Experimental and Computational Studies for the Synthesis of Functionalized Cyclopropanes from 2-Substituted Allylic Derivatives with Ethyl Diazoacetate

The catalytic asymmetric synthesis of highly functionalized cyclopropanes from 2-substituted allylic derivatives is reported. Using ethyl diazo acetate, the reaction, catalyzed by a chiral ruthenium complex (Ru(II)-Pheox), furnished the corresponding easily separable cis and trans cyclopropanes in moderate to high yields (32-97 %) and excellent ee (86-99 %). This approach significantly extends the portfolio of accessible enantioenriched cyclopropanes from an underexplored class of olefins. DFT calculations suggest that an outer-sphere mechanism is operative in this system.

Tracking SF5 I in the Iodopentafluorosulfanylation of Alkynes

In the vibrant field of SF5 chemistry, SF5 X reagents (X=F, Cl, Br) are at the heart of current investigations in radical pentafluorosulfanylation reactions. SF5 I is the missing link whose existence has not been reported despite its potential as SF5 donor. This study reports the formal addition of the hitherto unknown SF5 I reagent to alkynes by means of a combination of SF5 Cl/KI/18-crown-6 ether. The exclusive regio- and stereoselective synthesis of unprecedented (E)-1-iodo-2-(pentafluoro-λ6 -sulfanyl) alkenes was achieved. A consensus was reached through computational and mechanistic studies for the realistic formation of SF5 - anion but not SF5 I in solution and the rational involvement of SF5 ⋅ and iodine radicals in the iodo pentafluorosulfanylation reaction.

Synthesis of Indoles Using the Electrophilic Potential of Diazirines

The electrophilic potential of diazirines has been utilized to obtain N-substituted diaziridines that are directly hydrolyzed to produce monosubstituted hydrazines. The hydrazines can undergo the Fisher process with enolizable carbonyls to yield multiple indole derivatives in moderate to high yields. The N-metalated diaziridine intermediates can undergo isomerization prior to electrophilic substitution, to form N,N-disubstituted hydrazones. The latter react with enolizable carbonyls to produce N-protected indole derivatives in a single step. This protocol was used to efficiently synthesize indomethacin, an anti-inflammatory drug.

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.

N-Oxides as Control Element for the Direction of a Sigmatropic Rearrangement: Application as a Switch for Fluorescence

An exceptional level of control over the direction of the (2,3)-sigmatropic rearrangement between N-oxides and alkoxylamine is achieved by simply changing the solvent in which they are dissolved. Protic solvents like water, methanol and hexafluoroisopropanol favour the N-oxide form, while other solvents like acetone, acetonitrile and benzene favour the alkoxylamine. The reaction temperature and nature of the substituents on the alkene affect the rate of rearrangement. A N-oxide fragment was attached to two fluorescent molecules and acted as an on/off switch for their fluorescence. The conversion of alkoxylamines into the corresponding N-oxides has not previously been described and is here termed the 'Reverse Meisenheimer Rearrangement'.

Pulcherriminic acid modulates iron availability and protects against oxidative stress during microbial interactions

Siderophores are soluble or membrane-embedded molecules that bind the oxidized form of iron, Fe(III), and play roles in iron acquisition by microorganisms. Fe(III)-bound siderophores bind to specific receptors that allow microbes to acquire iron. However, certain soil microbes release a compound (pulcherriminic acid, PA) that, upon binding to Fe(III), forms a precipitate (pulcherrimin) that apparently functions by reducing iron availability rather than contributing to iron acquisition. Here, we use Bacillus subtilis (PA producer) and Pseudomonas protegens as a competition model to show that PA is involved in a peculiar iron-managing system. The presence of the competitor induces PA production, leading to precipitation of Fe(III) as pulcherrimin, which prevents oxidative stress in B. subtilis by restricting the Fenton reaction and deleterious ROS formation. In addition, B. subtilis uses its known siderophore bacillibactin to retrieve Fe(III) from pulcherrimin. Our findings indicate that PA plays multiple roles by modulating iron availability and conferring protection against oxidative stress during inter-species competition.

