Showing papers by "Vincent Robert published in 2018"

Helical Chirality Induces a Substrate-Selectivity Switch in Carbohydrates Recognitions.

Abstract: A new chiral hemicryptophane cage combining an electron-rich cyclotriveratrylene (CTV) unit and polar amine functions has been synthesized. The resolution of the racemic mixture has been performed by chiral HPLC, and the assignment of the absolute configuration of the two enantiomers has been achieved using ECD spectroscopy. In contrast with other hemicryptophane receptors, the two enantiomeric hosts display both remarkable enantioselectivities in the recognition of carbohydrates and good binding constants. Moreover, by switching the chirality of the CTV unit from M to P, a strong preference shift from glucose to mannose derivatives is observed.

Endohedral Functionalized Cage as a Tool to Create Frustrated Lewis Pairs

Abstract: A frustrated Lewis pair (FLP) system was obtained by confinement of the Lewis base partner, a Verkade's superbase, in a molecular cavity Whereas the model superbase lacking cavity displayed no catalytic activity in Morita-Baylis-Hillman (MBH) reactions, when associated to titanium (IV) chloride, the encaged superbase turns out to be an efficient catalyst under the same conditions The crucial role of the endohedral functionalized cage on catalytic performance was further demonstrated by the fact that model superbases with bulky substituents were much less efficient to produce active catalysts, as well as by inhibition and substrate selection experiments 31 P NMR spectroscopy and mass spectrometry experiments evidenced that no interaction between the Lewis acidic and basic partners occurred when the superbase was capped by a cycloveratrylene (CTV) unit, thus creating a true FLP active system

Positive Cooperative Effect in Ion‐Pair Recognition by a Tris‐urea Hemicryptophane Cage

Abstract: The synthesis of a hemicryptophane cage combining a CTV unit with a C3 symmetrical moiety bearing three urea functions is reported. This host was found to bind anions with higher binding constants than other previously reported hemicryptophanes. Due to its heteroditopic character this cage proved to be an efficient ion-pair receptor. The best cooperativity effect was observed for the tetramethylammonium bromide (TMABr) salt, which was confirmed and rationalized by DFT calculations.

Pairing-up viologen cations and dications: a microscopic investigation of van der Waals interactions

Abstract: The microscopic origin of van der Waals- and magnetic-interactions in 4,4′ methyl viologen cation-based units (MV+˙ and MV2+) was inspected using wave function (variational DDCI and perturbative MP2, CASPT2) and density functional theory (DFT) calculations. The analysis deepens the comprehension of the magnetic behavior of experimental bis-viologen cyclophanes ([CYC]2(+˙)), in which the MV+˙ units are connected through alkyl linkers of different lengths. The formation of the so-called long-multicenter bonds in such radical dimers, responsible for the quenching of the magnetic response, was analyzed in [MV2]2(+˙). Dynamical correlation effects, accessible from second-order perturbation corrections, were decisive in observing a bonding regime characterized by an equilibrium distance of 3.3 A and a 45 kJ mol−1 dissociation energy. At larger intermolecular distances, our calculations on [MV2]2(+˙) indicate that the singlet and triplet states are energetically competing (i.e. weak exchange interactions, JAB). Despite the absence of any clear bonding regime at the MP2 level, the puzzling association of two di-cations into [MV2]4+ is anticipated at 3.3 A using weakly screened point charges (e = 1.5) to account for the Coulomb interactions between the solvated subunits. The main conclusion is that both dispersion interactions and environment effects are required to overcome the Coulomb repulsion associated with doubly-charged species. All these data provide some complementary insights into the nature and amplitude of interactions between cation and dication units, and their relevance in various experimental manifestations.

Triple differential cross sections for the ionization of tetrahydrofuran by electron impact

Abstract: We provide triple differential cross sections (TDCS) for the ionization of tetrahydrofuran (THF) by single electron impact in coplanar asymmetric kinematics in the intermediate energy regime. For the C s and C2 THF isomers, the analysis is carried out using (i) the highest occupied molecular orbitals energies available from ab initio calculations, and (ii) the TDCSs computed from a Coulomb wave (CW) and a distorted wave (DW) to describe the ejected electron. The calculated TDCSs using both the CW and DW models are in better agreement with experimental data than those obtained using the molecular three-body distorted wave model. Hence, we show that the developed framework can be applied to a relatively complex molecule and that the proposed models successfully describe the ionization of both THF isomers.