A metal-free carbon catalyst for oxidative dehydrogenation of aryl cyclohexenes to produce biaryl compounds.

A metal-free route based on a carbon catalyst to synthesize biphenyls through oxidative dehydrogenation (ODH) of phenyl cyclohexene has been investigated. Among the samples examined, an air-oxidized active carbon exhibits the best activity with a 9.1 × 10-2 h-1 rate constant, yielding 74% biphenyl in 28 h at 140 °C under five bar O2 in anisole. The apparent activation energy is measured as 54.5 kJ⋅mol-1. The extended reaction scope, consisting of 15 differently substituted phenyl cyclohexenes, shows the wide applicability of the proposed method. The catalyst's good recyclability over six runs suggests this ODH method as a promising route to access the biaryl compounds. In addition, the reaction mechanism is investigated with a combination of X-ray photoelectron spectroscopy, functional group blocking, and model compounds of carbon catalysts and is proposed to be based on the redox cycle of the quinoidic groups on the carbon surface. Additional experiments prove that the addition of the catalytic amount of acid (methanesulfonic acid) accelerates the reaction. In addition, Hammett plot examination suggests the formation of a carbonium intermediate, and its possible structure is outlined.

Divergent Carbocatalytic Routes in Oxidative Coupling of Benzofused Heteroaryl Dimers: A Mechanistic Update.

Mildly thermal air or HNO3 oxidized activated carbons catalyse oxidative dehydrogenative couplings of benzo[b]fused heteroaryl 2,2'-dimers, e.g., 2-(benzofuran-2-yl)-1H-indole, to chiral 3,3'-coupled cyclooctatetraenes or carbazole-type migrative products under O2 atmosphere. DFT calculations show that the radical cation and the Scholl-type arenium cation mechanisms lead to different products with 2-(benzofuran-2-yl)-1H-indole, being in accord with experimental product distributions.

Carbocatalytic Oxidative Dehydrogenative Couplings of (Hetero)Aryls by Oxidized Multi-Walled Carbon Nanotubes in Liquid Phase.

HNO3 -oxidized carbon nanotubes catalyze oxidative dehydrogenative (ODH) carbon-carbon bond formation between electron-rich (hetero)aryls with O2 as a terminal oxidant. The recyclable carbocatalytic method provides a convenient and an operationally easy synthetic protocol for accessing various benzofused homodimers, biaryls, triphenylenes, and related benzofused heteroaryls that are highly useful frameworks for material chemistry applications. Carbonyls/quinones are the catalytically active site of the carbocatalyst as indicated by model compounds and titration experiments. Further investigations of the reaction mechanism with a combination of experimental and DFT methods support the competing nature of acid-catalyzed and radical cationic ODHs, and indicate that both mechanisms operate with the current material.

Photoreductive Removal of O-Benzyl Groups from Oxyarene N-Heterocycles Assisted by O-Pyridine-pyridone Tautomerism.

Facile photoreductive protocols have been developed to remove benzyl O-protective groups from oxyarene N-heterocycles at positions capable for 2-/4-O-pyridine-2-/4-pyridone tautomerism. Blue light irradiation, a [Ru] or [Ir] photocatalyst, and ascorbic acid in a water-acetonitrile solution debenzylates a variety of aryl N-heterocycles cleanly and selectively. Ascorbic acid has two functions in the reaction. On the one hand, it protonates the N-heterocycles that reduces their reduction potentials notably and on the other hand it acts as a sacrificial reductant. Reduction potentials and free energy barriers calculated at the CPCM-B3LYP/6-31+G** level can predict the reactivities of the studied substrates.

Synthesis, aromaticity and photophysical behaviour of ferrocene- and ruthenocene-appended semisynthetic chlorin derivatives.

Two novel synthetic strategies to covalently link a metallocene electron-donor unit to a chlorin ring are presented. In one approach, pyropheophorbide a is readily converted into its 13(1) -ferrocenyl dehydro derivative by nucleophilic addition of the ferrocenyl anion to the 13(1) -carbonyl group. In another approach, the corresponding 13(1) -pentamethylruthenocenyl derivative is synthesised from 13(1) -fulvenylchlorin by a facile ligand exchange/deprotonation reaction with the [RuCp*(cod)Cl] (Cp*=pentamethylcyclopentadienyl; cod=1,5-cyclooctadiene) complex. The resulting metallocene-chlorins exhibit reduced aromaticity, which was unequivocally supported by ring-current calculations based on the gauge-including magnetically induced current (GIMIC) method and by calculated nucleus-independent chemical shift (NICS) values. The negative ring current in the isocyclic E ring suggests the antiaromatic character of this moiety and also clarifies the spontaneous reactivity of the complexes with oxygen. The oxidation products were isolated and their electrochemical and photophysical properties were studied. The ruthenocene derivatives turned out to be stable under light irradiation and showed photoinduced charge transfer with charge-separation lifetimes of 152-1029 ps.

