Dendritic effect in polymer-supported catalysis of the intramolecular Pauson-Khand reaction

Heterogeneous Dendrimer-Based Catalysts

The present review compiles the advances in the dendritic catalysis within the last two decades, in particular concerning heterogeneous dendrimer-based catalysts and their and application in various processes, such as hydrogenation, oxidation, cross-coupling reactions, etc. There are considered three main approaches to the synthesis of immobilized heterogeneous dendrimer-based catalysts: (1) impregnation/adsorption on silica or carbon carriers; (2) dendrimer covalent grafting to various supports (silica, polystyrene, carbon nanotubes, porous aromatic frameworks, etc.), which may be performed in a divergent (as a gradual dendron growth on the support) or convergent way (as a grafting of whole dendrimer to the support); and (3) dendrimer cross-linking, using transition metal ions (resulting in coordination polymer networks) or bifunctional organic linkers, whose size, polarity, and rigidity define the properties of the resulted material. Additionally, magnetically separable dendritic catalysts, which can be synthesized using the three above-mentioned approaches, are also considered. Dendritic catalysts, synthesized in such ways, can be stored as powders and be easily separated from the reaction medium by filtration/centrifugation as traditional heterogeneous catalysts, maintaining efficiency as for homogeneous dendritic catalysts.

Data mining of amine dehydrogenases for the synthesis of enantiopure amino alcohols

Five amine dehydrogenases (AmDHs) derived from amino acid dehydrogenases have been identified and evaluated for the stereoselective amination of α-/β-functionalized carbonyl compounds to synthesize chiral amino alcohols.

Reactions of dicobalt octacarbonyl with dinucleating and mononucleating bis(imino)pyridine ligands

This work focuses on the application of dicobalt octacarbonyl (Co₂(CO)₈) as a metal precursor in the chemistry of formally low-valent cobalt with redox-active bis(imino)pyridine [NNN] ligands. The reactions of both mononucleating mesityl-substituted bis(aldimino)pyridine (L¹) and dinucleating macrocyclic xanthene-bridged di(bis(aldimino)pyridine) (L²) with Co₂(CO)₈ were investigated. Independent of the metal-to-ligand ratio (1 : 1 or 1 : 2 ligand to Co₂(CO)₈), the reaction of the dinucleating ligand L² with Co₂(CO)₈ produces a tetranuclear complex [Co₄(L²)(CO)₁₀] featuring two discrete [Co₂[NNN](CO)₅] units. In contrast, a related mononucleating bis(aldimino)pyridine ligand, L¹, produces different species at different ligand to Co₂(CO)₈ ratios, including dinuclear [Co₂(CO)₅(L¹)] and zwitterionic [Co(L¹)₂][Co(CO)₄]. Interestingly, [Co₄(L²)(CO)₁₀] features metal-metal bonds, and no bridging carbonyls, whereas [Co₂(CO)₅(L¹)] contains cobalt centers bridged by one or two carbonyl ligands. In either case, treatment with excess acetonitrile leads to disproportionation to the zwitterionic [Co[NNN](NCMe)₂][Co(CO)₄] units. The electronic structures of the complexes described above were studied with density functional theory. All the obtained bis(imino)pyridine complexes serve as catalysts for cyclotrimerization of methyl propiolate, albeit their reactivity is inferior compared with Co₂(CO)₈.

Multivalency as a key factor for high activity of selective supported organocatalysts for the Baylis-Hillman reaction

The polystyrene-supported N-alkylimidazole-based dendritic catalysts for the Baylis-Hillman reaction exhibit one of the strongest beneficial effects of multivalent architecture ever reported for an organocatalyst. The yields in the model reaction of methyl vinyl ketone with p-nitrobenzaldehyde are more than tripled when a non-dendritic catalyst is replaced by a second- or third-generation analogue. Moreover, the reaction of the less active substrates will not occur with the non-dendritic catalyst and will proceed to a significant extent only with the analogous catalysts of higher generations. A substantial additional enhancement of the reaction yield could be achieved by increasing the content of water in the reaction solvent. The plausible cause of the dendritic effect is the assistance of the second, nearby imidazole moiety in the presumably rate-determining proton transfer in the intermediate adduct, after the first imidazole unit induced the formation of the new carbon-carbon bond.

