI)CoBr]2, were isolated and characterized by 1H and 19F NMR spectroscopies, solution and solid-state magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, X-ray crystallography, and diffusion-ordered NMR spectroscopy (DOSY). One complex, [(4-CF3PhFI)CoBr]2, was explored as a single-component precatalyst for C(sp2)-C(sp3) Suzuki-Miyaura cross-coupling. Addition of potassium methoxide to [(4-CF3PhFI)CoBr]2 generated the corresponding (FI)cobalt(II) methoxide complex as determined by 1H and 19F NMR and EPR spectroscopies. These spectroscopic signatures were used to identify this compound as the resting state during catalytic C(sp2)-C(sp3) coupling. Variable time normalization analysis (VTNA) of in situ catalytic 19F NMR spectroscopic data was used to establish an experimental rate law that was first-order in a (FI)cobalt(II) precatalyst, zeroth-order in the alkyl halide, and first-order in an activated potassium methoxide-aryl boronate complex. These findings are consistent with turnover-limiting transmetalation that occurs prior to activation of the alkyl bromide electrophile. The involvement of boronate intermediates in transmetalation was corroborated by Hammett studies of electronically differentiated aryl boronic esters. Together, a cobalt(II)/cobalt(III) catalytic cycle was proposed that proceeds through a "boronate"-type mechanism.

Formal Cross-Coupling of Amines and Carboxylic Acids to Form sp3-sp2 Carbon-Carbon Bonds

Amines and carboxylic acids are abundant synthetic building blocks that are classically united to form an amide bond. To access new pockets of chemical space, we are interested in the development of amine-acid coupling reactions that complement the amide coupling. In particular, the formation of carbon-carbon bonds by formal deamination and decarboxylation would be an impactful addition to the synthesis toolbox. Here, we report a formal cross-coupling of alkyl amines and aryl carboxylic acids to form C(sp3)-C(sp2) bonds following preactivation of the amine-acid building blocks as a pyridinium salt and N-acyl-glutarimide, respectively. Under nickel-catalyzed reductive cross-coupling conditions, a diversity of simple and complex substrates are united in good to excellent yield, and numerous pharmaceuticals are successfully diversified. High-throughput experimentation was leveraged in the development of the reaction and the discovery of performance-enhancing additives such as phthalimide, RuCl3, and GaCl3. Mechanistic investigations suggest phthalimide may play a role in stabilizing productive Ni complexes rather than being involved in oxidative addition of the N-acyl-imide and that RuCl3 supports the decarbonylation event, thereby improving reaction selectivity.

Desymmetrization of Vicinal Bis(boronic) Esters by Enantioselective Suzuki-Miyaura Cross-Coupling Reaction

The development of an enantioselective catalytic Suzuki-Miyaura reaction that applies to meso 1,2-diborylcycloalkanes is described. This reaction provides a modular route to enantiomerically enriched substituted carbocycles and heterocycles that retain a synthetically versatile boronic ester. With appropriately constructed substrates, compounds bearing additional stereogenic centers and fully substituted carbon atoms can be generated in a straightforward fashion. Preliminary mechanistic experiments suggest that substrate activation arises from the cooperative effect of vicinal boronic esters during the transmetalation step.

Nickel-Catalyzed Intermolecular Asymmetric Addition of Aryl Iodides across Aldehydes

Enantioenriched alcohols comprise much of the framework of organic molecules. Here, we first report that chiral nickel complexes can catalyze the intermolecular enantioselective addition of aryl iodides across aldehydes to provide diverse optically active secondary alcohols using zinc metal as the reducing agent. This method shows a broad substrate scope under mild reaction conditions and precludes the traditional strategy through the pre-generation of organometallic reagents. Mechanistic studies indicate that an in situ formed arylnickel, instead of an arylzinc, adds efficiently to aldehydes, forming a new C-C bond and a chiral nickel alkoxide that may be turned over by zinc powder.

Iron-catalyzed domino coupling reactions of π-systems

The development of environmentally benign, inexpensive, and earth-abundant metal catalysts is desirable from both an ecological and economic standpoint. Certainly, in the past couple decades, iron has become a key player in the development of sustainable coupling chemistry and has become an indispensable tool in organic synthesis. Over the last ten years, organic chemistry has witnessed substantial improvements in efficient synthesis because of domino reactions. These protocols are more atom-economic, produce less waste, and demand less time compared to a classical stepwise reaction. Although iron-catalyzed domino reactions require a mindset that differs from the more routine noble-metal, homogenous iron catalysis they bear the chance to enable coupling reactions that rival that of noble-metal-catalysis. This review provides an overview of iron-catalyzed domino coupling reactions of π-systems. The classifications and reactivity paradigms examined should assist readers and provide guidance for the design of novel domino reactions.

Enantioselective Cross-couplings between Halide Derivatives and Organometallics by Using Iron and Cobalt Catalysts: Formation of C-C Bonds

This review highlights the recent achievements of iron- and cobalt-catalyzed enantioselective cross-couplings of halide derivatives with organometallic reagents for the construction of C-C bonds. Synthetic applications of enantioselective cross-couplings to natural products and biologically active compounds are also covered showing the power of these cross-couplings in organic synthesis.