Photogeneration of Chlorine Radical from a Self-Assembled Fluorous 4CzIPN•Chloride Complex: Application in C-H Bond Functionalization.

The chlorine radical is a strong HAT (Hydrogen Atom Transfer) agent that is very useful for the functionalization of C(sp3)-H bonds. Albeit highly attractive, its generation from the poorly oxidizable chloride ion mediated by an excited photoredox catalyst is a difficult task. We now report that 8Rf8-4CzIPN, an electron-deficient fluorous derivative of the benchmark 4CzIPN photoredox catalyst belonging to the donor-acceptor carbazole-cyanoarene family, is not only a better photooxidant than 4CzIPN, but also becomes an excellent host for the chloride ion. Combining these two properties ultimately makes the self-assembled 8Rf8-4CzIPN•Cl- dual catalyst highly reactive in redox-neutral Giese-type C(sp3)-H bond alkylation reactions promoted by the chlorine radical. Additionally, because of its fluorous character, the efficient separation/recovery of 8Rf8-4CzIPN could be envisioned.

Singlet Oxygen Responsive Molecular Receptor to Modulate Atropisomerism and Cation Binding.

In switchable molecular recognition, 1 O2 stimulus responsive receptors offer a unique structural change that is rarely exploited. The employed [4+2] reaction between 1 O2 and anthracene derivatives is quantitative, reversible and easily implemented. To evaluate the full potential of this new stimulus, a non-macrocyclic anthracene-based host was designed for the modular binding of cations. The structural investigation showed that 1 O2 controlled the atropisomerism in an on/off fashion within the pair of hosts. The binding studies revealed higher association constants for the endoperoxide receptor compared to the parent anthracene, due to a more favoured preorganization of the recognition site. The fatigue of the 1 O2 switchable hosts and their complexes was monitored over five cycles of cycloaddition/cycloreversion.

Chiral Anthranyl Trifluoromethyl Alcohols: Structures, Oxidative Dearomatization and Chiroptical Properties.

Chiral trifluoromethyl alcohol groups were introduced at the hindered ortho positions of 9,10-diphenylanthracenes to investigate their effects on the physical properties and reactivity towards oxidative dearomatization. In such compact structures, the position in different quadrants and the preferred orientation of the -CH(OH)CF3 groups were determined by the relative and absolute configurations of each stereoisomer, respectively. As a consequence, the stereochemistry governs the organization of the H-bonded molecules in single crystals (homochiral dimers vs ribbon), whereas in chlorinated solvents, they all behave as discrete compounds. Concerning their reactivity, the stereospecific dearomative oxidation of these molecules leads to 9,10-bis-spiro-isobenzofuran-anthracenes, when using organic single-electron transfer oxidants. The chiroptical properties of the alcohols and the corresponding dearomatized products were compared and showed an important modulation of the intensity.

One-pot synthesis of isosorbide from cellulose or lignocellulosic biomass: a challenge?

The catalytic conversion of (ligno)cellulose is currently subject of intense research. Isosorbide is one of the interesting products that can be produced from (ligno)cellulose as it can be used for the synthesis of a wide range of pharmaceuticals, chemicals, and polymers. Isosorbide is obtained after the hydrolysis of cellulose to glucose, followed by the hydrogenation of glucose to sorbitol that is then dehydrated to isosorbide. The one-pot process requires an acid and a hydrogenation catalyst. Several parameters are of importance during the direct conversion of (ligno)cellulose such as the acidity, the crystallinity and the particle size of cellulose as well as the nature of the feedstocks. This review highlights all these parameters and all the strategies employed to produce isosorbide from (ligno)cellulose in a one-pot process.

Chemo- and Regioselective Additions of Nucleophiles to Cyclic Carbonates for the Preparation of Self-Blowing Non-Isocyanate Polyurethane Foams.