Photoinduced charge transfer in a conformational switching chlorin dimer-azafulleroid in polar and nonpolar media.

In the present study, a biomimetic reaction center model, that is, a molecular triad consisting of a chlorin dimer and an azafulleroid, is synthesized and its photophysical properties are studied in comparison with the corresponding molecular dyad, which consists only of a chlorin monomer and an azafulleroid. As evidenced by (1) H NMR, UV/Vis, and fluorescence spectroscopy, the chlorin dimer-azafulleroid folds in nonpolar media into a C2 -symmetric geometry through hydrogen bonding, resulting in appreciable electronic interactions between the chlorins, whereas in polar media the two chlorins diverge from contact. Femtosecond transient absorption spectroscopy studies reveal longer charge-separated states for the chlorin dimer-azafulleroid; ≈1.6 ns in toluene, compared with the lifetime of ≈0.9 ns for the corresponding chlorin monomer-azafulleroid in toluene. In polar media, for example, benzonitrile, similar charge-separated states are observed, but the lifetimes are inevitably shorter: 65 and 73 ps for the dimeric and monomeric chlorin-azafulleroids, respectively. Nanosecond transient absorption and singlet oxygen phosphorescence studies corroborate that in toluene, the charge-separated state decays indirectly via the triplet excited state to the ground state, whereas in benzonitrile, direct recombination to the ground state is observed. Complementary DFT studies suggest two energy-minima conformations, that is, a folded chlorin dimer-azafulleroid, which is present in nonpolar media, and another conformation in polar media, in which the two hydrophobic chlorins wrap the azafulleroid. Inspection of the frontier molecular orbitals shows that in the folded conformation, the HOMO on each chlorin is equivalent and is shared owing to partial π-π overlap, resulting in delocalization of the conjugated π electrons, whereas the wrapped conformation lacks this stabilization. As such, the longer charge-separated lifetime for the dimer is rationalized by both the electron donor-acceptor separation distance and the stabilization of the radical cation through delocalization. The chlorin folding seems to change the photophysical properties in a manner similar to that observed in the chlorophyll dimer in natural photosynthetic reaction centers.

Gold(III)-catalyzed enynamine-cyclopentadiene cycloisomerization with chirality transfer: an experimental and theoretical study indicating involvement of dual Au(III) push-pull assisted cis-trans isomerism.

A synthetic approach for asymmetric ring-fused cyclopentadienes (Cps) with a chiral carbon at the ring junction has been established from chiral enynamines by achiral Au(III) catalysis. On the basis of experimental and theoretical data, the proposed mechanistic pathway from enynamines to Cps occurs via a Au(III) ene cis-trans isomerization step. Computational studies at DFT and NEVPT2 levels advocate that the cis-trans isomerization step proceeds via a dual Au(III) push-pull assisted intermediate with a low computed rotation barrier. The chirality transfer occurs through a helical-shaped transition state with allenic character. The scope of the catalysis encompasses sterically bulky enynamines including terpene natural products.

Regioselectivity of intermolecular Pauson-Khand reaction of aliphatic alkynes: experimental and theoretical study of the effect of alkyne polarization.

Generally judged poor electronic regioselectivity of alkyne insertion in intermolecular Pauson-Khand reaction (PKR) has severely restricted its synthetic applications. In our previous rational study concerning diarylalkynes (Fager-Jokela, E.; Muuronen, M.; Patzschke, M.; Helaja, J. J. Org. Chem. 2012, 77, 9134-9147), both experimental and theoretical results indicated that purely electronic factors, i.e., alkyne polarization via resonance effect, induced the observed modest regioselectivity. In the present work, we substantiate that the alkyne polarization via inductive effect can result notable, synthetically valuable regioselectivity. Computational study at DFT level was performed to disclose the electronic origin of the selectivity. Overall, the NBO charges of alkynes correlated qualitatively with regioisomer outcome. In a detailed computational PKR case study, the obtained Boltzmann distributions of the transition state (TS) populations correlate closely with experimental regioselectivity. Analysis of the TS-structures revealed that weak interactions, e.g., hydrogen bonding and steric repulsion, affect the regioselectivity and can easily override the electronic guidance.

Effect of mutual position of electron donor and acceptor on photoinduced electron transfer in supramolecular chlorophyll-fullerene dyads.

