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A one-step method for the conversion of nitroarenes into phenols under operationally simple, transition-metal-free conditions is described. This denitrative functionalisation protocol provides a concise and economical alternative to conventional three-step synthetic sequences. Experimental and computational studies suggest that unactivated nitroarenes may be substituted via an electron-catalysed radical-nucleophilic substitution (SRN1) chain mechanism.

The design and development of an oxime-based hydroxylation reagent, which can chemoselectively convert aryl halides (X = F, Cl, Br, I) into phenols under operationally simple, transition-metal-free conditions is described. Key to the success of this approach was the identification of a reducing oxime anion which can interact and couple with open-shell aryl radicals. Experimental and computational studies support the proposed radical-nucleophilic substitution chain mechanism., The design and development of an oxime-based hydroxylation reagent, which can chemoselectively convert aryl halides (X = F, Cl, Br, I) into phenols under operationally simple, transition-metal-free conditions is described.

We report a new general method for trapping short-lived radicals, based on a homolytic substitution reaction SH2′. This departure from conventional radical trapping by addition or radical-radical cross-coupling results in high sensitivity, detailed structural information, and general applicability of the new approach. The radical traps in this method are terminal alkenes possessing a nitroxide leaving group (e.g., allyl-TEMPO derivatives). The trapping process thus yields stable products which can be stored and subsequently analyzed by mass spectrometry (MS) supported by well-established techniques such as isotope exchange, tandem MS, and high-performance liquid chromatography-MS. The new method was applied to a range of model radical reactions in both liquid and gas phases including a photoredox-catalyzed thiol-ene reaction and alkene ozonolysis. An unprecedented range of radical intermediates was observed in complex reaction mixtures, offering new mechanistic insights. Gas-phase radicals can be detected at concentrations relevant to atmospheric chemistry.

A dinuclear, butadiene-bridged complex, trans-μ2:η2,η2-1,3-butadiene-bis(trichloroplatinate(ii)) (1) was unexpectedly obtained on photolysis of acetone solutions of (NBu4)2[PtCl6].

The formation of alginate gel from low molecular weight alginate and very low molecular weight alginate in the presence of divalent cations was investigated using Electron Paramagnetic Resonance (EPR) spectroscopy. The transition from sol to gel in the presence of divalent cations was monitored by the changes in the dynamics of spin labelled alginate. The immobilisation of the spin labelled alginate in the gel reflects the strength of interaction between the cation and alginate chain. Diffusion experiments showed that both the cation and alginate polyanion in the gel fibres can exchange with molecules in solution. In particular, we showed that dissolved alginate polyanions can replace alginates in the gel fibres, which can hence diffuse through the bulk of the gel. This illustrates the surprisingly highly dynamic nature of these gels and opens up the possibility of preparing multicomponent alginate gels via polyanion exchange process.

Stable benzotriazinyl radicals (Blatter's radicals) recently attracted considerable interest as building blocks for functional materials. The existing strategies to derivatize Blatter's radicals are limited, however, and synthetic routes are complex. Here, we report that an inexpensive, commercially available, analytical reagent Nitron undergoes a previously unrecognized transformation in wet acetonitrile in the presence of air to yield a new Blatter-type radical with an amide group replacing a phenyl at the C(3)-position. This one-pot reaction of Nitron provides access to a range of previously inaccessible triazinyl radicals with excellent benchtop stabilities. Mechanistic investigation suggests that the reaction starts with a hydrolytic cleavage of the triazole ring followed by oxidative cyclization. Several derivatives of Nitron were prepared and converted into Blatter-type radicals to test the synthetic value of the new reaction. These results significantly expand the scope of using functionalized benzotriazinyls as stable radical building blocks., Blatter's radicals are highly valuable stable organic free radicals, comprising a heterocyclic ring system functionalized with a heteroaryl or alkyl group at the C-3 position. Here, the authors report a synthetic strategy to obtain Blatter's radical substituted with an amine and amide group.

The polymerisation inhibitor, 2-nitrophenol reacts with radicals generated in self-initiated thermal styrene polymerisation to form the unexpected benzoxazine 1. We propose the mechanism for the formation of 1 and show that it is produced by a radical mediated C–H activation.