Polyurethane (PU) foams are indisputably daily essential materials found in many applications, notably for comfort (for example, matrasses) or energy saving (for example, thermal insulation). Today, greener routes for their production are intensively searched for to avoid the use of toxic isocyanates. An easily scalable process for the simple construction of self-blown isocyanate-free PU foams by exploiting the organocatalyzed chemo- and regioselective additions of amines and thiols to easily accessible cyclic carbonates is described. These reactions are first validated on model compounds and rationalized by DFT calculations. Various foams are then prepared and characterized in terms of morphology and mechanical properties, and the scope of the process is illustrated by modulating the composition of the reactive formulation. With impressive diversity and accessibility of the main components of the formulations, this new robust and solvent-free process could open avenues for construction of more sustainable PU foams, and offers the first realistic alternative to the traditional isocyanate route.

Singlet oxygen stimulus for switchable functional organic cages.

Molecular cages 1a and 2a incorporating a 9,10-diphenylanthracene (DPA) chromophore were synthesized through a templated ring-closure metathesis approach that allows variation in cavity size through the introduction of up to three different pillars. Reversible Diels-Alder reaction between the DPA moiety and photogenerated singlet oxygen smoothly converted 1a and 2a to the corresponding endoperoxide cages 1b and 2b, which are converted back to 1a and 2a upon heating. Endoperoxide formation constitutes a reversible covalent signal that combines structural changes in the interior of the cage with introduction of two additional coordination sites. This results in a large modulation of the binding ability of the receptors attributed to a change in the location of the preferred binding site owing to the added coordination by the endoperoxide oxygen lone pairs. Cages 1a and 2a form complexes with sodium and cesium whose association constants are modified by 4-20 fold for Na+ and 200-450 fold for Cs+ upon conversion to 1b and 2b. DFT calculations show that in the anthracene form, cages 1a and 2a can bind 2 metal cations in their periphery so that each cation is coordinated by 4 oxygens and one amine nitrogen, whereas the endoperoxide cages 1b and 2b bind cations centrally in a geometry that favors coordination to the endoperoxide oxygens.

Experimental and theoretical investigation on the OH + CH3C(O)CH3 reaction at interstellar temperatures (T=11.7-64.4 K).

The rate coefficient, k(T), for the gas-phase reaction between OH radicals and acetone CH3C(O)CH3, has been measured using the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique (T = 11.7-64.4 K). The temperature dependence of k(T = 10-300 K) has also been computed using a RRKM-Master equation analysis after partial revision of the potential energy surface. In agreement with previous studies we found that the reaction proceeds via initial formation of two pre-reactive complexes both leading to H2O + CH3C(O)CH2 by H-abstraction tunneling. The experimental k(T) was found to increase as temperature was lowered. The measured values have been found to be several orders of magnitude higher than k(300 K). This trend is reproduced by calculations, with a special good agreement with experiments below 25 K. The effect of total gas density on k(T) has been explored. Experimentally, no pressure dependence of k(20 K) and k(64 K) was observed, while k(50 K) at the largest gas density 4.47×1017 cm-3 is twice higher than the average values found at lower densities. The computed k(T) is also reported for 103 cm-3 of He (representative of the interstellar medium). The predicted rate coefficients at 10 K surround the experimental value which appears to be very close to the low pressure regime prevailing in the interstellar medium. For gas-phase model chemistry of interstellar molecular clouds, we suggest using the calculated value of 1.8×10-10 cm3 molecule-1 s-1 at 10 K and the reaction products are water and CH3C(O)CH2 radicals.

Organocatalytic Coupling of CO2 with Oxetane.

The organocatalytic coupling of CO2 with oxetanes is investigated under solvent-free conditions. The influence of the main reaction parameters (type of organocatalytic system, pressure, and temperature) on the yield, the product formed, and the selectivity of the reaction are discussed. An onium salt combined with a fluorinated alcohol promotes the efficient and selective organocatalytic synthesis of α,ω-hydroxyl oligocarbonates by coupling CO2 with oxetanes at 130 °C and at a CO2 pressure as low as 2 MPa. NMR characterizations were correlated with matrix-assisted laser desorption/ionization with time-of-flight mass spectrometer (MALDI-TOF) analyses for elucidating the structure of the oligomers. Online FTIR studies under pressure, NMR titrations, and DFT calculations allowed an in-depth understanding of the reaction mechanism. Finally, CO2 -based poly(carbonate-co-urethane)s were synthesized by step-growth polymerization of hydroxyl telechelic oligocarbonates with 4,4'-methylene diphenyl diisocyanate (MDI). The organocatalytic system described herein constitutes an innovative sustainable route to the selective preparation of hydroxyl telechelic carbonates of high interest for many applications, notably for the polyurethane business (especially for coatings or foams).

