Switchable Polymerization Triggered by Fast and Quantitative Insertion of Carbon Monoxide into Cobalt–Oxygen Bonds

A strategy that uses carbon monoxide (CO) as a molecular trigger to switch the polymerization mechanism of a cobalt Salen complex [salen=(R,R)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine] from ring-opening copolymerization (ROCOP) of epoxides/anhydrides to organometallic mediated controlled radical polymerization (OMRP) of acrylates is described. The key phenomenon is a rapid and quantitative insertion of CO into the Co−O bond, allowing for in situ transformation of the ROCOP active species (Salen)Co^III-OR into the OMRP photoinitiator (Salen)Co^III-CO₂R. The proposed mechanism, which involves CO coordination to (Salen)Co^III-OR and subsequent intramolecular rearrangement via migratory insertion has been rationalized by DFT calculations. Regulated by both CO and visible light, on-demand sequence control can be achieved for the one-pot synthesis of polyester-b-polyacrylate diblock copolymers (Đ<1.15).     

Ruthenium(II)-Catalyzed Asymmetric Inert C−H Bond Activation Assisted by a Chiral Transient Directing Group

A ruthenium(II)-catalyzed asymmetric intramolecular hydroarylation assisted by a chiral transient directing group has been developed. A series of 2,3-dihydrobenzofurans bearing chiral all-carbon quaternary stereocenters have been prepared in remarkably high yields (up to 98 %) and enantioselectivities (up to >99 % ee). By this methodology, a novel asymmetric total synthesis of CB2 receptor agonist MDA7 has been successfully developed.     

B-MWW Zeolite: The Case Against Single-Site Catalysis

Boron-containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron-containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B₂(OH)_xO_(3−x/2) (x=0–6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM-22 isomorphously substituted with boron (B-MWW). Using ¹¹B solid-state NMR spectroscopy, we show that the majority of boron species in B-MWW exist as isolated BO₃ units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B-MWW for ODH of propane falsifies the hypothesis that site-isolated BO₃ units are the active site in boron-based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium-based catalysts and provides an important piece of the mechanistic puzzle.     

Facile Synthesis of Hierarchical Nanosized Single-Crystal Aluminophosphate Molecular Sieves from Highly Homogeneous and Concentrated Precursors

The synthesis of hierarchical nanosized zeolite materials without growth modifiers and mesoporogens remains a substantial challenge. Herein, we report a general synthetic approach to produce hierarchical nanosized single-crystal aluminophosphate molecular sieves by preparing highly homogeneous and concentrated precursors and heating at elevated temperatures. Accordingly, aluminophosphate zeotypes of LTA (8-rings), AEL (10-rings), AFI (12-rings), and -CLO (20-rings) topologies, ranging from small to extra-large pores, were synthesized. These materials show exceptional properties, including small crystallites (30–150 nm), good monodispersity, abundant mesopores, and excellent thermal stability. A time-dependent study revealed a non-classical crystallization pathway by particle attachment. This work opens a new avenue for the development of hierarchical nanosized zeolite materials and understanding their crystallization mechanism.     

Impact of the Spatial Organization of Bifunctional Metal–Zeolite Catalysts on the Hydroisomerization of Light Alkanes

Improving product selectivity by controlling the spatial organization of functional sites at the nanoscale is a critical challenge in bifunctional catalysis. We present a series of composite bifunctional catalysts consisting of one-dimensional zeolites (ZSM-22 and mordenite) and a γ-alumina binder, with platinum particles controllably deposited either on the alumina binder or inside the zeolite crystals. The hydroisomerization of n-heptane demonstrates that the catalysts with platinum particles on the binder, which separates platinum and acid sites at the nanoscale, leads to a higher yield of desired isomers than catalysts with platinum particles inside the zeolite crystals. Platinum particles within the zeolite crystals impose pronounced diffusion limitations on reaction intermediates, which leads to secondary cracking reactions, especially for catalysts with narrow micropores or large zeolite crystals. These findings extend the understanding of the “intimacy criterion” for the rational design of bifunctional catalysts for the conversion of low-molecular-weight reactants.     

Pathway from N-Alkylglycine to Alkylisonitrile Catalyzed by Iron(II) and 2-Oxoglutarate-Dependent Oxygenases

N-alkylisonitrile, a precursor to isonitrile-containing lipopeptides, is biosynthesized by decarboxylation-assisted -N≡C group (isonitrile) formation by using N-alkylglycine as the substrate. This reaction is catalyzed by iron(II) and 2-oxoglutarate (Fe/2OG) dependent enzymes. Distinct from typical oxygenation or halogenation reactions catalyzed by this class of enzymes, installation of the isonitrile group represents a novel reaction type for Fe/2OG enzymes that involves a four-electron oxidative process. Reported here is a plausible mechanism of three Fe/2OG enzymes, Sav607, ScoE and SfaA, which catalyze isonitrile formation. The X-ray structures of iron-loaded ScoE in complex with its substrate and the intermediate, along with biochemical and biophysical data reveal that -N≡C bond formation involves two cycles of Fe/2OG enzyme catalysis. The reaction starts with an Fe^IV-oxo-catalyzed hydroxylation. It is likely followed by decarboxylation-assisted desaturation to complete isonitrile installation.     

Ruthenium-Catalyzed Direct Asymmetric Reductive Amination of Diaryl and Sterically Hindered Ketones with Ammonium Salts and H2

A Ru-catalyzed direct asymmetric reductive amination of ortho-OH-substituted diaryl and sterically hindered ketones with ammonium salts is reported. This method represents a straightforward route toward the synthesis of synthetically useful chiral primary diarylmethylamines and sterically hindered benzylamines (up to 97 % yield, 93–>99 % ee). Elaborations of the chiral amine products into bioactive compounds and a chiral ligand were demonstrated through manipulation of the removable and convertible -OH group.     

Iron(II)-Catalyzed Biomimetic Aerobic Oxidation of Alcohols

We report the first Fe^II-catalyzed biomimetic aerobic oxidation of alcohols. The principle of this oxidation, which involves several electron-transfer steps, is reminiscent of biological oxidation in the respiratory chain. The electron transfer from the alcohol to molecular oxygen occurs with the aid of three coupled catalytic redox systems, leading to a low-energy pathway. An iron transfer-hydrogenation complex was utilized as a substrate-selective dehydrogenation catalyst, along with an electron-rich quinone and an oxygen-activating Co(salen)-type complex as electron-transfer mediators. Various primary and secondary alcohols were oxidized in air to the corresponding aldehydes or ketones with this method in good to excellent yields.     

Lutidine‐Based Chiral Pincer Manganese Catalysts for Enantioselective Hydrogenation of Ketones

A series of Mn^I complexes containing lutidine‐based chiral pincer ligands with modular and tunable structures has been developed. The complex shows unprecedentedly high activities (up to 9800 TON; TON=turnover number), broad substrate scope (81 examples), good functional‐group tolerance, and excellent enantioselectivities (85–98 % ee) in the hydrogenation of various ketones. These aspects are rare in earth‐abundant metal catalyzed hydrogenations. The utility of the protocol have been demonstrated in the asymmetric synthesis of a variety of key intermediates for chiral drugs. Preliminary mechanistic investigations indicate that an outer‐sphere mode of substrate–catalyst interactions probably dominates the catalysis.