Electron paramagnetic resonance (EPR) spectroscopy and spin trapping were used to explore the mechanism of alcohol oxidation over gold catalysts. Reaction of secondary alcohols with supported and unsupported gold catalysts (e.g., Au/CeO(2), polymer-incarcerated Au nanoparticles, PPh(3)-protected Au nanoparticles) in the presence of spin traps led to the formation of a hydrogen spin adduct. Using isotope labeling, we confirmed that the hydrogen in the spin adduct originates from the cleavage of the C-H bond in the alcohol molecule. The formation of the hydrogen spin adduct most likely results from the abstraction of hydrogen from the Au surface by a spin trap. These results thus strongly suggest intermediate formation of Au-H species during alcohol oxidation. The role of oxygen in this mechanism is to restore the catalytic activity rather than oxidize alcohol. This was further confirmed by carrying out gold-catalyzed alcohol oxidation in the absence of oxygen, with nitroxides as hydrogen abstractors. The support (e.g., metal oxides) can activate oxygen and act as an H abstractor from the gold surface and hence lead to a faster recovery of the activity. Peroxyl radicals were also observed during alcohol oxidation, consistent with a free-radical autoxidation mechanism. However, this mechanism is likely to be a minor side reaction, which does not lead to the formation of an appreciable amount of oxidation products.

New nanocomposites containing functionalized acrylate monomers and Au nanoparticles (NPs, 1.5−3 nm core diameter, 1−2 wt %) have been developed for an all-optical fabrication of periodic bulk structures by holographic photopolymerization. The Au NPs were coated with ethyl 11-mercaptoundecanoate to ensure good solubility in low-polarity organic media. The materials with only 1.5 wt % Au NPs show unusually high amplitude of the refractive index modulation in the diffraction volume gratings (0.0073). Both volume and surface relief gratings are formed during the holographic exposure. The proposed mechanism of the refractive index contrast amplification (as compared to monomer mixture without NPs) includes interception of free radicals by Au nanoparticles. This slows down the free-radical photopolymerization, thus promoting the increase in lateral periodic redistribution of the components in the interference pattern. The redistribution of both NP and monomers provides high efficient grating formation with the ...

A study of the magnetic and electrochemical properties of a TEMPO-substituted disulfide diradical in three different environments was carried out: in solution, in the crystal, and as a self-assembled monolayer (SAM) on an Au(111) substrate, and the relationship between them was explored. In solution, this flexible diradical shows a strong spin-exchange interaction between the two nitroxide functions that depends on the temperature and solvent. Structural, dynamic, and thermodynamic information has been extracted from the EPR spectra of this dinitroxide. The magnetic interactions in the crystal include intra- and intermolecular contributions, which have been studied separately and shown to be antiferromagnetic in both cases. Finally, we demonstrate that both the magnetic and electrochemical properties are preserved upon chemisorption of the diradical on a gold surface. The resulting SAM displayed anisotropic magnetic properties, and angle-resolved EPR spectra of the monocrystal allowed a rough determination of the orientation of the molecules in the SAM., This work was supported by the CSIC-PIF RAPCAM (PIF-08-017-3), DGI grants CONSOLIDERC (CTQ2006-06333), POMAs (CTQ2010-19501), and BeWell (CTQ2013-40480-R), and the AGAUR (2009-SGR-00516 and 2014-SGR-17). CIBER-BBN is an initiative funded by the VI National R&D&i Plan 2008–2011, Iniciativa Ingenio 2010, Consolider Program, CIBER Actions and financed by the Instituto de Salud Carlos III with assistance from the European Regional Development Fund.

The properties of a gel consisting of a covalent network formed by the reaction of isocyanate end-capped polyethylene glycol (PEG) with β-cyclodextrin, were investigated by EPR spectroscopy. Spin-labelled cyclodextrin was incorporated into the cross-link points of the gel and at the chain ends. The dynamics of the gel fibres as reported by the spin label, was found to be sensitive to the H-bonding ability of the solvent, density of cross-links and temperature. Addition of spin probes (e.g., TEMPO and adamantane-TEMPO) to the unlabelled gel made it possible to characterise the solvent pools in the gel. While TEMPO was uniformly distributed throughout the solvent pools, the adamantane derivative was located at the gel fibre-solvent pool interface; these two probes thus reported on the different locations in the solvent pools. At low temperature, the gels were shown to prevent ice crystallisation in the solvent pools resulting in the formation of supercooled water. Both probes showed that the water froze at ca. 250 K, thus suggesting that the properties of the supercooled water are uniform across the solvent pools.