Organocatalytic Coupling of CO2 with a Propargylic Alcohol: A Comprehensive Mechanistic Study.

The metal-free coupling of a propargylic alcohol with CO2 catalysed by guanidine derivatives was investigated in detail through the combination of online kinetic studies by in situ attenuated-total reflection IR (ATR-IR) spectroscopy and DFT calculations. Bicyclic guanidines, namely 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) are effective catalysts for the conversion of 2-methyl-3-butyn-2-ol to α-methylene cyclic carbonate and oxoalkyl acyclic carbonate under mild reaction conditions. The lower selectivity of TBD in comparison with MTBD towards the formation of α-methylene cyclic carbonate was elucidated from DFT calculations and is related to the bifunctional activity (base/H-bond donor) of TBD decreasing the Gibbs free energy of the reaction path for the formation of the acyclic carbonate.

Quantum Tunneling Enhancement of the C + H2O and C + D2O Reactions at Low Temperature.

Recent studies of neutral gas-phase reactions characterized by barriers show that certain complex forming processes involving light atoms are enhanced by quantum mechanical tunneling at low temperature. Here, we performed kinetic experiments on the activated C((3)P) + H2O reaction, observing a surprising reactivity increase below 100 K, an effect that is only partially reproduced when water is replaced by its deuterated analogue. Product measurements of H- and D-atom formation allowed us to quantify the contribution of complex stabilization to the total rate while confirming the lower tunneling efficiency of deuterium. This result, which is validated through statistical calculations of the intermediate complexes and transition states has important consequences for simulated interstellar water abundances and suggests that tunneling mechanisms could be ubiquitous in cold dense clouds.

Comparison of electron and chemical ionization modes for the quantification of thiols and oxidative compounds in white wines by gas chromatography-tandem mass spectrometry.

A rapid, sensitive method for assaying volatile impact compounds in white wine was developed using gas chromatography-tandem mass spectrometry (GC-MS/MS) technology, with a triple quadrupole analyzer operating in chemical ionization and electron impact mode. This GC-MS/MS method made it possible to assay volatile thiols (3SH: 3-sulfanylhexanol, formerly 3MH; 3SHA: 3-sulfanylhexyl acetate, formerly 3MHA; 4MSP: 4-methyl-4-sulfanylpentan-2-one, formerly 4MMP; BM: benzenemethanethiol; E2SA: ethyl 2-sulfanylacetate; and 2FM: 2-furanmethanethiol) and odoriferous oxidation markers (Sotolon: 4,5-dimethyl-3-hydroxy-2(5)H-furanone, methional, and phenylacetaldehyde) simultaneously in dry white wines, comparing electron impact (EI) and chemical ionization (CI) modes. More molecular ions were produced by CI than protonated molecules, despite the greater fragmentation caused by EI. So, even using the best reactant gas giving the highest signal for thiols, EI was the best ionization mode, with the lowest detection limits. For all compounds of interest, the limits of quantification (LOQ) obtained were well below their detection thresholds (ranging from 0.5 to 8.5ng/L for volatile thiols and 65-260ng/L for oxidation markers). Recovery rates ranged from 86% to 111%, reproducibility (in terms of relative standard deviation; RSD) was below 18% in all cases, with correlation coefficients above 0.991 for all analytes. The method was successfully applied to the analysis of compounds of interest in Sauvignon Blanc wines from a single estate and ten different vintages.

Fluorinated Alcohols as Activators for the Solvent-Free Chemical Fixation of Carbon Dioxide into Epoxides.

The addition of fluorinated alcohols to onium salts provides highly efficient organocatalysts for the chemical fixation of CO2 into epoxides under mild experimental conditions. The combination of online kinetic studies, NMR titrations and DFT calculations allows understanding this synergistic effect that provides an active organocatalyst for CO2 /epoxides coupling.

Charge dissociation at interfaces between discotic liquid crystals: the surprising role of column mismatch.

