Photochemical Synthesis on Surfaces.
Synergistic Project funded by the Community of Madrid under grant ref. Y2020/NMT-6469.
PUBLICATIONS
del Río-Rodríguez et al., 2022, Chemical Science, 13, 6512-6518.
Herein, we report, a general, facile and environmentally friendly Minisci-type alkylation of N-heteroarenes under simple and straightforward electrochemical conditions using widely available alkyl halides as radical precursors. Primary, secondary and tertiary alkyl radicals have been shown to be efficiently generated and coupled with a large variety of N-heteroarenes. The method presents a very high functional group tolerance, including various heterocyclic-based natural products, which highlights the robustness of the methodology. This applicability has been further proved in the synthesis of various interesting biologically valuable building blocks. In addition, we have proposed a mechanism based on different proofs and pieces of electrochemical evidence.
del Río-Rodríguez et al., 2022, Chemical Communications, 58, 7277-7280.
The photocatalytic generation of α-amino radicals is combined with chiral isothiourea derived α,β-unsaturated acyl ammonium intermediates. The reaction proceeds via a [3+2] radical-polar crossover mechanism to generate γ-lactams in good yields and enantioselectivities. The enantioselective radical conjugate addition was carried out under batch and flow conditions.
González-Muñoz et al., 2022, Journal of Catalysis, 413, 274-283.
A simple one-step synthesis of a heterogeneous photocatalyst based on the encapsulation of the quinolinate-platinum complex 1 in the inner cavity of Single Walled Carbon Nanotubes (Pt1@oSWNT) is presented. This strategy generates a robust and stable catalyst for photooxidations performed in water media. Pt1@oSWNT is characterized spectroscopically and microscopically to have encapsulated a 2.5 % wt. of the unaltered complex. This material, which only contained 0.04 mol% of 1, is able to chemoselectively yield a wide variety of sulfoxides in water in the presence of air under 385 nm irradiation. The catalyst also presents affinity for polyaromatic substrates, increasing the reaction rate as a function of the number of condensed rings, achieving turnover frequencies as high as 8187 h-1. Moreover, the heterogeneous catalyst could be recycled for more than six consecutive reaction runs without losing its catalytic activity or detecting catalyst leaching.
del Río-Rodríguez et al., 2022, Chemistry - A European Journal, 28, 54, e202201644.
A nickel-catalysed reductive cross-coupling reaction between benzyl sulfonium salts and benzyl bromides is reported. Simple, stable and readily available sulfonium salts have shown their ability as leaving groups in cross electrophile coupling, allowing the formation of challenging sp3-sp3 carbon-carbon bonds, towards the synthesis of interesting dihydro stilbene derivatives. In addition, benzyl tosyl derivatives have been demonstrated to be suitable substrates for the reductive cross-coupling via in situ formation of the corresponding sulfonium salt.
Sciarretta et al., 2022, ACS Applied Materials Interfaces, 14, 30, 34975-34984.
The immobilization of TiO2 nanoparticles on graphene acid (GA), a conductive graphene derivative densely functionalized with COOH groups, is presented. The interaction between the carboxyl groups of the surface and the titanium precursor leads to a controlled TiO2 heterogenization on the nanosheet according to microscopic and spectroscopic characterizations. Electronic communication shared among graphene and semiconductor nanoparticles shifts the hybrid material optical features toward less energetic radiation but maintaining the conductivity. Therefore, GA-TiO2 is employed as heterogeneous photocatalyst for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles using ketoacids and hydrazides as substrates. The material presented enhanced photoactivity compared to bare TiO2, being able to yield a large structural variety of oxadiazoles in reaction times as fast as 1 h with full recyclability and stability. The carbocatalytic character of GA is the responsible for the substrates condensation and the GA-TiO2 light interaction ability is able to photocatalyze the cyclization to the final 1,3,4-oxadiazoles, demonstrating the optimal performance of this multifunctional photocatalytic material.
López-Magano et al., 2022, Applied Catalysis Environmental: B, 317, 121791.