EPR spectroscopy and spin-trapping experiments showed that polymer-encapsulated Au nanoparticles promote aldehyde oxidation via a radical pathway by initiating formation of acyl radicals.

During asymmetric cyanohydrin synthesis catalysed by vanadium(V)oxo(salen) complexes, the catalysts are reduced to vanadium(IV)oxo(salen) as determined by EPR spectroscopy; the reducing agent is cyanide which is oxidised to cyanogen via a non-radical mechanism.

Monolayer-protected, Gd(3+)-functionalised gold nanoparticles with enhanced spin-lattice relaxivity (r(1)) were prepared; adsorption of polyelectrolytes on these materials further increased r(1) and ligand exchange with a biotin-derivatised disulfide led to a prototype avidin-targeted contrast agent.

Stirring of perthiolated beta-cyclodextrin in water yields cross-linked hollow capsules ca. 50 nm in diameter, which can be used for encapsulation and controlled release of large molecules as shown using Reichardt's dye.

Functionalisation of PAMAM dendrimers with a small number of thiol groups makes them good ligands for CdSe/ZnSe nanoparticles; the particles coated with thiolated dendrimers have good cell permeability and are potent transfection agents.

Au nanoparticles protected with a polymerisable ligand were incorporated into bulk macroporous polymers; etching the metal core resulted in disulfide-lined, nanometre-scale cavities capable of recognising similarly-sized Au particles.

Spin-trapping experiments show the formation of sulfur-centered radicals during a ligand-exchange reaction between Ph3P-protected Au nanoparticles and alkanethiols in air (see picture). Oxidation of the alkanethiols by molecular O2 adsorbed on the nanoparticles is proposed as the key step. The feasibility of such a process is demonstrated by the Au-nanoparticle-catalyzed oxidation of BH4- and tBuOOH with air.

Gold nanoparticles with diameters of ca. 13 nm were synthesised by UV irradiation of a supramolecular organogel into which HAuCl4 and tetraoctylammonium bromide had been diffused—the gel network plays an essential role in nanoparticle stabilisation.

This Account reports the synthesis and characterization of dendrimer-encapsulated metal nanoparticles and their applications to catalysis. These materials are prepared by sequestering metal ions within dendrimers followed by chemical reduction to yield the corresponding zerovalent metal nanoparticle. The size of such particles depends on the number of metal ions initially loaded into the dendrimer. Intradendrimer hydrogenation and carbon−carbon coupling reactions in water, organic solvents, biphasic fluorous/organic solvents, and supercritical CO2 are also described.

The kinetics of alkaline hydrolysis of ester groups in self-assembled monolayers (SAMs) were monitored by a combination of atomic force microscopy (AFM) on the nanometer scale and FT-IR spectroscop...

Solutions of alkanethiol-protected Au nanoparticles undergo a slow aging process over a period of several days, which dramatically reduces their reactivity in a place-exchange reaction with disulfides. This behavior was tentatively attributed to the reorganization of the most reactive binding sites (e.g., defect sites) on the nanoparticle surface.

An EPR study of a place-exchange reaction of a diradical disulfide with butanethiol-protected Au nanoparticles showed that the two branches of the disulfide molecule do not adsorb adjacent to each other on the Au surface. The most likely mechanism includes adsorption of only one branch of the disulfide molecule in the exchange process. The exchange reaction was found to be zeroth-order with respect to the diradical, indicative of a dissociative "SN1"-type mechanism.

Oxidation of gold nanoparticles protected by thiolated β-cyclodextrin molecules, leads to formation of water-soluble polycyclodextrin nanocapsules held together by S–S bonds.

Formation of organogels from covalently linked polyethylene glycols and cyclodextrins and their encapsulation/release properties were monitored with EPR spectroscopy using spin labels and spin probes; changes in molecular tumbling rates provided information on spin probe/label location inside the gel network, and molecular properties of the gel.