In this study we have explored the influence of mutual position of chlorin electron donor and fullerene C60 electron acceptor on photoinduced electron transfer. Two zinc-chlorin-aza-[18]crown-6 compounds and three pyrrolidino[60]fullerenes with alkyl aminium and varying coordinative moieties were synthesized and used for self-assembling of a set of complexes via two-point binding. The aza[18]crown6 moieties were connected to chlorins via amide linker either at 13(4) or 17(4) position, hence, being attached on different sides of the chlorin plane. Furthermore, in the former case, the linker holds the crown closely spaced, whereas, in the latter, the linker gives more space and conformational freedom for the crown with respect to the chlorin macrocycle. The coordinative moieties at fullerene site, 3-pyridine, 4-pyridine, and 3-furan, were built by utilizing the Prato reaction. The two-point binding drove the molecules into specific complex formation by self-assembling; aminium ion was chelated by crown ether, while zinc moiety of chlorin was coordinated by pyridine and furan. Such pairing resulted in distinct supramolecular chlorin-fullerene dyads with defined distance and orientation. The performed computational studies at DFT level in solution, with TPSS-D3/def2-TZVP//def2-SVP, indicated different geometries and binding energies for the self-assembling complexes. Notably, the computations pointed out that for all the studied complexes, the donor-acceptor distances and binding energies were dictated by chirality of pyrrolidino ring at C60. The selective excitation of chlorin chromophore revealed efficient emission quenching in all dyads. The ultrafast spectroscopy studies suggested a fast and efficient photoinduced charge transfer in the dyads. The lifetimes of the charge separated states range from 55 to 187 ps in o-dichlorobenzene and from 14 to 60 ps in benzonitrile. Expectedly, the electron transfer rate was found to be critically dependent on the donor-acceptor distance; additionally, the mutual orientation of these entities was found to have significant contribution on the rate.

Light-harvesting nanorods based on pheophorbide-appending cellulose.

In contrast to the success in artificial DNA- and peptide-based nanostructures, the ability of polysaccharides to self-assemble into one-, two-, and three-dimensional nanostructures are limited. Here, we describe a strategy for designing and fabricating nanorods using a regioselectively functionalized cellulose derivative at the air-water interface in a stepwise manner. A semisynthetic chlorophyll derivative, pyro-pheophorbide a, was partially introduced into the C-6 position of the cellulose backbone for the design of materials with specific optical properties. Remarkably, controlled formation of cellulose nanorods can be achieved, producing light-harvesting nanorods that display a larger bathochromic shift than their solution counterparts. The results presented here demonstrate that the self-assembly of functionalized polysaccharides on surfaces could lead the nanostructures mimicking the naturally occurring chloroplasts.

Tautomeric switching and metal-cation sensing of ligand-equipped 4-hydroxy-/4-oxo-1,4-dihydroquinolines.

Novel 4-hydroxyquinoline (4HQ) based tautomeric switches are reported. 4HQs equipped with coordinative side arms (8-arylimino and 3-piperidin-1-ylmethyl groups) were synthesized to access O- or N-selective chelation of Zn(2+) and Cd(2+) ions by 4HQ. In the case of the monodentate arylimino group, O chelation of metal ions induces concomitant switching of phenol tautomer to the keto form in nonpolar or aprotic media. This change is accompanied by selective and highly sensitive fluorometric sensing of Zn(2+) ions. In the case of the bidentate 8-(quinolin-8-ylimino)methyl side arm, NMR studies in CD(3) OD indicated that both Cd(2+) and Zn(2+) ions afford N chelation for 4HQ, coexisting with tautomeric switching from quinolin-4(1H)-one to quinolin-4-olate. In corroboration, UV/Vis-monitored metal-ion titrations in toluene and methanol implied similar structural changes. Additionally, fluorescence measurements indicated that the metal-triggered tautomeric switching is associated with compound signaling properties. The results are supported by DFT calculations at the B3LYP 6-31G* level. Several X-ray structures of metal-free and metal-chelating 4HQ are presented to support the solution studies.

Electronic regioselectivity of diarylalkynes in cobalt-mediated Pauson-Khand reaction: an experimental and computational study with para- and meta-substituted diarylalkynes and norbornene.