We present the design and synthesis of a new covalent triazine framework with high content of N atoms bearing triazine and phenanthroline moieties. This material displays interesting luminescence phenomena and enhanced photoredox activity as a consequence of synergistic combination of both N-containing aromatic fragments. As catalytic application, the reduction of a variety of brominated and challenging chlorinated aromatic structures, including persistent organic pollutants such as polybrominated diphenyl ethers, has been performed under light irradiation and room temperature conditions. These chemical transformations follow a photoredox mechanism involving the formation of aryl radicals, that were trapped with different radical acceptors in order to afford the formation of new C-C, C-B and C-P bonds.
Daliran et al., 2022, Chemical Society Reviews, 51, 7810-7882.
Although C–H functionalization is one of the simplest reactions, it requires the use of highly active and selective catalysts. Recently, C–H-active transformations using porous materials such as crystalline metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) as well as amorphous porous-organic polymers (POPs) as new emerging heterogeneous catalysts have attracted significant attention due to their promising activity and potential material tunability. These porous solids offer exceptional structural uniformity, facile tunability and permanent porosity. In addition, tuning the catalytic selectivity of these porous materials can be achieved through engineering their site microenvironments, such as metal node substitution, linker changes, node/linker functionalization, and pore modification. The present review provides an overview of the current state of the art on MOFs, COFs and POPs as advanced catalysts for various C–H bond activation reactions, providing details about their chemo-, regio-, and stereo-selectivity control, comparing their performance with that of other catalysts, triggering additional research by showing the present limitations and challenges in this area, and providing a perspective for future developments.
Rodríguez et al., 2022, Green Chemistry, 24, 6613-6618.
α-Aminoamides are reiterative molecular subunits in transcendental molecules, which bear attractive functionalities for (bio)synthetic purposes. Herein, their preparation by harnessing flow photocatalysis is described, which provides significant improvements over other catalytic methods in terms of efficiency while outperforming an analogous batch setup. A reaction time of five minutes, operational simplicity and absence of purification steps for the isolation of final products evidence an enhanced performance.
Martínez and Alonso, 2022, Physical Reviews A, 105, 062820.
The mass spectra of metal clusters obtained by gas aggregation and laser vaporization techniques show variations of the abundance with cluster size, and the best-known manifestation is the magic numbers. The case of nanoalloys adds structural and chemical richness, and asymmetry in the atomic concentrations is sometimes observed. This is the case with TixCy and SixCy clusters, for which the experiments performed until now reveal an enrichment of the clusters in carbon. The structure of the mass spectrum arises when hot clusters evaporate atoms to cool down. By performing density functional calculations, complemented by a thermochemical formalism to obtain Gibbs free energies, we have found that for each cluster in the families TixCy and SixCy with x = 1–4 and y = 1–4, it is easier to remove titanium or silicon atoms as compared to removing carbon atoms, and this explains the carbon enrichment systematically observed in the experiments. The conclusion is that the broad compositional trends expected in the formation of alloy nanoclusters can be anticipated from calculations of the evaporation energies of the alloy components, and this may be particularly interesting in the field of catalysis.
López-Magano et al., 2023, Advanced Materials, accepted article.
Organic photochemistry developed intensely in the 1980s in which the nature of excited electronic states and the energy and electron transfer processes were thoroughly studied and finally well-understood. This knowledge from molecular organic photochemistry can be transferred to the design of covalent organic frameworks (COFs) as active visible light photocatalyts. Covalent organic frameworks (COFs) constitute a new class of crystalline porous materials with substantial application potentials. Featured with outstanding structural tunability, large porosity, high surface area, excellent stability and unique photoelectronic properties, COFs have been studied as potential candidates in various research areas (e.g., photocatalysis). This review was aimed to provide state-of-the-art insights into the design of COF photocatalysts (pristine, functionalized, and hybrid COFs) for organic transformations. The catalytic reaction mechanism of COF-based photocatalysts and the influence of dimensionality and crystallinity on heterogenous photocatalysis performance was also discussed, followed by perspectives and prospects on the main challenges and opportunities in future research of COFs and COF-based photocatalysts.
Murillo et al., 2022, Journal of Materials Chemistry C, 10, 18004-18016.