Iodine(III)-Mediated Enantioselective Oxidative Contraction of Dihydropyranones

The γ-butyrolactone motif is promising in pharmaceuticals and natural products with various biological activities. The oxidative contraction of dihydropyranones mediated by hypervalent iodine (HVI) reagents is an efficient approach of preparing this motif. We show that, using readily available chiral HVI reagents, numerous enantioenriched γ-butyrolactones are accessible. The method provides generally high enantioselectivities and modest to high yields. The resulting chiral iodoarene is easily recovered and reused for the reaction multiple times, without loss of yield or enantioselectivities.

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.

We report a mild palladium-catalyzed method for the selective allylation of 4-alkylpyridines in which highly basic pyridylic anions behave as soft nucleophiles.

Activation of a Metal-Halogen Bond by Halogen Bonding

In recent years, the non-covalent interaction of halogen bonding (XB) has found increasing application in organocatalysis. However, reports of the activation of metal-ligand bonds by XB have so far been limited to a few reactions with elemental iodine or bromine. Herein, we present the activation of metal-halogen bonds by two classes of inert halogen bond donors and the use of the resulting activated complexes in homogenous gold catalysis. The only recently explored class of iodolium derivatives were shown to be effective activators in two test reactions and their activity could be modulated by blocking of the Lewis acidic sites. Bis(benzimidazolium)-based halogen bonding activators provided even more rapid conversion, while the non-iodinated reference compound showed little activity. The role of halogen bonding in the activation of metal-halogen bonds was further investigated by NMR experiments and DFT calculations, which support the mode of activation occurring via halogen bonding.

Lewis Acidity Scale of Diaryliodonium Ions toward Oxygen, Nitrogen, and Halogen Lewis Bases

Equilibrium constants for the associations of 17 diaryliodonium salts Ar₂I⁺X^- with 11 different Lewis bases (halide ions, carboxylates, p-nitrophenolate, amines, and tris(p-anisyl)phosphine) have been investigated by titrations followed by photometric or conductometric methods as well as by isothermal titration calorimetry (ITC) in acetonitrile at 20 °C. The resulting set of equilibrium constants K_I covers 6 orders of magnitude and can be expressed by the linear free-energy relationship lg K_I = s_I LA_I + LB_I, which characterizes iodonium ions by the Lewis acidity parameter LA_I, as well as the iodonium-specific affinities of Lewis bases by the Lewis basicity parameter LB_I and the susceptibility s_I. Least squares minimization with the definition LA_I = 0 for Ph₂I⁺ and s_I = 1.00 for the benzoate ion provides Lewis acidities LA_I for 17 iodonium ions and Lewis basicities LB_I and s_I for 10 Lewis bases. The lack of a general correlation between the Lewis basicities LB_I (with respect to Ar₂I⁺) and LB (with respect to Ar₂CH⁺) indicates that different factors control the thermodynamics of Lewis adduct formation for iodonium ions and carbenium ions. Analysis of temperature-dependent equilibrium measurements as well as ITC experiments reveal a large entropic contribution to the observed Gibbs reaction energies for the Lewis adduct formations from iodonium ions and Lewis bases originating from solvation effects. The kinetics of the benzoate transfer from the bis(4-dimethylamino)-substituted benzhydryl benzoate Ar₂CH-OBz to the phenyl(perfluorophenyl)iodonium ion was found to follow a first-order rate law. The first-order rate constant k_obs was not affected by the concentration of Ph(C₆F₅)I⁺ indicating that the benzoate release from Ar₂CH-OBz proceeds via an unassisted S_N1-type mechanism followed by interception of the released benzoate ions by Ph(C₆F₅)I⁺ ions.

Denitrogenative Hydrotrifluoromethylation of Benzaldehyde Hydrazones: Synthesis of (2,2,2-Trifluoroethyl)arenes

Reacting hydrazones of arylaldehydes with Togni's CF₃ -benziodoxolone reagent, in the presence of potassium hydroxide and cesium fluoride, induces a denitrogenative hydrotrifluoromethylation event to produce (2,2,2-trifluoroethyl)arenes. This novel reaction was tolerant to many electronically-diverse functional groups and substitution patterns, as well as naphthyl- and heteroaryl-derived substrates. Advantages of this process include the easy access to hydrazone precursors on a large scale, speed and operational simplicity, and being transition metal-free.