Both steric and electronic factors of substituted alkynes are known to guide α/ß-cyclopentenone regioselectivity in the cobalt-mediated Pauson-Khand reaction (PKR). In synthetic applications of the PKR, the steric factors can often override or render possible electronic effects. This study examined alkyne-dependent electronic regioselectivity of cyclopentenone formation in PKR with norbornene and sterically equivalent, but electronically unsymmetrical, meta- and para-substituted diarylethynyls to unveil the role of electronic effects alone. In agreement with the literature reports, EDG para-substituted aryls, to some extent, favored the cyclopentenone α-regioisomer, while the EWG-substituted aryls correspondingly preferred the ß-regioisomer. The cooperation of EGW and EDG in diaryl-substituted alkynes did not lead to any increased regioselectivities that could be expected by a "push-pull" effect. Both EWG and EDG meta-substituted aryls preferred the ß-regioisomer, which was demonstrated by 3,5-dimethoxy- and 3,5-bis(trifluoromethyl)-1-phenylethynyls that yielded 1/1.6 and 1/2.0 α/ß-regioselectivities, respectively. Theoretically, inspection of Hammett values of α-alkyne carbons gave qualitatively satisfactory prediction for para-substituted aryls but correlated only weakly with meta-substituted effects. Computational investigations at the DFT level revealed a correlation between NBO charges and the regioselectivity. Overall, the results suggest that the polarity of an alkyne, also designated by the relative polarization of aryl α-carbons, dictates the regioselectivity in the absence of steric effects.

Phenanthrenequinone-Sensitized Photocatalytic Synthesis of Polysubstituted Quinolines from 2-Vinylarylimines.

Visible-light-excited 9,10-phenanthrenequinone (PQ*) was used as a photocatalyst for the synthesis of polysubstituted quinolines via the electrocyclization of 2-vinylarylimines. Up to quantitative yields of 2,4-disubstituted quinolines were received after 1 h of excitation with blue LEDs at room temperature when MgCO3 was used as an additive in DCM. On the basis of experimental and DFT studies, we propose that PQ* induces one-electron oxidation of the imine substrate that triggers the electrocyclization mechanism.

Dual H-bond activation of NHC-Au(i)-Cl complexes with amide functionalized side-arms assisted by H-bond donor substrates or acid additives.

Novel approach with amide-tethered H-bond donor NHC ligands enabled Au(i)-catalysis via H-bonding. The plain NHC-Au(i)-Cl complex catalysed conversions of terminal N-propynamides to oxazolines, and enyne cycloisomerization with an acid additive, in DCM at RT. DFT calculations enlightened the function of the side-arm in the activation.

Zn pyro-pheophorbide a--fulleronicotine dyad; supramolecular self assembled donor-acceptor system for photoinduced charge separation.

Zn pyro-pheophorbide a and fulleronicotine form in nonpolar solvents a supramolecular self-assembled electron donor-acceptor dyad, which performs fast photoinduced charge separation (0.5 ps) and slow recombination (>1 ns) as evidenced by photochemical studies.

Hydrogen bond driven self-assembled C2-symmetric chlorin syn dimers; unorthodox models for chlorophyll 'special pairs' in photosynthetic reaction centres.

Amide linked pyro-pheophorbide a dimers, equipped with a suitable length of linkage, assemble in non-polar solvents by internal hydrogen bonding into C2-symmetric stacked structures as evidenced by UV-vis, fluorescence, IR, CD, 1H NMR spectroscopy and DFT molecular modelling studies.

Visible-Light-Photocatalyzed Reductions of N-Heterocyclic Nitroaryls to Anilines Utilizing Ascorbic Acid Reductant.

A photoreductive protocol utilizing [Ru(bpy)3]2+ photocatalyst, blue light LEDs, and ascorbic acid (AscH2) has been developed to reduce nitro N-heteroaryls to the corresponding anilines. Based on experimental and computational results and previous studies, we propose that the reaction proceeds via proton-coupled electron transfer between AscH2, photocatalyst, and the nitro N-heteroaryl. The method offers a green catalytic procedure to reduce, e.g., 4-/8-nitroquinolines to the corresponding aminoquinolines, substructures present in important antimalarial drugs.

Noncovalent attachment of pyro-pheophorbide a to a carbon nanotube.

Pyrene mediated noncovalent attachment of a chlorophyll derivative, pyro-pheophorbide a, to a soluble single wall carbon nanotube is reported and the resultant CD, UV-Vis absorbance, fluorescence and 1H NMR spectra are discussed.

Novel crown ether and salen metal chelation driven molecular pincers.

The synthesis of novel metal chelation driven molecular pincers based on crown ether and salen ligand substructures are described. Their functionality was monitored by fluorescence spectroscopy using pyrene groups as fluorescence probes. The pincer was shown to function reversibly with respect to metal chelation with Zn2+ and also to chelate ditopically with 100 mol % of Zn2+ and 100 mol % Li+ or 200 mol % of Li+ ion.