Supported Nickel Nanoparticles as Catalyst in Direct sp3 C-H Alkylation of 9H-Fluorene Using Alcohols as Alkylating Agent

Herein, we report an inexpensive first-row transition metal Ni heterogeneous catalytic system for the Csp 3-mono alkylation of fluorene using alcohols as alkylating agents via borrowing hydrogen strategy. The catalytic protocol displayed versatility with high yields of the desired products using various types of primary alcohols, including aryl/hetero aryl methanols, and aliphatic alcohols as alkylating agents. The catalyst Ni NPs@N-C was synthesized via high-temperature pyrolysis strategy, using ZIF-8 as the sacrificial template. The Ni NPs@N-C catalyst was characterized by XPS, HR-TEM, HAADF-STEM, XRD and ICP-MS. The catalyst is stable even in the air at room temperature, displayed excellent activity and could be recycled 5 times without appreciable loss of its activity.

Iron-Catalyzed sp3 C-H Alkylation of Fluorene with Primary and Secondary Alcohols: A Borrowing Hydrogen Approach

The utilization of earth-abundant, cheap, and nontoxic transition metals in important catalytic transformations is essential for sustainable development, and iron has gained significant attention as the most abundant transition metal. A mixture of FeCl2 (3 mol %), phenanthroline (6 mol %), and KOtBu (0.4 eqivalent) was used as an effective catalyst for the sp3 C-H alkylation of fluorene using alcohol as a nonhazardous alkylating partner, and eco-friendly water was formed as the only byproduct. The substrate scope includes a wide range of substituted fluorenes and substituted benzyl alcohols. The reaction is equally effective with challenging secondary alcohols and unactivated aliphatic alcohols. Selective mono-C9-alkylation of fluorenes with alcohols yielded the corresponding products in good isolated yields. Various postfunctionalizations of C-9 alkylated fluorene products were performed to establish the practical utility of this catalytic alkylation. Control experiments suggested a homogeneous reaction path involving borrowing hydrogen mechanism with the formation and subsequent reduction of 9-alkylidene fluorene intermediate.

Cobalt-Catalyzed Divergence in C(sp3)-H Functionalization of 9H-Fluorene: A Streamlined Approach Utilizing Alcohols

Sustainable chemical production demands the creation of innovative catalysts and catalytic technologies. While the development of coherent and robust catalytic systems using earth-abundant transition metals is essential, it remains a significant challenge. Herein, an expedient divergence strategy for tandem dehydrogenative C(sp3)-H alkylation and cyclization reactions of 9H-fluorene using a newly developed N,N-bidentate cobalt catalytic system is developed. This method capitalizes on the use of abundant and readily accessible alcohol. Demonstrating wide-ranging applicability, the catalytic protocol successfully integrated a diverse array of fluorenes and alcohols. This includes benzylic, heteroaromatic, and aliphatic primary and secondary alcohols, amassing a total of 75 distinct examples. When applied to sterically bulky alcohols, the reaction preferentially undergoes alkenylation, yielding a tetrasubstituted olefin as the main product. In the case of diols, the expected outcome is Dual-fluorescence at both terminal positions. This process leads to difluorination, followed by a cyclization step, culminating in the formation of a relatively unprecedented spiro compound. The scalability of this method has been validated through gram-scale synthesis. Control experiments have shed light on the reaction mechanism, indicating that it progresses through an unsaturated intermediate and adheres to the borrowing hydrogen pathway.

Copper-catalyzed C(sp3)-H alkylation of fluorene with primary and secondary alcohols using a borrowing hydrogen method

Despite the limited success of copper-catalyzed alkylations, (NNS)CuCl proved to be an effective catalyst for the sp3 C-H alkylation of fluorene with alcohols. Various primary alcohols and challenging secondary alcohols were successfully used. The practical applicability of the method was effectively tested with several post-functionalization reactions. This copper-catalyzed alkylation of fluorene involved a borrowing hydrogen mechanism.

