1,721,173 research outputs found
Organocatalytic Asymmetric Hydrophosphination of α,β-Unsaturated Aldehydes
No full textThe first organocatalytic asymmetric functionalization of aldehydes with a P-based compound has been successfully developed; the use of a simple chiral secondary amine has enabled the highly chemo- and enantioselective conjugate addition of diphenyl phosphine to α,β-unsaturated aldehydes, providing a direct route to chiral β-phosphine aldehyde intermediates. The synthetic utility of the asymmetric hydrophosphination (AHP) strategy was exemplified in a rapid one-pot (two step) synthesis of highly enantioenriched 1,3-aminophoshines, potentially useful P,N-ligands
Photoredox radical conjugate addition of dithiane-2-carboxylate promoted by an iridium(III) phenyl-tetrazole complex: a formal radical methylation of Michael acceptors
No full textA readily accessible iridium(iii) phenyl-tetrazole complex ([Ir(ptrz)2(tBu-bpy)]+, 2; Hptrz = 2-methyl-5-phenyl-tetrazole; tBu-bpy = 4,4′-di-tert-butyl-2,2′-bipyridine) is shown to be a versatile catalyst for a new photocatalytic Michael reaction. Under light irradiation in the presence of 2, a dithiane 2-carboxylic acid, obtained by simple hydrolysis of a commercially available ethyl ester, generates a 1,3-dithiane radical capable of performing addition to a variety of Michael acceptors (e.g., unsaturated ketones, esters, amides and malonates). This broad scope reaction with high yields is a formal photo-redox addition of the elusive methyl radical and the adducts obtained can be starting materials for a variety of functionalized products. The excited-state oxidation potential of catalyst 2 allows selective formation of radicals only from α-heterosubstituted carboxylates. Chemical modification of this metal complex can tune the electrochemical properties, opening a route to new highly selective catalytic photo-oxidation reactions
Iridium(III) Complexes with Fluorinated Phenyl-tetrazoles as Cyclometalating Ligands: Enhanced Excited-State Energy and Blue Emission
No full textFive cationic iridium(III) complexes with fluorinated cyclometalating tetrazole ligands [Ir(dfptrz)2L]+, where Hdfptrz = 5-(2,4-difluorophenyl)-2-methyl-2H-tetrazole and L = 2,2′-bypiridine (1F), 4,4′-ditert-butyl-2,2′-bipyridine (2F), 1,10-phenantroline (3F), 4,4′-bis(dimethylamino)-2,2′-bipyridine (4F), and tert-butyl isocyanide (5F), were prepared following a one-pot synthetic strategy based on a bis-cyclometalated solvato complex obtained via silver(I)-assisted cyclometalation, which was then reacted with the proper ancillary ligand to get the targeted complexes. The X-ray crystal structures of 2F and 4F were determined, showing that the tetrazole ligands are in a trans arrangement with respect to the iridium center. Electrochemical and photophysical properties, along with density functional theory calculations, allowed a full rationalization of the electronic properties of 1F–5F. In acetonitrile solution at 298 K, complexes 1F–3F, equipped with bipyridine and phenanthroline ligands, exhibit strong vibronically structured luminescence bands in the blue region with photoluminescence quantum yields (PLQYs) in the range 56–76%. This behavior is radically different from the nonfluorinated analogues reported previously, which emits in the green region from 3MLCT excited states. 4F shows relatively strong emission (PLQY = 40%) of charge transfer character centered on the amino-bipyridine ancillary ligand, whereas the emission of 5F is very weak (PLQY = 0.6%), further blue-shifted and attributed to the lowest ligand-centered (3LC) triplet state of the tetrazolyl cyclometalated moiety. A similar photophysical behavior is observed in PMMA at 298 K, whereas in a 77 K matrix, all of the compounds are strong emitters. This novel fluorinated phenyl-tetrazole cyclometalating ligand provides the corresponding iridium(III) complexes with a combination of excited-state energy and redox potentials that make them very promising as photoredox catalysts
A chelating diisocyanide ligand for cyclometalated Ir(III) complexes with strong and tunable luminescence
No full textWe report the synthesis, structural characterisation and detailed photophysical description
of three cationic cyclometalated iridium(III) complexes (2–4) bearing a chelating
diisocyanide as the ancillary ligand (1 1⁄4 2,200-diisocyano-1,10:30,100-terphenyl). All
compounds display irreversible reduction and oxidation potentials and emit from a
triplet excited state centred on the cyclometalating ligands with lifetimes of several
dozen microseconds, as commonly observed for other iridium(III) isocyanide complexes
and further confirmed by DFT calculations. Room-temperature photoluminescence can
be tuned from blue to orange upon variation of the cyclometalating ligands, and the
related quantum yields range from around 30% in acetonitrile solution to nearly 80% in
solid-state, as for complex 3 embedded in a 1% w/w poly(methyl methacrylate) matrix
Luminescent methacrylic copolymers with side-chain cyclometalated iridium(III) complexes
No full textA series of eight copolymers made of a poly (methyl methacrylate) backbone with a luminescent iridium(III)-based pendant is presented. These materials are obtained by radical copolymerization of different ratios of methyl methacrylate (MMA) with the reactive Ir-based MMA monomer [Ir(ppy)2(ptmma)]Cl (Hppy = 2-phenylpyridine, ptmma = 11-(4-pyridin-2-yltriazol-1-yl)undecyl 2-methylprop-2-enoate). The products are characterized by NMR, IR and DSC methods. The specific ratio among the two monomers, dictates the concentration of the emissive iridium-based centers along the polymer backbone, which affects the overall luminescence behavior of the material. The position of the absorption and emission maxima of the iridium-based monomer and of the related polymeric compounds are virtually identical, pinpointing the optically innocent character of the polymer matrix. Photophysical