The formation of radicals in the gas phase oxidation of cyclohexane over metal oxides was studied using spin trapping and EPR spectroscopy; the results confirmed that during the catalytic initiation, the oxygen responsible for the radicals formation originates from the lattice of the metal oxide.

N-Heterocyclic carbene coated Au and Pd nanoparticles have been prepared by a ligand exchange reaction; although carbenes quantitatively displaced the thioether and phosphine ligands from the nanoparticle surface, the resultant nanoparticles spontaneously leached metal complexes and aggregated in solution.

Here we describe the spontaneous assembly of fatty acids onto the surface of amine-terminated, poly(amidoamine) (PAMAM) dendrimers (Scheme 1). This process, which is driven by acidbase chemistry and ion pairing, results in the extraction of dendrimers from aqueous solutions to nonpolar phases. Dendrimer-encapsulated guest molecules, such as the hydrophilic dyes and catalytically active metal nanoparticles described here, can thus be easily solubilized in organic solvents. Dendrimers are monodisperse, hyperbranched polymers possessing a very high concentration of surface functional groups.1 Many of the properties of dendrimers, including their solubility, are strongly influenced by the nature of these terminal functionalities.2,3 For example, dendrimers terminated in hydrophobic groups are soluble in nonpolar solvents, while those having hydrophilic groups are soluble in polar solvents such as water and low-molecular-weight alcohols.4-6 Terminal groups are normally covalently bonded to the body of dendrimers, but there have been a few reports of electrostatic binding of charged molecules to the surface of dendrimer polyions.1,7-10 We now demonstrate that complete surface modification of dendrimers can be accomplished by spontaneous, acid-base self-assembly, which eliminates the need for chemical synthesis and purification. A similar approach has previously been reported for solubilization of ionic polymers11,12 and DNA13,14 in organic solvents. The electrostatic self-assembly process we report is reversible, and therefore the dendrimers and whatever guests they may contain can be easily shuttled between hydrophilic and hydrophobic phases by adjustment of the pH of the aqueous phase. Fourth-generation, amine-terminated PAMAM dendrimers (G4NH2) readily dissolve in toluene or heptane containing dodecanoic acid. The amount of G4-NH2 that can be dissolved in 1% dodecanoic acid/toluene corresponds to about 1 dendrimer per 70-80 molecules of acid15 which suggests an approximately 1:1 stoichiometry between the fatty acid and each of the 64 terminal amine groups present on the dendrimers. Transmission FT-IR spectroscopy of this toluene solution (Figure 1) indicates that solubilization is accompanied by proton transfer from the acid to the terminal amine groups of the dendrimers and that ionization is essentially complete. Evidence for this comes from the nearly complete disappearance of the dodecanoic acid carboxyl peak at 1710 cm-1 (part a of Figure 1) and the appearance of the asymmetric carboxylate peak at 1557 cm-1 (part c of Figure 1) upon addition of dendrimer.16 We conclude that the acid molecules arrange themselves around the dendrimer in a composite structure that resembles an inverted micelle having a hydrophilic dendritic interior and a hydrophobic alkyl-chain-dominated exterior that lends solubility to the ensemble (Scheme 1). In the presence of large excesses of dodecanoic acid, proton transfer extends to the tertiary amine groups within the dendrimer interior (see Supporting Information). Dendrimers with covalently grafted hydrophobic terminal groups have previously been shown to dissolve in nonpolar solvents and solubilize encapsulated guest molecules.3,4,6 The present system, however, offers two significant advantages. First, commercially available dendrimers can be used directly to prepare solutions in nonpolar solvents without the need for chemical synthesis or separation. Second, since acid-base interactions are reversible, addition of HCl to the aqueous phase or simple dilution of the organic phase with pure solvent leads to the transfer of G4-NH2 back into aqueous layer. We report here two examples * To whom correspondence should be addressed. Telephone: (409) 8455629. Fax: (409) 845-1399. E-mail: crooks@tamu.edu. (1) For a recent review, see Zeng, F. W.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681. (2) Hawker, C. J.; Chu, F. Macromolecules 1996, 29, 4370. (3) Cooper, A. I.; Londono, J. D.; Wignall, G.; McClain, J. B.; Samulski, E. T.; Lin, J. S.; Dobrynin, A.; Rubinstein, M.; Burke, A. L. C.; Frechet, J. M. J.; DeSimone, J. M. Nature 1997, 389, 368. (4) Jansen, J. F. G. A.; Meijer, E. W. Macromol. Symp. 1996, 27. (5) Schenning, A. P. H. J.; Elissen-Roman, C.; Weener, J.-W.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (6) Sayed-Sweet, Y.; Hedstrand, D. M.; Spinder, R.; Tomalia, D. A. J. Mater. Chem. 1997, 7, 1199. (7) Li, Y.; Dubin, P. L.; Spindler, R.; Tomalia, D. A. Macromolecules 1995, 28, 8426. (8) Jockusch, S.; Turro, N. J.; Tomalia, D. A. Macromolecules 1995, 28, 8, 7416. (9) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515. (10) Watkins, D. M.; Sayed-Sweet, Y.; Klimash, J. W.; Turro, N. J.; Tomalia, D. A. Langmuir 1997, 13, 3136. (11) Kabanov, A. V.; Sergeev, V. G.; Foster, M. S.; Kasaikin, V. A.; Levashov, A. V.; Kabanov, V. A. Macromolecules 1995, 28, 3657. (12) Bakeev, K. N.; Shu, Y. M.; Zezin, A. B.; Kabanov, V. A.; Lezov, A. V.; Mel’nikov, A. B.; Kolomiets, I. P.; Rjumtsev, E. I.; MacKnight, W. J. Macromolecules 1996, 29, 1320. (13) Sergeyev, V. G.; Mikhailenko, S. V.; Pyshkina, O. A.; Yaminsky, I. V.; Yoshikawa, K. J. Am. Chem. Soc. 1999, 121, 1780. (14) Mel’nikov, S. M.; Lindman, B. Langmuir 1999, 15, 1923. (15) This value was calculated from the maximum amount of neat G4NH2 which completely dissolves in 1% dodecanoic acid/toluene mixture after 5 min of sonication. (16) The νa(CO2) peak at 1557 cm-1 overlaps with the amide II band. In a similar system with the PAMAM dendrimer replaced by a poly(propyleneimine) Cascade dendrimer which does not contain any amide bonds, we have clearly observed the appearance of the νa(CO2) peak. Scheme 1 4910 J. Am. Chem. Soc. 1999, 121, 4910-4911