The semiconducting and self-assembling properties of columnar discotic liquid crystals have stimulated intense research toward their application in organic solar cells, although with a rather disappointing outcome to date in terms of efficiencies. These failures call for a rational strategy to choose those molecular design features (e.g., lattice parameter, length and nature of peripheral chains) that could optimize solar cell performance. With this purpose, in this work we address for the first time the construction of a realistic planar heterojunction between a columnar donor and acceptor as well as a quantitative measurement of charge separation and recombination rates using state of the art computational techniques. In particular, choosing as a case study the interface between a perylene donor and a benzoperylene diimide acceptor, we attempt to answer the largely overlooked question of whether having well-matching donor and acceptor columns at the interface is really beneficial for optimal charge separation. Surprisingly, it turns out that achieving a system with contiguous columns is detrimental to the solar cell efficiency and that engineering the mismatch is the key to optimal performance.

Gas-phase kinetics of the hydroxyl radical reaction with allene: absolute rate measurements at low temperature, product determinations, and calculations.

The gas phase reaction of the hydroxyl radical with allene has been studied theoretically and experimentally in a continuous supersonic flow reactor over the range 50 ≤ T/K ≤ 224. This reaction has been found to exhibit a negative temperature dependence over the entire temperature range investigated, varying between (0.75 and 5.0) × 10(-11) cm(3) molecule(-1) s(-1). Product formation from the reaction of OH and OD radicals with allene (C(3)H(4)) has been investigated in a fast flow reactor through time-of-flight mass spectrometry, at pressures between 0.8 and 2.4 Torr. The branching ratios for adduct formation (C(3)H(4)OH) in this pressure range are found to be equal to 34 ± 16% and 48 ± 16% for the OH and OD + allene reactions, respectively, the only other channel being the formation of CH(3) or CH(2)D + H(2)CCO (ketene). Moreover, the rate constant for the OD + C(3)H(4) reaction is also found to be 1.4 times faster than the rate constant for the OH + C(3)H(4) reaction at 1.5 Torr and at 298 K. The experimental results and implications for atmospheric chemistry have been rationalized by quantum chemical and RRKM calculations.

Silylboranes as new sources of silyl radicals for chain-transfer reactions.

Various silylboranes, which were outfitted with a catecholborane moiety at one end and a (Me(3)Si)(3)Si moiety at the other end of a carbon chain, were prepared through the hydroboration of the corresponding unsaturated silanes. The C-centered radical species generated from these silylboranes efficiently cyclized to provide, through a 5-exo intramolecular homolytic substitution at the silicon center, the corresponding silacycle and a Me(3)Si radical that was subsequently trapped by sulfonyl acceptors. These cyclizations proceeded at unprecedented rates, due, in part, to a strong gem-dialkyl effect that was attributable to the presence of bulky substituents on a quaternary center located on the chain. In parallel, we designed arylsilylboranes that produced silyl radicals through a 1,5-hydrogen transfer. Such silyl radicals may be valuable radical chain carriers, for instance, in oximation reactions of alkyl halides. Finally, computational studies allowed calculation of activation barriers of the homolytic substitution step and additionally illustrated that the overall reaction mechanism involved a transition state in which the attacking carbon center, the central silicon atom, and the Me(3)Si leaving group were collinear.

Allylsilanes in "tin-free" oximation, alkenylation, and allylation of alkyl halides.

Tin-free oximation, vinylation, and allylation of alkyl halides have been developed by using allylsilanes as di-tin surrogates. Initiation of the radical process with a peroxide provides the silyl radical, which can abstract a halogen from the corresponding alkyl halide. The resulting carbon-centered radical then adds to various acceptors, including a sulfonyloxime, a vinylsulfone, and an allylsulfone, leading to formation of the desired products along with the corresponding allylsulfone resulting from the reaction of the PhSO(2) radical with the allylsilane precursor. Better results were generally obtained with methallylsilane 1b than with 1a. This observation was rationalized by invoking the higher nucleophilicity of 1b and the faster ß-fragmentation of the corresponding ß-silyl radical intermediate. Calculation of the energy barrier for the ß-fragmentation of a series of ß-silyl radicals at the DFT level supported this hypothesis. Finally, a second version of these oximation and vinylation reactions, based on the utilization of 3-tris(trimethylsilyl)silylthiopropene, was devised, affording the desired oximes and olefins in reasonable yields. This strategy allowed the title reaction to be performed under milder conditions (AIBN, benzene, 80 °C), as a result of the easier ß-fragmentation of the C-S bond as compared with the C-Si bond.