In this work, we have synthesized a new luminescent and semiconducting coordination polymer (CP) based on [CuI(PyzBr)]n staircase double Cu(I)–I chains where the 2-bromopyrazine ligand (PyzBr) was selected as the N-donor ligand due to its wide commercial availability in liquid whereas copper iodide (CuI) is a promising p-type wide bandgap semiconductor for optoelectronic applications. The obtained CP has a reversible luminescent response to temperature and pressure. Upon decreasing the temperature, its emission increases drastically. Structure determination by single-crystal X-ray diffraction at 250 and 100 K allows us to understand its response to temperature. It also presents an electrical response to pressure. Its electrical conductivity increases with quasi-hydrostatic pressure. This result has been corroborated by spin-polarized single-point DFT band structure calculations. In addition, a discontinuity is found in the conductivity near 5 GPa, supporting the existence of a phase transition which is corroborated by an important structural transformation around these pressures. This conclusion is consistent with optical-absorption experiments, which show that the electronic band gap decreases with pressure and also has a discontinuity at ≈5 GPa. Its structural changes are directly related to the high Cu(I)–I chain structural flexibility, which allows the shortening of the Cu⋯Cu distances via cuprophilic interactions.
Ruiz del Árbol et al., 2023, Chemical Communications, accepted manuscript.
We have grown two different metal-organic frameworks with either honeycomb or Kagome structure on Cu(111) using para-aminophenol molecules and native surface adatoms. Although structurally different, both frameworks are made up from the same chemical species, the critical role being played by the reaction conditions during their growth. This work highlights the importance of the balance between thermodynamics and kinetics in the final structure of surface-supported metal-organic networks.
López-Magano et al., 2022, Advanced Sustainable Systems, 6 (3), 2100409.
The phenanthroline unit in an imine-based covalent organic framework (Phen-COF) offers a robust coordination site for Pd(OAc)2 centers. Coordination of palladium centers is demonstrated by a variety of techniques, including X-ray photoelectron spectroscopy and total X-ray fluorescence. The stable phenanthroline-Pd(II) coordination avoids leaching of metal centers to the reaction medium, where deactivation processes through nanoparticle formation limits the catalytic activities observed for homogeneous systems. Thus, because of isolation and immobilization of catalytic sites in the Pd@Phen-COF the performance of material, the catalytic outputs are dramatically increased with respect to the performance observed for analogous molecular catalysts. This concept is applied in this work to CC cross-coupling reactions under mild and environmentally benign conditions. The activities found for Suzuki-Miyaura and Mizoroki-Heck reactions allow thousands of turnover numbers in the transformation of a wide scope of precursors with a high degree of recyclability. The results reported in this work contribute to the design of greener protocols for transformations that have a crucial role in the industrial synthesis of high-added value fine chemicals.
Sánchez-Fuente et al., 2021, Catalysts, 11 (12), 1426
Reaction between concentrated solutions of phenotiazine containing trialdehyde building block 4,4′,4″-(10-phenothiazine-3,7,10-triyl)tribenzaldehyde and (1R,2R)-cyclohexane-1,2-diamine results in the formation of a yellow transparent film. Exhaustive characterization of this material indicates that it is the result of the assembly of a linear polymer resulting from the linking of imine-based macrocycles. Phenotiazine units confer to this plastic the optical properties characteristic of photocatalytic materials. The transparency of the obtained material enabled the performance of solvent-free photocatalytic processes. This concept is illustrated by the oxidation of liquid organic sulfides, which can be performed in a recyclable manner. According to selective quenching experiments, such processes are the result of the energy transfer to oxygen molecule, generating singlet oxygen that is able to activate the sulfide molecules directly.
Mollari et al., 2022, Chemical Communications 58, 1334.
Herein, a visible-light mediated strategy unlocking a family of cyclic β-amino carbonyl derivatives bearing three contiguous stereogenic centres is introduced. The overall reactivity relies on the performance of the substrate-catalyst complex to assist both the enantiocontrol and the photoredox tasks. This transformation led to an enantioselective [3 + 2] photocycloaddition between coordinated α,β-unsaturated acyl imidazoles and cyclopropylamine derivatives.