Iodine(III)-Mediated Contraction of 3,4-Dihydropyranones: Access to Polysubstituted γ-Butyrolactones

Functionalized γ-butyrolactones are privileged structures in the field of medicinal chemistry; they are found in numerous natural products and synthetic compounds with diverse biological activities. The oxidative ring contraction of 3,4-dihydropyran-2-one derivatives represents a promising yet underappreciated strategy to access these compounds. To the best of our knowledge, very few examples of this strategy have been reported, with limited investigation of the influence of stereogenic centers on the starting dihydropyranones. We investigated the iodine(III)-mediated contraction of a representative set of dihydropyranone derivatives. The method gives rapid access to functionalized γ-butyrolactones in good yields. The reaction scope was investigated, and the method was found to support various levels of substituents, even enabling access to sterically congested quaternary centers. The stereoselectivity was investigated using chiral substrates and a chiral iodine(III) reagent.

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.

Iodine(III)-mediated synthesis of chiral α-substituted ketones: recent advances and mechanistic insights

The development of iodine(III)-mediated synthetic transformations has received growing interest, in particular to mediate enantioselective processes. In this class, the α-tosyloxylation of ketone derivatives using iodine(III) is a particularly powerful one, as it yields α-tosyloxy ketones – versatile chiral precursors that enable rapid access to numerous α-chiral ketones through nucleophilic displacement. Despite years of research from numerous groups, the enantioselectivities for this transformation have remained modest. Using quantum chemical calculations, we have uncovered a possible rational for the lack of selectivity. With these computational insights, we have developed an alternative experimental strategy and achieved unprecedented levels of selectivities. Applying this newfound knowledge, we have recently developed a new method to access α-halo ketones from non-ketonic precursors.

Mechanistic insights on the iodine(III)-mediated α-oxidation of ketones

The synthesis of α-substituted carbonyl compounds is of great importance due to their ubiquity in both natural and man-made biologically active compounds. The field of hypervalent iodine chemistry has been a great contributor to access these molecules. For example, the α-oxidation of carbonyl compounds has been one of the most investigated iodine(III)-mediated stereoselective transformations. Yet, it is also the transformation that has met the most challenge in terms of achieving high stereoselectivities. The different mechanistic pathways of the iodine(III)-mediated α-tosyloxylation of ketones have been investigated. The calculations suggest an unprecedented iodine(III)-promoted enolization process. Indications that iodonium intermediates could serve as proficient Lewis acids are reported. This concept could have broad impact and foster new developments in the field of hypervalent iodine chemistry.

Copper-catalyzed O-arylation of N-protected 1,2-aminoalcohols using functionalized trivalent organobismuth reagents

The O-arylation of 1,2-aminoalcohols using functionalized triarylbismuth reagents is reported.

Synthesis and characterization of combined cross-linked laccase and tyrosinase aggregates transforming acetaminophen as a model phenolic compound in wastewaters

Laccase (EC 1.10.3.2) and tyrosinases (EC 1.14.18.1) are ubiquitous enzymes present in nature as they are known to originate from bacteria, fungi, plants, etc. Both laccase and tyrosinase are copper-containing phenoloxidases requiring readily available O2 without auxiliary cofactor for their catalytic transformation of numerous phenolic substrates. In the present study, laccase and tyrosinase have been insolubilized as combined crosslinked enzyme aggregates (combi-CLEA) using chitosan, a renewable and biodegradable polymer, as crosslinker. The combi-CLEA, with specific activity of 12.3 U/g for laccase and 167.4 U/g for tyrosinase, exhibited high enzymatic activity at pH5-8 and temperature at 5-30°C, significant resistance to denaturation and no diffusional restriction to its active site based upon the Michaelis-Menten kinetic parameters. Subsequently, the combi-CLEA was applied to the transformation of acetaminophen as a model phenolic compound in samples of real wastewaters in order to evaluate the potential efficiency of the biocatalyst. In batch mode the combi-CLEA transformed more than 80% to nearly 100% of acetaminophen from the municipal wastewater and more than 90% from the hospital wastewater. UPLC-MS analysis of acetaminophen metabolites showed the formation of its oligomers as dimers, trimers and tetramers due to the laccase and 3-hydroxyacetaminophen due to the tyrosinase.