Advantages of polymer-supported multivalent organocatalysts for the Baylis-Hillman reaction over their soluble analogues

Immobilization of well-defined catalytic units onto insoluble support promises significant benefits, but frequently results in a reduced activity and selectivity of the heterogenized catalysts. Recently, we showed that introduction of a dendritic spacer between the support and the units could remedy the compromised activity and/or selectivity of heterogenized catalysts and, in particular, of the systems based on N-alkylated imidazoles. These catalysts exhibit an outstanding multivalency effect on the activity in the Baylis-Hillman reaction, while preserving very high chemoselectivity. In order to better understand this remarkable effect, we decided to synthesize and examine soluble analogues of the supported systems. These soluble catalysts display poor chemoselectivity, although it improves with the increase of the dendritic generation. Though the consumption of the limiting aldehyde reactant (conversion) displays the opposite trend, experiments demonstrated that the chemoselectivity is generation-dependent rather than conversion-dependent. A hydrophobic “pocket” effect was implicated as responsible for the differences between the polystyrene-bound and the soluble catalysts. An MS analysis of the crude reaction mixture revealed that the formation of multiple adducts, which incorporate several enone and several nitrobenzaldehyde fragments into a single molecular structure (as opposed to one-to-one stoichiometry of the Baylis-Hillman reaction), is responsible for the decline in the chemoselectivity.

Extremely efficient catalysis of carbon-carbon bond formation using "click" dendrimer-stabilized palladium nanoparticles

This article is an account of the work carried out in the authors' laboratory illustrating the usefulness of dendrimer design for nanoparticle palladium catalysis. The "click" synthesis of dendrimers constructed generation by generation by 1-->3 C connectivity, introduces 1,2,3-triazolyl ligands insides the dendrimers at each generation. Complexation of the ligands by Pd(II) followed by reduction to Pd(0) forms dendrimer-stabilized Pd nanoparticles (PdNPs) that are extremely reactive in the catalysis of olefin hydrogenation and C-C bond coupling reactions. The stabilization can be outer-dendritic for the small zeroth-generation dendrimer or intra-dendritic for the larger first- and second-generation dendrimers. The example of the Miyaura-Suzuki reaction that can be catalyzed by down to 1 ppm of PdNPs with a "homeopathic" mechanism (the less, the better) is illustrated here, including catalysis in aqueous solvents.

Supported N-alkylimidazole-decorated dendrons as heterogeneous catalysts for the Baylis–Hillman reaction

The activity of supported N-alkylimidazole catalysts could be dramatically enhanced by introduction of a dendritic spacer and further improved by addition of water as a cosolvent.

Synthesis of large ring macrocycles (12-18) by recyclable palladium-complexed dendrimers on silica gel catalyzed intramolecular cyclocarbonylation reactions

Intramolecular cyclocarbonylation reactions with palladium-complexed dendrimers on silica gel as catalysts are very effective for the synthesis of twelve- to eighteen-membered ring macrocycles. This process can tolerate a wide variety of functional groups, including halide, ether, ketone, and ester. The heterogeneous dendritic catalysts facilitate excellent substrate reactivity, affording oxygen-, nitrogen-, or sulfur-containing tricyclic heterocycles in 70-92 % yields. Importantly, these systems are easily recovered by simple filtration and reused several times with only a slight loss of activity.

Pd Catalysis on Dendronized Solid Support: Generation Effects and the Influence of the Backbone Structure

Recent studies revealed that catalysts, prepared on dendronized support, frequently exhibit enhanced activity and selectivity as compared to their non-dendronized analogues. Regretfully, in early studies of the supported dendritic catalysis, no particular attention was paid to the coordinative nature of the dendritic backbone. In this study, we functionalized Wang polystyrene support with three types of dendritic templates: poly(aril benzyl ether), poly(aryl benzyl thioether), and poly(aryl benzyl amine). These dendronized resins were further decorated with phosphine ligands on the periphery and complexed with a Pd(0) catalytic precursor. The catalysis of the Heck and Suzuki reactions of bromobenzene with the first to third generation supported dendritic catalysts was examined and compared to that of the non-dendritic analogues. All of the examined reactions revealed a positive dendritic effect, reflected in up to 5-fold increase in yield, in the most prominent case. The reasons for the observed effect are the proximity of the ligating sites translated into reduced cross-linking and, probably, the increased distance of the catalyst from the polymer matrix. We proved, however, that the latter could not be achieved with a linear spacer. Although the Suzuki reaction was rather insensitive to the backbone structure, the Heck reaction catalysis at 80 degrees C exhibited substantial sensitivity to the nature of the dendritic backbone, with the polyether structure demonstrating the best outcome. This is the first demonstration of the influence of the coordinative ability of the backbone on the activity of a supported dendritic catalyst.

Enhanced catalytic properties of copper in O- and N-arylation and vinylation reactions, using phosphorus dendrimers as ligands

Imino pyridine-capped phosphorus dendrimers were found to strongly enhance the catalytic activity of copper in arylations and vinylations of O- and N-nucleophiles. These reactions could indeed be performed under very mild conditions, often the mildest to date. Interestingly, such performances could not be obtained when using the monomeric imino pyridine chelating ligand alone. To the best of our knowledge, this constitutes a rare example of such a positive dendritic effect in the field of organometallic catalysis.

