Finding heterogeneous catalysts that are superior to homogeneous ones for selective catalytic transformations is a major challenge in catalysis. Here, we show how micropores in metal-organic frameworks (MOFs) push homogeneous catalytic reactions into kinetic regimes inaccessible under standard conditions. Such property allows branched selectivity up to 90% in the Co-catalysed hydroformylation of olefins without directing groups, not achievable with existing catalysts. This finding has a big potential in the production of aldehydes for the fine chemical industry. Monte Carlo and density functional theory simulations combined with kinetic models show that the micropores of MOFs with UMCM-1 and MOF-74 topologies increase the olefins density beyond neat conditions while partially preventing the adsorption of syngas leading to high branched selectivity. The easy experimental protocol and the chemical and structural flexibility of MOFs will attract the interest of the fine chemical industries towards the design of heterogeneous processes with exceptional selectivity.
Abstract. Iron-catalyzed alkyl-aryl Kumada coupling has developed into an efficient synthetic method, yet its mechanism remains vague. Here, we apply a bisoxazolinylphenylamido pincer ligand (Bopa) to stabilize the catalytically active Fe center, resulting in isolation and characterization of well-defined iron complexes whose catalytic roles are probed and confirmed. Reactivity studies of the iron complexes identifies an Fe(II) "ate" complex, [Fe(Bopa-Ph)(Ph) 2 ] -, as the active species for the oxidative addition of alkyl halide. Experiments using radicalprobe substrates and DFT computations reveal a bimetallic and radical mechanism for the oxidative addition. The kinetics of the coupling of an alkyl iodide with PhMgCl indicates that formation of the "ate" complex, rather than oxidative addition, is the turnover determining step. This work provides insights in iron-catalyzed cross coupling reactions of alkyl halides.3
Two functions are important: Three examples of well‐defined bifunctional iron catalysts that are very efficient in the hydrogenation of ketones are described (see scheme). These remarkable studies will contribute significantly to the development of more environmentally friendly and sustainable reduction reactions in the near future.
The
influence of metal–organic frameworks (MOFs) as additives
is herein described for the reaction of n-alkyl aldehydes
in the presence of methylvinylketone and triphenylphosphine. In the
absence of a MOF, the expected Morita–Baylis–Hillman
product, a β-hydroxy enone, is observed. In the presence of
MOFs with UMCM-1 and MOF-5 topologies, the reaction is selective to
Aldol-Tishchenko products, the 1 and 3 n-alkylesters
of 2-alkyl-1,3-diols, which is unprecedented in organocatalysis. The
(3-oxo-2-butenyl)triphenylphosphonium zwitterion, a commonly known
nucleophile, is identified as the catalytic active species. This zwitterion
favors nucleophilic character in solution, whereas once confined within
the framework, it becomes an electrophile yielding Aldol-Tishchenko
selectivity. Computational investigations reveal a structural change
in the phosphonium moiety induced by the steric confinement of the
framework that makes it accessible and an electrophile.
Put through the pincer: A recent study revealed the structure of the Ni‐containing active site of lactate racemase. The Ni is coordinated by a SCS pincer ligand derived from a nicotinic acid mononucleotide.
a b s t r a c tThe paramagnetic complex bis(oxazolinylphenyl)amine-Fe(III)Cl 2 is investigated by means of solid-state proton NMR at 18.8 T (800 MHz) using magic-angle spinning at 65 kHz. Spin echoes that are excited and refocused by combs of rotor-synchronized pulses in the manner of 'Delays Alternating with Nutation for Tailored Excitation' (DANTE) allow one to characterize different chemical environments that severely overlap in conventional MAS spectra. Such sequences combine two apparently contradictory features: an overall bandwidth exceeding several MHz, and very selective irradiation of a few kHz within inhomogeneously broadened sidebands. The experimental hyperfine interactions correlate well with DFT calculations.
The production of biodiesel derived from microalgae is among the most forthcoming technologies that provide an ecologic alternative to fossil fuels. Herein, a method was developed that enables the direct extraction and conversion of algal oil to biodiesel without prior isolation. The reaction occurs in aqueous media catalyzed by immobilized Candida antarctica lipase B (Novozyme 435). Zwitter-type ionic liquids were used as cocatalyst to improve the selectivity and reactivity of the enzyme. In a model reaction with sunflower oil, 64% biodiesel was obtained. Applying this method to a slurry of whole-cell Chlorella zof ingiensis in water resulted in 74.8% of lipid extraction, with 27.7% biotransformation products and up to 16% biodiesel. Factors that reduced the lipase activity with whole-cell algae were subsequently probed and discussed. This "in situ" method shows an improvement to existing methods, since it integrates the oil extraction and conversion into an one-pot procedure in aqueous conditions. The extraction is nondisruptive, and is a model for a greener algae to biodiesel process.
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