On the interaction between supercritical CO2 and epoxides combining infrared absorption spectroscopy and quantum chemistry calculations.

The nature and strength of the interactions occurring between epoxides and CO(2) have been investigated by combining infrared spectroscopy with quantum chemistry calculations. A series of infrared absorption experiments on four model epoxide molecules highly diluted in supercritical CO(2) have been performed at constant temperature T = 40 °C for various CO(2) pressures varying from 1 to 30 MPa. Then, we carried out a theoretical analysis based on quantum chemistry calculations using Density Functional Theory (B3PW91 and CAM-B3LYP) and ab initio (MP2) computational methods. A very good agreement between experimental and calculated vibrational frequency shifts of the epoxide ring vibrations group was obtained using the CAM-B3LYP functional, hence validating the calculated optimized geometries of the epoxide-CO(2) complexes. Whatever the epoxide considered, CO(2) is found to be on average above the oxygen atom of the epoxy ring and interacts with the carbon atom of CO(2) through a Lewis acid-Lewis base type of interaction. The substituents on the epoxide ring are found to influence the stability of the epoxide-CO(2) complexes mainly because of the partial charge on the oxygen atom that is sensitive to the nature of the substituent.

Supramolecular organization and charge transport properties of self-assembled π-π stacks of perylene diimide dyes.

Molecular dynamics (MD) simulations have been coupled to valence bond/Hartree-Fock (VB/HF) quantum-chemical calculations to evaluate the impact of diagonal and off-diagonal disorder on charge carrier mobilities in self-assembled one-dimensional stacks of a perylene diimide (PDI) derivative. The relative distance and orientation of the PDI cores probed along the MD trajectories translate into fluctuations in site energies and transfer integrals that are calculated at the VB/HF level. The charge carrier mobilities, as obtained from time-of-flight numerical simulations, span several orders of magnitude depending on the relative time scales for charge versus molecular motion. Comparison to experiment suggests that charge transport in the crystal phase is limited by the presence of static defects.

Gas-phase kinetics of hydroxyl radical reactions with C3H6 and C4H8: product branching ratios and OH addition site-specificity.

Products of the reaction of OH radicals with propene, trans-2-butene, and 1-butene have been investigated in a fast flow reactor, coupled with time-of-flight mass spectrometry, at pressures between 0.8 and 3.0 Torr. The product determination includes H atom abstraction channels as well as site-specific OH addition. The OH radicals are produced by the H + NO(2) → OH + NO reaction or by the F + H(2)O → OH + HF reaction, the H or F atoms being produced in a microwave discharge. The gas mixture is ionized using single photon ionization (SPI at 10.54 eV), and products are detected using time-of-flight mass spectrometry (TOF-MS). The H atom abstraction channels are measured to be <2% for OH + propene, 8 ± 3% for OH + 1-butene, and 3 ± 1% for OH + trans-2-butene. Analysis of ion fragmentation patterns leads to 72 ± 16% OH addition to the propene terminal C atom and 71 ± 16% OH addition to the 1-butene terminal C atom. The errors bars represent 95% confidence intervals and include estimated uncertainties in photoionization cross sections.

Effect of the dynamical disorder on the second-order nonlinear optical responses of helicity-encoded polymer strands.

By combining classical samplings with quantum chemistry semiempirical time-dependent Hartree-Fock calculations, the high impact of dynamic fluctuations on the NLO properties of helical strands has been evidenced. In particular, these fluctuations are responsible for relative variations of approximately 20% in the hyper-Rayleigh responses in both pyridine-pyrimidine (py-pym) and hydrazone-pyrimidine (hy-pym) strands. Then, dynamical disorder has an even more important impact on the electric-field-induced second harmonic generation responses, whose variations can reach 2 (py-pym) or 5 (hy-pym) times their mean value. This work also highlights the relationships between the unit cell nature and helical conformation of foldamers and their second-order NLO responses. In particular, the octupolar symmetry of the hyper-Rayleigh depolarization ratios is related to the helix periodicity of three unit cells per turn in both compounds.