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.

Cu(II) Coordination Polymers for the Selective Oxidation of Biomass-Derived Veratryl Alcohol in Green Solvents: A Sustainable Catalytic Approach.

Four air-stable one-dimensional copper(II) coordination polymers (CP1-CP4) with azide linkers were synthesized using tridentate NNS and NNN ligands. Single-crystal X-ray diffraction (XRD) analysis confirmed the molecular structures of CP1, CP3, and CP4. In the presence of TEMPO, all four coordination polymers demonstrated effective catalytic activity for the selective aerobic oxidation of veratryl alcohol, a biomass model compound, under base-free conditions. CP4 exhibited the best catalytic efficiency. Oxidations were conducted at ambient temperature (40 °C) utilizing air as a sustainable oxidant. Selective oxidation of veratryl alcohol to veratraldehyde was also conducted in the presence of a catalytic amount of base (5 mol %), and enhanced reactivity was observed. The green solvents, acetone, and water, were used to maximize sustainability. The optimized reaction conditions were applied to broaden the substrate scope of various lignin model alcohols and substituted benzylic alcohols with wide electronic variability. CP4 exhibited high recyclability, consistently providing quantitative yields even after ten consecutive runs. The catalytic protocol demonstrated sustainability and environmental compatibility, as evidenced by a low E-factor (4.29) and a high Eco-scale score (90). Based on experimental evidence and theoretical calculations, a plausible catalytic cycle was proposed. Finally, the sustainability credentials of the different optimized reaction protocols were evaluated using the CHEM21 green metrics toolkit.

Iron- and base-catalyzed C(α)-alkylation and one-pot sequential alkylation-hydroxylation of oxindoles with secondary alcohols.

The utilization of economical and environmentally benign transition metals in crucial catalytic processes is pivotal for sustainable advancement in synthetic organic chemistry. Iron, as the most abundant transition metal in the Earth's crust, has gained significant attention for this purpose. A combination of FeCl2 (5 mol%) in the presence of phenanthroline (10 mol%) and NaOtBu (1.5 equivalent) proved effective for the C(α)-alkylation of oxindole, employing challenging secondary alcohol as a non-hazardous alkylating agent. The C(α)-alkylation of oxindole was optimized in green solvent or under neat conditions. The substrate scope encompasses a broad array of substituted oxindoles with various secondary alcohols. Further post-functionalization of the C(α)-alkylated oxindole products demonstrated the practical utility of this catalytic alkylation. One-pot C-H hydroxylation of alkylated oxindoles yielded 3-alkyl-3-hydroxy-2-oxindoles using air as the most sustainable oxidant. Low E-factors (3.61 to 4.19) and good Eco-scale scores (74 to 76) of these sustainable catalytic protocols for the alkylation and one-pot sequential alkylation-hydroxylation of oxindoles demonstrated minimum waste generation. Plausible catalytic paths are proposed on the basis of past reports and control experiments, which suggested that a borrowing hydrogen pathway is involved in this alkylation.

Nickel-Catalyzed α-Alkylation of Arylacetonitriles with Challenging Secondary Alcohols.

Nickel(II) complex 1 was utilized as a sustainable catalyst for α-alkylation of arylacetonitriles with challenging secondary alcohols. Arylacetonitriles with a wide range of functional groups were tolerated, and various cyclic and acyclic secondary alcohols were utilized to yield a large number of α-alkylated products. The plausible mechanism involves the base-promoted activation of precatalyst 1 to an active catalyst 2 (dehydrochlorinated product) which activates the O-H and C-H bonds of the secondary alcohol in a dehydrogenative pathway.

Cobalt-Catalyzed Chemodivergent Synthesis of Cyclic Amines and Lactams from Ketoacids and Anilines Using Hydrosilylation.

Here, commercially available Co2(CO)8 was utilized as an efficient catalyst for chemodivergent synthesis of pyrrolidines and pyrrolidones from levulinic acid and aromatic amines under slightly different hydrosilylation conditions. 1.5 and 3 equiv of phenylsilane selectively yielded pyrrolidone and pyrrolidine, respectively. Various ketoacids and amines were successfully tested. Plausible mechanism involves the condensation of levulinic acid and amine to form an imine, which cyclizes to 3-pyrrolidin-2-one followed by reduction to pyrrolidone. The final reduction of pyrrolidone gave pyrrolidine.