This article discusses recent publications on chemical reactivity in large organic structures, including supramolecular systems (e.g., host–guest complexes), dendrimers, colloids (e.g., micelles, vesicles, microemulsions), nanoparticles and thin films. Although these systems are very different structurally, the main factors affecting organic reactivity in these assemblies are similar. These effects are: (i) proximity of functional groups in organised assemblies that may facilitate reactions between them (“proximity effect”); (ii) partitioning of the reagents between the bulk solvent and the organised assembly, which determines the local concentration of reagents in the vicinity of the functional group (“solvent effect”); (iii) unique properties of the interface between the organic assembly and the bulk solution (“interface effect”); (iv) restricted and/or anisotropic mobility of organic molecules in the organised assembly (“steric effect”). This article aims to describe each effect using examples from different types of organic assemblies. The review covers literature published in 2006 and early 2007.

The self-assembly of dendritic building blocks containing multiple terminal alkenes on their surfaces yields soft gel-phase materials--subsequent Grubbs' metathesis leads to covalent cross-linking between the alkenes and the formation of robust swellable gels.

A series of physical organic chemistry experiments suitable for second- or third-year undergraduate students is presented. 1H NMR spectroscopy is used to monitor the enolization of butanone and calculate rate constants, activation parameters, and primary and secondary isotope effects for this reaction. The experiments teach students to handle and interpret NMR spectra and offer a broad range of topics for discussion, including isotope effects, kinetics, activation parameters, regiochemistry, and reaction mechanisms.

L-Lysine based dendrimers containing a disulfide moiety at the core were synthesized. These novel ligands were used to stabilize Au nanoparticles via a one-phase chemical reduction method. The size of the resultant nanoparticles was found to decrease with increasing dendritic generation. Furthermore, the thermal stability of the nanoparticles in solution was shown to be controlled by the extent of branching in the protecting dendritic shell.