investigations show that the emission quantum yields (PLQYs) of the iridium-based emitters improve upon increasing the fraction of the inert MMA co-monomer, due to a progressive decrease in self-absorption and self-quenching, approaching the infinite-dilution limit at the relatively low [MMA]:[Ir] molar ratio of 500, with PLQY = 0.51 ± 0.05. The concentration-quenching rate process follows a power-law proportional to R–(2.6±0.9), where R is the calculated average distance between the emitting units of the copolymers. Such a deviation from the expected dipole-dipole interaction (i.e., R−6 dependence) suggests the partial formation of aggregates leading to the occurrence of static self-quenching in the solid matrix, as the concentration of the iridium centers increases. These results offer valuable hints for the formulation of strongly emitting hybrid materials in which a high concentration of triplet luminophores is embedded in organic polymers solely serving as solid transparent scaffolds
Reaction of dicarbonates with carboxylic acids catalyzed by weak Lewis acids: General method for the synthesis of anhydrides and esters
No full textThe reaction between carboxylic acids (RCOOH) and dialkyl dicarbonates [(R1OCO)2O], in the presence of a weak Lewis acid such as magnesium chloride and the corresponding alcohol (R1OH) as the solvent, leads to the esters RCOOR1 in excellent yields. The mechanism involves a double addition of the acid to the dicarbonate, affording a carboxylic anhydride [(RCO)2O], R1OH and carbon dioxide. The esters arise from the attack of the alcohols on the anhydrides. Exploiting the lesser reactivity of tert-butyl alcohol in comparison with other alcohols, a clean synthesis of both carboxylic anhydrides and esters has been set up. In the former reaction, an acid/Boc2O molecular ratio of 2:1 leads to the anhydride in good to excellent yields, depending on the stability of the resulting anhydride to the usual workup conditions. In the latter reaction, stoichiometric mixtures of the acid and Boc2O are allowed to react with a twofold excess of a primary alcohol, secondary alcohol or phenol (R 2OH) to give the corresponding esters (RCOOR2). Purification of the products is particularly easy since all byproducts are volatile or water soluble. A very easy chromatography is required only in the case of nonvolatile alcohols. A broad variety of sensitive functional groups is tolerated on both the acid and the alcohol, in particular a high chemoselectivity is observed. In fact, no transesterification processes occur with the acid-sensitive acetoxy group and methyl esters. © Georg Thieme Verlag Stuttgart
Aminocatalytic Enantioselective anti-Mannich Reaction of Aldehydes with in Situ Generated N-Cbz and N-Boc Imines
No full textwe have developed the first aminocatalyzed
anti-selective Mannich reaction of aldehydes with N-Cbz- and
N-Boc-protected imines generated in situ from stable aamido
sulfones 2. Besides the high level of efficiency and
stereocontrol achieved, this approach introduces important
synthetic advantages, by avoiding the requirement to preform
the N-carbamoyl imines
Photocurable Itaconic Acid-Functionalized Star Polycaprolactone in Biobased Formulations for Vat Photopolymerization
Full text linkPhotopolymerization-based three-dimensional (3D) printing techniques, such as stereolithography (SLA), are becoming more popular due to their ability to create complex geometries and material properties that are difficult to achieve with other methods. However, a major issue with photopolymerization-based techniques is that they rely on the use of harmful (meth)acrylic acid esters derived from fossil fuels. To address this, researchers have been exploring the use of biobased alternatives such as itaconic acid, which is a building block obtained through fermentation and has a photocurable double bond. Additionally, aliphatic polyesters, such as polycaprolactone (PCL) and its hyperbranched star-like polymers, have been discovered to be biodegradable and low cost and have intricate 3D structures. However, their solid thermoplastic features make them unsuitable for vat photopolymerization-based techniques. This study proposes the one-pot synthesis of a liquid itaconic acid-functionalized branched polycaprolactone macromer, named sorbitol hexa(pentacaprolactone methyl itaconate), SH(PCI). The synthesized (meth)acrylic-free low-molecular-weight polymer was then used to create photocurable resins that could be 3D-printed into biobased thermosets with high resolution and printability. The polymer and the printed object were tested and characterized for structural, mechanical, and thermal properties, and 10 different resins were proposed with biobased contents of up to 97%, which are among the highest reported in the literature so far. These resins lead to printed materials that cover a broad range of mechanical properties, including elastic moduli ranging from 22.1 to 218 MPa, elongations at breaks from 3.9 to 20%, and tensile strengths from 3.6 to 11.6 MPa
Magnesium perchlorate as efficient Lewis acid for the Knoevenagel condensation between beta-diketones and aldehydes
No full textA new protocol for the Knoevenagel condensation between beta-diketones and aliphatic and aromatic aldehydes promoted by
Mg(ClO4)2 under mild conditions is reported
Quaternary Stereogenic Carbons in Complex Molecules by an Asymmetric, Organocatalytic, Triple Cascade Reaction
No full textA novel organocatalytic triple cascade that allows the stereoselective construction of all-carbon quaternary sterogenic centers in complex organic molecules has been disclosed. The method provides a flexible and direct access to cyclohexene carbaldehydes with three or four stereogenic carbon atoms with high diastereomeric and complete enantiomeric control, and can be extended to the preparation of enantiopure cyclohexanes with five chiral centers and two quaternary carbons
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