Photocatalytic degradation of carbamazepine and three derivatives using TiO₂ and ZnO: effect of pH, ionic strength, and natural organic matter

Removal of pharmaceuticals (PhCs) by photocatalysis is a promising avenue in water treatment. The efficiency of these treatments on PhC derivatives compared to their parent molecules remains poorly documented. The present study investigates the efficiency of photodegradation catalyzed by TiO₂ and ZnO nanoparticles on the removal of carbamazepine (CBZ) and three of its derivatives; carbamazepine epoxide (CBZ-E), acridine (AI), and acridone (AO). The effects of environmental parameters such as pH, ionic strength, and natural organic matter content on photodegradation efficiency (transformation after 6h and kinetics) were tested. We report that the efficiency of the catalysts (TiO2 and ZnO) can be very different when comparing CBZ and its derivatives (CBZ-E, AI and AO). TiO₂ was more efficient than ZnO at degrading CBZ and CBZ-E. For AI and AO, no significant differences were observed between the two catalysts. We also report that environmental parameters have contrasting effects on the efficiency of the photodegradation of CBZ compared to its derivatives. Changing pH and organic matter content had the most contrasted effects; the photodegradation of CBZ and CBZ-E was significantly affected by pH (especially in presence of TiO₂ NPs) and by the presence of natural organic matter. In contrast, the photodegradation of AI and AO was not affected by pH and organic matter. Only the photodegradation of CBZ was clearly affected by IS and solely at very high IS (1M). Overall, our results highlight that TiO₂ and ZnO catalysts present contrasted efficiency on the removal of CBZ when compared to its derivatives (CBZ-E, AI and AO). Our results also show that the effect of environmental parameters on the efficiency of the photodegradation of CBZ derivatives cannot be predicted based on the behavior of the parent molecule (CBZ).

Diazirines as potent electrophilic nitrogen sources: application to the synthesis of pyrazoles

Even after more than 50 years since its discovery, the electrophilic potential of diazirines was never truly exploited. This longstanding limitation has been resolved. N-Monosubstituted diaziridines and hydrazones are obtained by nucleophilic additions. They release, under hydrolysis conditions, the corresponding monosubstituted hydrazines. The latter were converted to pyrazoles in high yields. The adamantanone can be recovered in 80-100% yields. This work demonstrates the potential of diazirines as electrophilic nitrogen sources with recoverable protecting groups.

Enabling nucleophilic substitution reactions of activated alkyl fluorides through hydrogen bonding

It was discovered that the presence of water as a cosolvent enables the reaction of activated alkyl fluorides for bimolecular nucleophilic substitution reactions. DFT calculations show that activation proceeds through stabilization of the transition structure by a stronger F···H2O interaction and diminishing C-F bond elongation, and not simple transition state electrostatic stabilization. Overall, the findings put forward a distinct strategy for C-F bond activation through H-bonding.

1-Fluoro-3,3-dimethyl-1,3-dihydro-1λ(3)-benzo[d][1,2]iodoxole

The asymmetric unit of the title compound, C(9)H(10)FIO, contains two independent mol-ecules which are weakly bound by inter-molecular O⋯I inter-actions [3.046 (4) and 2.947 (4) Å]. The two covalent I-F bonds are slightly longer than the two I-O bonds.

Formal intramolecular (4 + 1)-cycloaddition of dialkoxycarbenes: control of the stereoselectivity and a mechanistic portrait

The stereoselective synthesis of 5-5, 6-5, and 7-5 fused O-heterocyclic compounds is reported. The key reaction is a formal intramolecular (4 + 1)-cycloaddition involving a dialkoxycarbene and an electron-deficient diene where the stereoselectivity is dependent on the length of the tether. An analysis of the stereochemical outcome of this reaction sheds light on its complex mechanistic picture. High-level calculations were used to support the proposed mechanistic portrait.

Theoretical bond dissociation energies of halo-heterocycles: trends and relationships to regioselectivity in palladium-catalyzed cross-coupling reactions

Selectivity of the palladium-catalyzed cross-coupling reactions of heterocycles bearing multiple identical halogens is mainly determined by the relative ease of oxidative addition. This is related to both the energy to distort the carbon halogen bond to the transition-state geometry (related to the CX bond-dissociation energy) and to the interaction between the heterocycle pi* (LUMO) and PdL(2) HOMO (J. Am. Chem. Soc. 2007, 129, 12664). The computed bond dissociation energies of a larger series of halo-heterocycles have been explored with B3LYP and higher accuracy G3B3 calculations. Quantitative trends in bond dissociation energies have been identified for five- and six-membered chloro and bromo substituted heterocycles with N, O, and S heteroatoms.