Polymer-supported proline-decorated dendrons: dendritic effect in asymmetric aldol reaction

The yield and enantioselectivity of an asymmetric aldol reaction, catalyzed by a proline derivative immobilized on polystyrene via dipolar cycloaddition, are remarkably improved by the dendronization of the support.

Polymer-Supported Palladacycles: Efficient Reagents for Synthesis of Benzopyrans with Palladium Recovery. Relationship among Resin Loading, Pd:P Ratio, and Reactivity of Immobilized Palladacycles

[reaction: see text] Oxapalladacycles were immobilized on polystyrene-divinylbenzene supports and treated with 3-aryl-2-propynoates or 1-alkyl-1,2-propadienes to afford 2H-1-benzopyrans in yields superior to those for solution-phase experiments. Isolation of benzopyrans was facilitated, and 71-80% of the palladium was recovered. Effects of resin loading with phosphorus and palladium were studied, and the optimum immobilized palladacycles featuring a medium loading with P (1.35 mmol P/g) and a high loading with Pd (Pd:P ratio 1:1.7) were identified. Resins with higher swelling capacities were more reactive.

Synthesis of dendritic catalysts and application in asymmetric transfer hydrogenation

Frechet-type core-functionalized chiral diamine-based dendritic ligands and hybrid dendritic ligands condensed from polyether wedge and Newkome-type poly(ether-amide) supported multiple ligands were designed and synthesized. The solubility of hybrid dendrimers was found to be finely controlled by the polyether dendron. The catalytic efficiency and recovery use of dendritic ruthenium complexes were compared in the transfer hydrogenation of acetophenone. The core-functionalized dendritic catalysts demonstrated much better recyclability, which verified the stabilizing effects of the bulky polyether wedge on the catalytically active complex. Moreover, the dendritic catalysts were applied in the asymmetric transfer hydrogenation of ketones, enones, imine, and activated olefin, and moderate to excellent enantioselectivitiy was achieved comparable to that of monomeric catalysts.

Generation effects on the microstructure and product distribution in ethylene polymerization promoted by dendritic nickel catalysts

Carbosilane dendrimers Gn-[(ONNMe2)NiBr2]m, containing up to sixteen terminal pyridylimine nickel complexes, have been studied as catalysts for polymerization of ethylene; the microstructure and the oligomer/polymer distribution are significantly affected by the generation of the dendritic precursor.

Photochemical and photophysical properties of a poly(propylene amine) dendrimer functionalised with E-stilbene units

A second generation poly(propylene amine) dendrimer functionalized at the periphery with eight E-stilbene and eight 4-tert-butylbenzenesulfonyl units has been prepared. The absorption spectrum, fluorescence spectrum and decay, E<==>Z photoisomerization, and photocyclization of the Z-isomer of the stilbene units have been investigated in air equilibrated acetonitrile solutions. For comparison purposes, a reference compound of the peripheral dendrimer units, namely 4-tert-butyl-N-propyl-N-(4-styryl-benzyl)-benzenesulfonamide, has also been studied. The quantum yield of the E-->Z photoisomerization reaction (0.30) and the fluorescence quantum yield of the E isomer (0.014) are substantially smaller for the units appended to the dendrimer compared to those of the reference compound (0.50 and 0.046, respectively). The presence of a red tail and the biexponential decay of the emission band of the dendrimer indicate formation of excimers between the stilbene units appended at the poly(propylene amine) dendritic structure. Under the experimental conditions used (lambda(exc)= 313 nm), a Z/E photostationary state (around 9 : 1 for both reference compound and dendrimer ) is reached in the time scale of minutes. On continuing irradiation, other photoreactions take place in the time scale of hours: the stilbene moiety of compound undergoes photocyclization to phenanthrene (quantum yield 0.015), whereas in dendrimer photocyclization to phenanthrene is accompanied by other processes, including a photoreaction involving the internal amine groups.

The Pauson-Khand reaction, a powerful synthetic tool for the synthesis of complex molecules

There are still some synthetic chemists who hesitate to use metal-mediated or -catalysed reactions. The Pauson-Khand reaction (PKR) is a powerful transformation that has now been sufficiently well developed to be routinely considered when planning a synthesis, especially of polycyclic complex molecules. This tutorial review aims to encourage the use of this process explaining the best ways of performing a PKR both in the stoichiometric and the catalytic version, showing the scope of the process and its limitations. Additionally, asymmetry can be introduced in the reaction using several strategies, which will be discussed. The most recent examples of the synthetic applications of the PKR in natural product synthesis will give the reader an idea of the great usefulness of this reaction.

Remarkable dendritic effect in the polymer-supported catalysis of the Heck arylation of olefins

[reaction: see text] Phosphine-palladium complexes, immobilized on polystyrene, demonstrated a remarkable increase in catalytic activity and selectivity in the Heck reaction upon the introduction of a dendritic spacer between the support and phosphine. For some reactions an up to 5-fold increase in yield is observed.