Unravelling a bench-stable zinc-amide compound as highly active multitasking catalyst for radical-mediated selective alk(en)ylation of unactivated carbocycles under mild conditions

The direct functionalization of unactivated organic moieties via C-C bond formation has long fascinated synthetic chemists. Although base-metal systems are steadily emerging in this area, achieving multitasking activity in a single catalyst to execute several such functionalizations under mild conditions is challenging. To address this, we herein report an effective protocol for the selective C-alk(en)ylation of indene/fluorene with alcohol as a green alkylating agent employing a naturally abundant and eco-friendly zinc-derived compound, for the first time. Notably, this study unveils the unique potential of a bench-stable Zn compound bearing an amidated imidazolium salt towards C-C bond-forming reactions utilizing an array of alcohols, ranging from aliphatic to aromatic and, attractively, even secondary alcohols. Moreover, this readily scalable protocol, which proceeds via an underdeveloped radical-mediated borrowing hydrogen protocol (an aldehyde is generated from an alcohol, and subsequent condensation with indene/fluorene provides the corresponding alkenylated products) established based on a range of control experiments, works effortlessly under mild conditions using a low catalyst loading. Notably, this approach affords remarkable selectivity towards alkylated or alkenylated products with a high level of functional group tolerance and chemoselectivity. Crucially, the catalytic activity of these Zn compounds can be attributed to their hydrogen atom transfer (HAT) capability, while their selectivity towards different products can be understood in terms of employed reaction conditions. Lastly, the synthetic utility of obtained products was showcased by their late-stage functionalization to access unsymmetrical 9,9-disubstituted fluorenes, which are potentially useful for various optoelectronic applications.

Noninnocent Azo-Aromatic Cobalt(II)-Catalyzed sp3 C-H Alkylation of Fluorenes with Alcohols

Herein, employing well-defined redox noninnocent cobalt(II) complexes an efficient sp3 C-H alkylation of fluorenes using alcohols as alkylating agents to result in alkylated fluorenes is reported. The catalytic protocol was versatile with various fluorenes and benzyl alcohols. It also showed very good functional group tolerance with both alcohols and fluorenes. Moreover, an efficient single-step and simultaneous di C-C as well as both C-C and the C-N alkylation reaction of fluorenes was observed with this catalytic protocol. Such selective single-step dialkylation of fluorenes is indeed beneficial. Several control experiments, deuterium labeling, and 1H NMR kinetic studies have revealed a ligand radical-based borrowing hydrogen mechanism involving the azo-aromatic complexes of cobalt as catalysts for the alkylation of fluorenes.

Electron transfer catalysis mediated by 3d complexes of redox non-innocent ligands possessing an azo function: a perspective

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes.

A Catecholaldimine-Based NiII-Complex as an Effective Catalyst for the Direct Conversion of Alcohols to trans-Cinnamonitriles and Aldehydes

A nickel(II) complex [Ni(HL)₂] 1 was synthesized by treatment of a new catecholaldimine-based ligand with NiCl₂·6H₂O in methanol at room temperature. Complex 1 showed excellent catalytic activity where aromatic and heterocyclic alcohols were rapidly converted into trans-cinnamonitrile in a one-pot manner via oxidative olefination in the presence of KOH. The potential of the disclosed catalyst and the results obtained for the direct conversion of alcohols to two different functionalities (trans-cinnamonitrile and aldehydes) are well supported by DFT studies.

N-Heterocyclic Carbene-Supported Nickel-Catalyzed Selective (Un)Symmetrical N-Alkylation of Aromatic Diamines with Alcohols

The "borrowing hydrogen" (BH) approach for the N-alkylation of phenylenediamines using alcohols as coupling partners is highly challenging due to the selectivity issue of the generated products. Furthermore, the development of base-metal systems that can potentially substitute precious metals with competitive activity is a major challenge in BH catalysis. We present herein an efficient protocol for the N,N'-di-alkylation of aromatic diamines using an in situ-generated Ni-NHC complex from NiCl₂ and the ligand L1, which gave access to a wide range of N,N'-di-alkylated orthophenylene diamines (rather than the generally observed benzimidazole derivatives), meta- and para-phenylene diamines along with 2,6-diamino pyridine derivatives in good to excellent yields. Moreover, the catalyst system was also successful in the derivatization of a clinically important drug molecule, Dapsone. Notably, the present protocol could be applied effectively to synthesize unsymmetrically substituted N,N'-di-alkylated diamines via sequential alkylation and is the first report in the base-metal system to the best of our knowledge. Diverse control experiments including the deuterium incorporation studies suggest that the present protocol proceeds via a BH sequence.