Valle-Amores et al., 2022, Journal of Catalysis, 406, 174-183.
The carbocatalytic synthesis of azines (N-N linked diimines) by mild-oxidized multiwalled carbon nanotubes catalyst (oxMWNT) is presented. The material, just with a 5 %wt. loading, is able to carry out a smooth room-temperature metal-free condensation of aldehydes and hydrazine, without external additives, to obtain a wide library of symmetric and also asymmetric azines in excellent yields, even in gram scale, with an excellent selectivity for aromatic substrates. This methodology allows the synthesis of azines with application in nonlinear optics, and the organic materials and biological active compounds crafting. oxMWNT catalysed the reaction in just 3 h with full recyclability upon the recovery of the catalyst. In addition, due to the inherent oxMWNT oxidative capacity in the presence of nitric acid, we have also developed the one-pot synthesis of azines starting from alcohols.
Guerrero-Corella et al., 2022, ACS Organic & Inorganic Au, 2 (3) 197-204.
The activation of molecules through intramolecular hydrogen-bond formation to promote chemical reactions appears as a suitable strategy in organic synthesis, especially for the preparation of chiral compounds under metal and organocatalytic conditions. The use of this interaction has enabled reactivity enhancement of reagents, as well as stabilization of the chemical species and enantiocontrol of the processes.
Nova-Fernández et al., 2022, Chemical Communications, 58, 4611-4614.
A sustainable strategy for the alkylation of heterocycles is presented. The protocol relies on the in-situ generation and further in-line use of alkyl zinc sulfinates through a continuous-flow system. The environmentally friendly character of the protocol is assured by the use of a green solvent mixture, the presence of a metal free oxidant and low waste generation.
Jiménez-Almarza et al., 2022, ACS Applied Materials Interfaces, 14 (14), 16258-16268.
Three materials containing a photoactive unit, 10-phenyl phenothiazine (PTH), have been studied for the visible light-mediated oxidative coupling of amines. In particular, the materials considered are assembled through the condensation of extended polyimine, polyhydrazone, or polytriazine frameworks. These three materials present different stabilities in the presence of strong nucleophiles such as amines, which is a key factor for efficient catalytic performance. In the series of materials reported herein, the triazine-based material shows the optimal compromise between activity and stability when studied for the oxidative coupling of amines, achieving imine products. Accordingly, while significant leaching of molecular active fragments is ruled out for triazine-based polymers, other materials of the series show a significant chemical erosion as a result of the reaction with the amine substrates. Consequently, only a triazine-based material allows performing several catalytic cycles (up to seven) with yields higher than 80%. The applicability of this heterogeneous catalyst has been proven with a variety of substrates, confirming its stability and obtaining diverse imine coupling products with excellent yields.
Salaverri et al., 2022, Advanced Synthesis & Catalysis, 364 (10), 1689-1694.
The application of proton coupled electron transfer (PCET) processes in organic synthesis has opened the door to new radical intermediates for synthesis such as alkyl radicals in remote positions to a ketone. Herein, we present the addition of these remote alkyl radicals to electron deficient double bonds under photoorganocatalyzed and very mild conditions. The method is not only applicable to diactivated double bonds, but monoactivated ones are also accessible using more stabilized alkyl radicals, and alkyl chains of any length can be introduced. The final products can be easily converted into more complex structures via a one-pot process, and the activating functional groups were transformed in the more versatile methyl esters. Mechanistic investigations support a mechanistic proposal based on a PCET process.
Salaverri et al., 2022, Organic Letters, 24 (17), 3123–3127.
Herein, we report the enantioselective addition of remote alkyl radicals, generated from the ring opening of unstrained cycloalkanols by a proton-coupled electron transfer (PCET) process, to 2-acyl imidazoles previously coordinated to a rhodium-based chiral Lewis acid. High yields and enantioselectivites up to 99% are achieved in 1 h. Mechanistic investigations support the formation of the remote alkyl radical by a PCET process, and theoretical studies explain the observed stereochemistry in the addition step.