Hydrosilylation of Terminal Alkynes Catalyzed by an Air-Stable Manganese-NHC Complex.

In recent years, catalysis with base metal manganese has received a significant amount of interest. Catalysis with manganese complexes having N-heterocyclic carbenes (NHCs) is relatively underdeveloped in comparison to the extensively investigated manganese catalysts possessing pincer ligands (particularly phosphine-based ligands). Herein, we describe the synthesis of two imidazolium salts decorated with picolyl arms (L1 and L2) as NHC precursors. Facile coordination of L1 and L2 with MnBr(CO)5 in the presence of a base resulted in the formation manganese(I)-NHC complexes (1 and 2) as an air-stable solid in good isolated yield. Single-crystal X-ray analysis revealed the structure of the cationic complexes [Mn(CO)3(NHC)][PF6] with tridentate N,C,N binding of the NHC ligand in a facile fashion. Along with a few known manganese(I) complexes, these Mn(I)-NHC complexes 1 and 2 were tested for the hydrosilylation of terminal alkynes. Complex 1 was proved to be an effective catalyst for the hydrosilylation of terminal alkynes with good selectivity toward the less thermodynamically stable ß-(Z)-vinylsilanes. This method provided good regioselectivity (anti-Markovnikov addition) and stereoselectivity (ß-(Z)-product). Experimental evidence suggested that the present hydrosilylation pathway involved an organometallic mechanism with manganese(I)-silyl species as a possible reactive intermediate.

Efficient α-Alkylation of Arylacetonitriles with Secondary Alcohols Catalyzed by a Phosphine-Free Air-Stable Iridium(III) Complex.

A well-defined and readily available air-stable dimeric iridium(III) complex catalyzed α-alkylation of arylacetonitriles using secondary alcohols with the liberation of water as the only byproduct is reported. The α-alkylations were efficiently performed at 120 °C under solvent-free conditions with very low (0.1-0.01 mol %) catalyst loading. Various secondary alcohols including cyclic and acyclic alcohols and a wide variety of arylacetonitriles bearing different functional groups were converted into the corresponding α-alkylated products in good yields. Mechanistic study revealed that the reaction proceeds via alcohol activation by metal-ligand cooperation with the formation of reactive iridium-hydride species.

Catalytic Synthesis of N-Heterocycles via Direct C(sp3)-H Amination Using an Air-Stable Iron(III) Species with a Redox-Active Ligand.

Coordination of FeCl3 to the redox-active pyridine-aminophenol ligand NNOH2 in the presence of base and under aerobic conditions generates FeCl2(NNOISQ) (1), featuring high-spin FeIII and an NNOISQ radical ligand. The complex has an overall S = 2 spin state, as deduced from experimental and computational data. The ligand-centered radical couples antiferromagnetically with the Fe center. Readily available, well-defined, and air-stable 1 catalyzes the challenging intramolecular direct C(sp3)-H amination of unactivated organic azides to generate a range of saturated N-heterocycles with the highest turnover number (TON) (1 mol% of 1, 12 h, TON = 62; 0.1 mol% of 1, 7 days, TON = 620) reported to date. The catalyst is easily recycled without noticeable loss of catalytic activity. A detailed kinetic study for C(sp3)-H amination of 1-azido-4-phenylbutane (S1) revealed zero order in the azide substrate and first order in both the catalyst and Boc2O. A cationic iron complex, generated from the neutral precatalyst upon reaction with Boc2O, is proposed as the catalytically active species.

Redox-Active-Ligand-Mediated Formation of an Acyclic Trinuclear Ruthenium Complex with Bridging Nitrido Ligands.

Coordination of a redox-active pyridine aminophenol ligand to Ru(II) followed by aerobic oxidation generates two diamagnetic Ru(III) species [1 a (cis) and 1 b (trans)] with ligand-centered radicals. The reaction of 1 a/1 b with excess NaN3 under inert atmosphere resulted in the formation of a rare bis(nitrido)-bridged trinuclear ruthenium complex with two nonlinear asymmetrical Ru-N-Ru fragments. The spontaneous reduction of the ligand centered radical in the parent 1 a/1 b supports the oxidation of a nitride (N(3-) ) to half an equivalent of N2 . The trinuclear omplex is reactive toward TEMPO-H, tin hydrides, thiols, and dihydrogen.