Gold(I)-Mediated Isomerization of Spring-loaded 1,7-Enynes seen through the Lens of Density Functional Theory.

We propose a mechanism for the previously reported formation of benzobicyclo[3.2.0]heptane derivatives from 1,7-enyne derivatives bearing a terminal cyclopropane. -> A mechanism for the previously reported formation of benzobicyclo[3.2.0]heptane derivatives from 1,7-enyne derivatives bearing a terminal cyclopropane is proposed.

Direct π-Activation vs. O-Activation in Halogen-Bonding Catalysis.

Halogen bond donors had an increasing impact on catalysis in recent years, and their development is highly active. In particular, numerous iodine-based halogen bond donors have been developed and used to promote various reactions through coordinating carbonyl groups, a ubiquitous mode of activation in catalysis. We now report computational data which strongly suggests that an alternative activation mode, through direct π-complexation, is operative for unsaturated carbonyl substrates. Calculations also suggest that solvent polarity could have a clear impact on the preference of the mode of activation, raising the possibility of a mechanistic manifold switch through solvent variation. These findings could profoundly impact the development of the next generation of halogen-bond donor catalysts.

A Quantum-chemical Analysis on the Lewis Acidity of Diarylhalonium Ions.

Cyclic diaryliodonium compounds like iodolium derivatives have increasingly found use as noncovalent Lewis acids in the last years. They are more stable toward nucleophilic substitution than acyclic systems and are markedly more Lewis acidic. Herein, this higher Lewis acidity is analyzed and explained via quantum-chemical calculations and energy decomposition analyses. Its key origin is the change in energy levels and hybridization of iodine's orbitals, leading to both more favorable electrostatic interaction and better charge transfer. Both of the latter seem to contribute in similar fashion, while hydrogen bonding as well as steric repulsion with the phenyl rings play at best a minor role. In comparison to iodolium, bromolium and chlorolium are less Lewis acidic the lighter the halogen, which is predominantly based on less favorable charge-transfer interactions.

Crystal Structures of Lignocellulosic Furfuryl Biobased Polydiacetylenes with Hydrogen-Bond Networks: Influencing the Direction of Solid-State Polymerization through Modification of the Spacer Length.

We present the topochemical polymerization of two lignocellulosic biobased diacetylenes (DAs) that only differ by an alkyl spacer length of 1 methylene (n = 1) or 3 methylene units (n = 3) between the diyne and carbamate functionalities. Their crystalline molecular organizations have the distinctive feature of being suitable for polymerization in two potential directions, either parallel or skewed to the hydrogen-bonded (HB) network. However, single-crystal structures of the final polydiacetylenes (PDAs) demonstrate that the resulting orientation of the conjugated backbones is different for these two derivatives, which lead to HB supramolecular polymer networks (2D nanosheets) for n = 1 and to independent linear PDA chains with intramolecular HBs for n = 3. Thus, spacer length modification can be considered a new strategy to influence the molecular orientation of conjugated polymer chains, which is crucial for developing the next generation of materials with optimal mechanical and optoelectronic properties. Calculations were performed on model oligodiacetylenes to evaluate the cooperativity effect of HBs in the different crystalline supramolecular packing motifs and the energy profile related to the torsion of the conjugated backbone of a PDA chain (i.e., its ability to adopt planar or helical conformations).

CalcUS: An Open-Source Quantum Chemistry Web Platform.

Computational chemistry is an increasingly active field due to the improvement of computing resources and theoretical tools. However, its use remains usually limited to technically inclined users due to the technical challenges of preparing, launching, and analyzing calculations. In this context, we have developed CalcUS, an open-source platform to streamline quantum chemistry studies. Its objective is to democratize access to computational chemistry by providing a user-friendly web interface to simplify running and analyzing quantum mechanical calculations. It is freely available, expandable, and customizable. It promotes connectivity to multiple software packages and algorithms, thus providing state-of-the-art techniques to all practitioners. We propose CalcUS as a standalone tool and infrastructure to support other open-source packages.

Iodine(III)-Based Halogen Bond Donors: Properties and Applications.

Halogen bonding, the non-covalent interaction of Lewis bases with an electron-deficient region of halogen substituents, received increased attention recently. Consequently, the design and evaluation of numerous halogen-containing species as halogen bond donors have been subject to intense research. More recently, organoiodine compounds at the iodine(III) state have been receiving growing attention in the field. Due to their electronic and structural properties, they provide access to unique binding modes. For this reason, our groups have been involved in the development of such compounds, in the quantification of their halogen bonding strength (through the evaluation of their Lewis acidities), as well as in the evaluation of their activities as catalysts in several model reactions. This account will describe these contributions.

Iodine-Catalyzed Synthesis of Substituted Furans and Pyrans: Reaction Scope and Mechanistic Insights.

Substituted pyrans and furans are core structures found in a wide variety of natural products and biologically active compounds. Herein, we report a practical and mild catalytic method for the synthesis of substituted pyrans and furans using molecular iodine, a simple and inexpensive catalyst. The method described is performed under solvent-free conditions at an ambient temperature and atmosphere, thus offering a facile and practical alternative to currently available reaction protocols. A combination of experimental studies and density functional theory calculations revealed interesting mechanistic insights into this seemingly simple reaction.

A low cost, high accuracy method for halogen bonding complexes.

The ONIOM scheme M052X/[Def2TZVP+Def2TZVPD.ECP(I)]:AM1 is shown to represent halogen bond (XB) geometries nearly as well as DFT while being more than two orders of magnitude faster in systems containing >40 atoms. This finding is shown to hold for 40 XB donors, which cover most known backbones, and for a range of neutral and anionic Lewis bases. Complexation free energies can be accurately computed using these geometries and a single-point energy calculation at the DFT level. This approach circumvents the unfavorable scaling of computing time associated with modeling large systems involving halogen bonding.

Pyridylic anions are soft nucleophiles in the palladium-catalyzed C(sp3)-H allylation of 4-alkylpyridines.

We report a mild palladium-catalyzed method for the selective allylation of 4-alkylpyridines in which highly basic pyridylic anions behave as soft nucleophiles. This method exploits alkylidene dihydropyridines, which are semi-stable intermediates readily formed using a 'soft-enolization' approach, in a new mechanistic manifold for decarboxylative allylation. Notably, the catalytic generation of pyridylic anions results in a substantially broader functional group tolerance compared to other pyridine allylation methods. Experimental and theoretical mechanistic studies strongly suggest that pyridylic anions are indeed the active nucleophiles in these reactions, and that they participate in an outer-sphere reductive elimination step. This finding establishes a new pK a boundary of 35 for soft nucleophiles in transition metal-catalyzed allylations.

Investigation of Iodonium Trifluoroborate Zwitterions as Bifunctional Arene Reagents.

The synthesis of a new family of iodonium zwitterions, in which the formal anion is a trifluoroborate moiety, is reported. These reagents present very good stability and have high resistance toward benzyne formation. Their structures were confirmed by X-ray crystallographic analysis and were further investigated using DFT calculations. QTAIM analysis supports an ionic, noncovalent, I+···BF3- interaction, in accordance with a true zwitterionic nature. Preliminary results of synthetic applications, the arylation of phenolates and trifluoroborate group functionalization, are reported.